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English Pages 390 Year 1984
T
TH
T
S H
SICS
RENORMALIZATION An introduction to
renormalization, the renormalization group, and the operator-product expansion JOHN
C.COLLINS
Illinois Institute of Technology
The right of the University of Cambridge to print and sell all manner of books was granted by Henry VIII in 1534. The University has printed and published continuously since 1584.
CAMBRIDGE UNIVERSITY PRESS CAMBRIDGE LONDON
NEW YORK
MELBOURNE
NEW ROCHELLE SYDNEY
Published by the Press Syndicate of the University of Cambridge The Pitt Building, Trumpington Street, Cambridge CB2 1RP 32 East 57th Street, New York, NY 10022, USA 296 Beaconsfield Parade, Middle Park, Melbourne 3206, Australia ? Cambridge University Press 1984 First published 1984 Printed in Great Britain at the University Press, Cambridge Library of Congress catalogue card number: 83-23927
British Library cataloguing in publication data Collins, John C. Renormalization.—(Cambridge monographs on mathematical physics) 1. Renormalization (Physics) 2. Quantum field theory I. Title 530.T43
QC174.45
ISBN 0 521 24261 4
This work was supported in part by the US Department of Energy under contract number DE-AC02-80ER10712.
Contents
1
Introduction
2
Quantum field theory
2.1
Scalar field theory
1 4 4 6 9
2.2
Functional-integral
2.3
Renormalization
2.4
Ultra-violet regulators Equations of motion for Green's functions
12
2.5 2.6
Symmetries
15
2.7
Ward identities
19
2.8
Perturbation theory
21
solution
13
2.9
Spontaneously broken symmetry
25
2.10
Fermions
27
2.11
Gauge theories
28
2.12
Quantizing gauge theories BRS invariance and Slavnov-Taylor identities
29
34
2.15
Feynman rules for gauge theories Other symmetries of (2.11.7)
2.16
Model field theories
36
2.13 2.14
32
35
38
3
Basic examples
3.1
One-loop self-energy in
3.1.1
Wick rotation
39
3.1.2
Lattice
40
3.1.3
Interpretation of divergence
41
3.1.4
Computation Higher order
43 45
3.3.1
Degree of divergence c/>3 aid 6
3.3.2
Why may Z be zero and yet contain divergences?
48
3.3.3
49
3.4
Renormalizability and non-renormalizability Renormalization group
3.4.1
Arbitrariness in a renormalized
3.4.2
Renormalization prescriptions Dimensional regularization Minimal subtraction
3.2 3.3
3.5 3.6
</>3 theory
38
44 47
=
graph
50 50 52 53 56
V
Contents
VI
3.6.1
Definition
3.6.2
d
3.6.3
Renormalization group and minimal subtraction
=
56
6
57 58
3.6.4
Massless theories
59
3.7
Coordinate space
59
4
Dimensional regularization
62
4.1
Definition and axioms
64
4.2
Continuation to small d
68
4.3
Properties
73
4.4
Formulae for Minkowski space
81
4.4.1
83
4.5
Schwinger parameters Feynman parameters Dirac matrices
4.6
7 s and eK^v
86
5
Renormalization
88
5.1
Divergences and subdivergences Two-loop self-energy in (c/>3)6 Fig. 5.1.2 Differentiation with respect to external momenta Fig. 5.1.3 Renormalization of Feynman graphs One-particle-irreducible graph with no subdivergences General case
4.4.2
5.2 5.2.1 5.2.2 5.2.3 5.3 5.3.1 5.3.2
81
83
89 93 94 97 99 101 102 103 105
5.3.3
Application
5.3.4
Summary
106
5.4
Three-loop example
106
5.5
Forest formula
109
5.5.1
Formula
109
5.5.2
Proof
110
5.6
Relation to if
112
5.7
Renormalizability
116
5.7.1
Renormalizability and non-renormalizability
116
5.7.2
Cosmological term
118
5.7.3
Degrees of renormalizability
118
5.7.4
Non-renormalizability
120
5.7.5
Relation of renormalizability to dimension of coupling
121
5.7.6
Non-renormalizable theories of physics
122
5.8
Proof of locality of counterterms; Weinberg's theorem Degree of counterterms equals degree of divergence
125
5.8.1 5.8.2
R(G) is finite if d(G) is negative
127
5.8.3
Asymptotic behavior Oversubtractions Mass-shell renormalization
130
5.9 5.9.1
of general formulae
126 129
and oversubtraction
130
Contents 5.9.2
Remarks
5.9.3
Oversubtraction
vn
131 on 1 PR
graphs
131
5.10
Renormalization without regulators: the BPHZ scheme
133
5.11
Minimal subtraction
135
5.11.1
Definition
135
5.11.2
MS renormalization
137
5.11.3
Minimal subtraction with other regulators
137
6
Composite operators
138
6.1
Operator-product expansion
139
6.2
Renormalization of composite operators: examples
142
6.2.1
Renormalization of
6.2.2
c/>2 c/>2(x)(/>2(>!)
142
Renormalization of
6.3
Definitions
146
6.4
Operator mixing Tensors and minimal subtraction
150
6.6
Properties
152
6.7
Differentiation with respect to parameters 'm<f Relation of renormalizations of <f>2 and m2
166
6.5
6.8
7 7.1 7.1.1 7.1.2
145
149
Renormalization group Change of renormalization prescription Change of parametrization
163
168 169 169
7.1.3
Renormalization-prescription dependence Low-order examples
7.2
Proof of RG invariance
176
7.3
Renormalization-group equation Renormalization-group coefficients
180
183
7.3.3
RG equation Solution
184
7.4
Large-momentum behavior of Green's functions
185
7.4.1
Generalizations
187
7.5
Varieties of high- and low-energy behavior Asymptotic freedom
187 189
7.5.3
Maximum accuracy in Fixed point theories
7.5.4
Low-energy behavior of massless theory
190 191
7.6
Leading logarithms, etc.
193
7.6.1
193
7.6.2
Renormalization-group logarithms Non-renormalization-group logarithms
7.6.3
Landau ghost
196
7.7
Other theories
197
7.8
200
7.9
Other renormalization prescriptions Dimensibnal transmutation
7.10
Choice of cut-off procedure
206
7.3.1
7.3.2
7.5.1 7.5.2
an
asymptotically
171 172
180
free theory
187
196
203
Contents
Vlll
c/>4 self-energy
7.10.1
Example:
7.10.2
RG coefficients
7.10.3
Computation of g0 and Z; asymptotically free case
7.10.4
Accuracy needed for g0
7.10.5 7.10.6
ml Non-asymptotically
7.11
Computing
free
case
208 208 209 213 216 216
renormalization factors using dimensional
regularization
7.12
Renormalization group for composite operators
8
Large-mass expansion
8.1
A model
217 219
8.2
Power-counting
8.2.1
Tree
8.2.2
Finite graphs with heavy loops
222 224 225 225 226
8.2.3
Divergent one-loop graphs
226
8.2.4
More than
229
8.4.2
loop General ideas Renormalization prescriptions with manifest decoupling Dominant regions Proof of decoupling Renormalization prescription R* with manifest decoupling Definition of R*
8.4.3
IR finiteness of C*(T)
237
8.4.4
Manifest decoupling for R*
239
8.4.5
Decouplmg theorem
239
8.5
Renormalization-group analysis
8.5.1
Sample calculation
8.5.2
Accuracy
9
Global symmetries
244
9.1
Unbroken symmetry
245
9.2
247
9.2.1
Spontaneously broken symmetry Proof of invariance of counterterms
9.2.2
Renormalization of the current
251
9.2.3
divergences Renormalization methods Generation of renormalization
252
8.3 8.3.1 8.3.2 8.4 8.4.1
9.3 9.3.1
graphs
one
230 231 232 233 233 236
240 242 243
Infra-red
249
253
conditions by Ward identities
254
10
Operator-product expansion
257
10.1
Examples
258
10.1.1
Cases with
10.1.2 10.1.3 10.1.4
divergences Divergent example
258
Momentum space Fig. 10.1.3 inside bigger
262
no
graph
260 262
Contents
IX
269
10.4
Strategy of proof Proof Construction of remainder Absence of infra-red and ultra-violet divergences in r(T) R(F) r(T) is the Wilson expansion Formula for C(U) General case
10.5
Renormalization group
274
11
Coordinate space
277
11.1
Short-distance singularities of free propagator
278
11.1.1
Zero temperature
278
11.1.2
Non-zero temperature Construction of counterterms in low-order graphs
280 286
11.4
Flat-space renormalization External fields
12
Renormalization of gauge theories
293
12.1
Statement of results
296
12.2
298
12.2.1
Proof of renormalizability Preliminaries
12.2.2
Choice of counterterms
300
12.2.3
Graphs with external derivatives
301
12.2.4
302
12.2.5
Graphs finite by equations of motion Gluon self-energy
12.2.6
BRS transformation of
303
12.2.7
Gluon self-interaction
12.2.8
SRc
12.2.9
Quark-gluon interaction,
12.3
More general theories
307
12.3.1
Bigger
307
12.3.2
Scalar matter
12.3.3
Spontaneous symmetry
12.4
Gauge dependence
12.4.1
Change of c,
310
12.4.2
Change of Fa
313
Krgauge
314
10.2 10.3 10.3.1 10.3.2 10.3.3 10.3.4
11.2 11.3
12.5
—
263 266 266 270 272 273
281 291
298
302
A^
304 304
Sip, Sip; introduction of PMail
gauge group
305
307 308
breaking
309
of counterterms
12.6
Renormalization of gauge-invariant operators
316
12.6.1
Caveat
319
12.7
Renormalization-group equation
319
12.8 12.9
Operator-product expansion Abelian theories: with and without photon
12.9.1
BRS treatment of massive
12.9.2
Elementary
12.9.3
Ward identities
321
photon theory with photon
treatment of abelian
322
mass
323 mass
324 325
Contents
X
326
12.9.5
Counterterms proportional to A2 and d-A2 Relation between e0 and Z3
12.9.6
Gauge dependence
327
12.9.7
Renormalization-group equation Unitary gauge for massive photon
329
12.10 13
Anomalies
331
13.1
Chiral transformations
333
13.2 13.3
Definition of y5 Properties of axial currents
12.9.4
327
330
334 338
13.3.1
Non-anomalous currents
338
13.3.2
Anomalous currents
338
13.3.3
Chiral gauge theories Supersymmetric theories
339
Ward identity for bare axial current Renormalization of operators in Ward identities
339
One-loop calculations Non-singlet axial current has
342
13.3.4 13.4 13.4.1 13.5 13.6
339
no
anomaly-
340
346
13.6.2
Reduction of anomaly Renormalized current has
13.7
Three-current Ward
13.7.!
General form of anomaly
349
13.7.2
One-loop value
351
13.7.3
Higher orders
352
14
Deep-inelastic scattering
354
14.1
Kinematics, etc.
355
14.2
Parton model
358
14.3
359
14.4
Dispersion relations and moments Expansion for scalar current
14.5
Calculation of Wilson coefficients
364
14.5.1
Lowest-order Wilson coefficients
364
14.5.2
Anomalous dimensions
365
14.5.3
Solution of RG
14.6
OPE for
14.6.1
Wilson coefficients
13.6.1
14.7 14.8
346 no anomalous dimension
identity:
equation
-
the triangle anomaly
non-singlet
vector and axial currents
347 349
361
367 367
electromagnetic case Parton interpretation of Wilson expansion W4 and W5 References
368
Index
377
-
369 371 372
1 Introduction
The structure of a quantum field theory often simplifies when
one
considers
involving large momenta or short distances. These important in improving one's ability to calculate predictions from the theory, and in essence form the subject of this book. The first simplification to be considered involves the very existence of the theory. The problem is that there are usually ultra-violet divergences caused by large fluctuations of the field(s) on short distance scales. These manifest themselves in Feynman graphs as divergences when loop momenta go to infinity with the external momenta fixed. The simplification is that the divergences can be cancelled by renormalizations of the parameters of the action. Consequently our first task will be to treat the ultra-violet processes
simplifications are
renormalizations. Renormalization is essential, for otherwise most field theories do not exist. We will then expose the methods needed to handle high-energy/short- distance problems. The aim is to be able to make testable predictions from a
strong interaction theory,
improve the rate of convergence of the weakly coupled theory. The simplifications
or to
perturbation expansion generally take the form of a factorization of a cross-section or of amplitude, each factor containing the dependence of the process phenomena that happen on one particular distance scale. Such a in
a
an on
perturbation expansion for a process are large when the process involves widely different distance scales.
factorization is useful, because the coefficients of the
The industry called 'perturbative QCD' consists of deriving such factorization theorems for strong interactions (Mueller (1981)) and exploring their phenomenological consequences. We will only study the earliest of
these factorizations, the operator product expansion of Wilson (1969). We will also discuss the theorems that describe the behavior of a theory when of its fields get large (Appelquist & Carazzone (1975) and Witten These large-mass theorems have their main uses in weak (1976)). the
masses
interaction theories. The presence of ultra-violet
divergences,
by renormalization counterterms,
means
1
even
though they
are
cancelled
that in any process there
are
2
Introduction
contributions from quantum fluctuations on every distance scale. This is both a complication and an opportunity to find interesting physics. The
complication is that the derivation of factorization theorems is made difficult. The opportunity is given by the observable phenomena that directly result from the existence of the divergences. A standard example is given by the scaling violations in deep-inelastic scattering. It is the renormalization group (Stueckelberg & Petermann (1953) and Gell-Mann & Low (1954)) that is the key technique in disentangling the complications. The infinite parts of the counterterms are determined by the requirement that they cancel the divergences, but the finite parts are not so determined. In fact, the partition of a bare coupling g0 into the sum of a finite renormalized coupling gR and a singular counterterm Ag is arbitrary.
One can reparametrize the theory by transferring a finite amount from gR to Ag without changing the physics: the theory is invariant.
renormalization-group
This trivial-sounding observation is in practice very useful, and far from trivial. Suppose one has some graph whose renormalized value is large (so that it is
inadequate
to use a few low orders of the
perturbation theory
to
compute the corresponding quantity). Then in appropriate circumstances it is possible to adjust the partition of g0 (viz., g0 gR + Ag) so that the =
counterterm Ag cancels not only the
large piece
but also the
excessively divergence graph's finite part. The large piece is now in the lowest higher orders. Construction of factorization theorems of
of the
order instead of
the sort reviewed by Mueller (1981) provides many circumstances where this trick is applicable. Without it the factorization theorems would be
almost
powerless.
that the subjects of renormalization, the renormalization group, and the operator product expansion are intimately linked, and we will treat We
see
explain the general methods that examples we will examine but in many other situations. We will not aim at complete rigor. However there are many pitfalls and traps ready to ensnare an unwary physicist. Thus a precise set of concepts and notations is necessary, for many of the dangers are essentially combinatorial. The appropriate basis is then that of Zimmermann (1969, them all in this book. The aim will be to are
applicable not only
to the
1970, 1973a, 1973b). One other problem is that of choice of an ultra-violet cut off. From a fundamental point of view, the lattice cut-off seems best as it appears in non-
perturbative treatments using the functional integral (e.g., Glimm & Jaffe (1981)). In perturbation theory one can arrange to use no regulator whatsoever (e.g., Piguet & Rouet (1981)). In practice, dimensional reg-
3
Introduction
deservedly become very popular. This consists of replacing physical space-time dimensionality 4 by an arbitrary complex number d. The main attraction of this method is that virtually no violence is done to the structure of a Feynman graph; a second attraction is that it also regulates infra-red divergences. The disadvantage is that the method has not been formulated outside of perturbation theory (at least not yet). Much of the treatment in this book, especially the examples, will be based on the ularization has
the
use
of dimensional
strongly
that
none
regularization.
However it cannot be
of the fundamental results depend
on
emphasized this choice.
too
2
Quantum field theory Most of the work in this book will be
strictly perturbative. However it is important not to consider perturbation theory as the be-all and end-all of field theory. Rather, it must be looked on only as a systematic method of approximating a complete quantum field theory, with the errors under control. So in this chapter we will review the foundations of quantum field theory starting from the functional integral. The purpose of this review is partly to set out the results on which the rest of the book is based. It will also introduce our notation. We will also list a
number of standard field theories which will be used throughout the book. Some examples are physical theories of the real world; others are simpler theories whose only purpose will be to illustrate methods in the absence of
complications. integration is not absolutely essential. Its use is to provide a systematic basis for the rest of our work: the functional integral gives an explicit solution of any given field theory. Our task will be to investigate a certain class of properties of the solution. The
use
For
more
of functional
details the reader should consult
a
standard textbook
on
field
theory. Of these, probably the most complete and up-to-date is by Itzykson & Zuber (1980); this includes a treatment of the functional integral method. Other useful references include: Bjorken & Drell (1966), Bogoliubov & Shirkov (1980), Lurie (1968), and Ramond (1981).
2.1 Scalar field theory The simplest quantum field theory is that of a single real scalar field t/>(xM). The theory is defined by canonically quantizing a classical field theory. This
classical theory is
specified by <?
=
a
Lagrangian density:
(dct>)2/2-P((t>\
(2.1.1)
from which follows the equation of motion D</> + P'(0) 4
=
O.
(2.1.2)
2.1 Scalar
5
field theory
P(0) is a function of </>(x), which we generally take to be a polynomial like P(4>) m24>2/2 + #4/4!, and P'((/>) dP/dt/>. (Note that we use units with h c 1.) In the Hamiltonian formulation of the same theory, we define a canonical Here
--=
=
=
=
momentum field: n(x)
=
d^/dcj)
=
<j)
=
(2.1.3)
dtfr/dt,
and the Hamiltonian
-J-
d3x(;r2/2 + V02/2 + P(0)).
(2.1.4)
Physically, we require that a theory have a lowest energy state.
If it does not
unstable against decay into a lower energy state plus a collection of particles. If the function P(t/>) has no minimum, then the then all states
are
catastrophic situation exists (Baym (I960)). require P((j>) to be bounded below. in the Quantization proceeds Heisenberg picture by reinterpreting (j>(x) as a hermitian operator on a Hilbert space satisfying the canonical equal- formula (2.1.4) implies that just such Thus
a
the function
we
time commutation relations, i.e.,
[4>(xU(y) ̄\
=
WMy)]
=
y'
oj
l"'
given by (2.1.4) so the equation of motion (2.1.2) Heisenberg equation of motion
The Hamiltonian is still
follows from the
i34>/dt
=
(2.1.6)
\_4>9ir].
theory is specified by stating what the space of states is by giving the manner in which <f> acts on the states. We will construct a solution by use of the functional integral. It should be noted that <j)(x) is in general not a well-behaved operator, but rather it is an operator-valued distribution. Physically that means that one cannot measure 0(x) at a single point, but only averages of (/>(x) over a space-time region. That is, A solution to the
and
4>f
=
0(x)/(x)d4x,
for any complex-valued function /(x), is
an
(2.1.7)
operator. Now, products of
always make sense (e.g., <5(x)2). In particular, the Hamiltonian H involves products of fields at the same point. Some care is needed to define these products properly; this is, in fact, the subject of
distributions do
not
renormalization, to be treated shortly. The following properties of the theory
are
standard:
6
Quantum field theory
(1) The theory has
a
|0>,
Poincare-invariant ground state
called the
vacuum.
(2) The
states and the action of
(j>
on
them
can
be reconstructed from the
time-ordered Green's functions
GN(x1,...,xN)
=
<0|T^(x1)...^(xN)|0>.
(2.1.8)
The T-ordering symbol means that the fields are written in order of increasing time from right to left. The Green's functions have appropriate causality properties, etc., so (3) that they are the Green's functions of a physically sensible theory. Mathematically, these properties are summarized by the Wightman axioms (Streater &
Wightman (1978)).
Bose symmetry of the
(/>-field
symmetric under interchange of motion of
<f>
means
that the Green's functions
and from the commutation relations
of motion for the Green's functions. The
For
a
general (N
+
l)-point Green's function,
=
we
of
be derived equations example is
can
simplest
DyG2(y9x) + <0| TP'(d>(y))ct>(x)\Oy
are
equations
any of the x's. From the
-
i<5<4>(*
-
y).
(2.1.9)
have N <5-functions
on
the
right:
□,GN+1(y,x1,...,xiV) + <O|TP'(^))0(x1)...^(xN)|O> =
-i£ S?{y-xJ)GN_l(xl9...9xJ_l9xJ+i9...9xN).
(2.1.10)
j=i
summarizes both the equations of motion and the commutation relations. Solving the theory for the Green's functions means in
This
equation
essence
solving this
set of
functions that are the easiest
theory
can
be calculated
2.2
coupled equations. It is in fact the Green's objects to compute. All other properties of the
once
the Green's functions
Functional-integral
are
known.
solution
quantum field theory is a non-trivial problem in two cases are elementary: free field theory (P m2(/>2/2), consistency. Only 1 is a rather \. The case d and the case of one space-time dimension, d The solution of
a
=
=
=
just the quantum mechanics of a particle with Heisenberg position operator <f>(t) in a potential P((/>). (In Section 2.1, we explained the case d 4. It is easy to go back and change the formulae to be valid for a general value of d) trivial field theory, for it is =
2.2 For the
case
of
Functional-integral solution
(/>4 theory, P((/>)
7
with
(2.2.1)
m2</>2/2 + #74!,
=
are rigorously known to exist if d 2 or 3 (Glimm & Jaffa (1981)). If d > 4 then no non-trivial solution exists (Aizenman (1981)). The case d 4 is difficult; the difficulty is to perform renormalization of the ultra-violet
solutions
=
=
divergences beyond perturbation theory. As we will see the theory at d 4 is 'exactly renormalizable' in perturbation theory; this is the most interesting case. For the most part we will ignore the difficulties in going beyond perturbation theory. We will return to this problem in Section 7.10 =
when
we
discuss the application of the renormalization group outside of
perturbation theory. If we ignore, temporarily, the renormalization problem, then a solution for the theory can be found in terms of a functional integral. The formula for the Green's functions is written
GN(xl9...,xN)
=
as
jr
[d^]eis^M(x1)...^l(xJV).
(2.2.2)
(See Chapter 9 of Itzykson &Zuber (1980), or see Glimm &Jaffe(1981).)On the
right-hand
integration is
side of this equation ^(x) represents a classical field, and the the value of ^(x) at every space-time point. The result of
over
the integral in (2.2.2) is the N-point Green's function for the corresponding quantum field, (/>. In the integrand appears the classical action, which is
S|>4]= d4*^- The normalization factor Jf is to
^
Equivalent
to
(2.2.2) is
=
the
(2-2-3)
give <0|0>
{[[^]eiSM1j integral
=
1,
so
that
\
for the
(2.2.4)
generating functional
of
Green's functions:
Z[J]
=
Jf
I[(L4]exp jiS[v4]
+
\d*xJ(x)A(x) 1,
(2.2.5)
where J(x) is an arbitrary function. Functionally differentiating with respect to J(x)
gives
the Green's functions, e.g.,
<0|T^(,)|0>
=
^^^ZW|^o)
It is somewhat delicate to make over A.
The
principal steps
are:
precise
the definition of the
(2.2.6) integration
8
theory
Quantum field
(1) 'Wick-rotate' time
to
imaginary
values: t
Euclidean. The exponent in the
"SEudM= With
our
metric,
we
-
integral
=
—
i-r,
[drd3x[-dA2/2
have 3 A2
=
-
so
that space-time is
is then:
(dA/dr)2
—
(2.2.7)
P(A)l
+
VA2. We may subtract
P(A); this subtraction gives an overall factor in the functional integral, and it cancels between the integral and the normalization factor (2.2.4). Therefore the Euclidean action 5Eucl is positive definite. The factor exp(-SEucl) gives much out from ¥£ the minimum value of
better convergence for large A and for
rapidly varying A than does in Minkowski exp(iS) space. (2) Replace space-time by a finite lattice. We may choose a cubic lattice with spacing a. Its points are then x" where the n^s are
box
=
n'a.
integers. They are bounded to keep x inside a spatial T/2 to + 7/2. The keep t within a range
of volume V and to
—
integral
t[dA}A(xl)...A(xN)exp(-SEucl[A]) is of
now an
absolutely convergent ordinary integral
variables.
The
action
approximation. (3) Take the continuum limit
5Hucl
a ->
infinite time T. (4) Analytically continue back
is
given
over a
(2.2.8) finite number
obvious
its
discrete
0, and the limits of infinite volume V and
to Minkowski
space-time.
The difficulties occur at step 3. Taking the limits of infinite T and V gives divergences of exactly the sort associated with taking the thermodynamic limit of a partition function see below. Further divergences occur when -
the continuum limit
(2.2.2) gives or
as
an
a ->
overall normalization factor which goes to
the number of
absorbed
by
0 is taken. In addition, the canonical derivation of
space-time points
goes to
infinity as a ->0 infinity; this factor is
the normalization Jf.
The limits of infinite volume and time
are
under
good
control. They
are
literally thermodynamic limits of a classical statistical mechanical system in four spatial dimensions. Recall, for example, that in write
S[A]=g ̄l
cj)4 theory
\d*x(dA2/2-m2A2/2-A4/4\)
one can
(2.2.9)
2.3 Renormalization =
gulA.
Thus the integral
J<
where A
f [cL4] exp(
9
-
S[/l]) is
proportional to
f[(L3]exp{-(l/fif)S[yl-^ff-l]}.
(2.2.10)
This is the partition function of a classical system at temperature l/g, when the phase space is spanned by the field A, and when the energy of a given configuration is
^EuclMJ
 ̄
d4x(
-
3A2/2 + m2A2/2 + A*/4\).
identity between Euclidean field theory and certain classical statistical mechanics systems has been fruitful both in working out the rigorous The
mathematical treatment of quantum field theory (Glimm & Jaffe (1981)) and in finding new ways to treat thermodynamic problems (Wilson & Kogut (1974)). As is particularly emphasized in Wilson's work, there is a lot of cross-fertilization between field
theory
and the theory of
transitions. The methods of the renormalization group fields, and the continuum limit in field theory can be
phase
are common to
both
usefully regarded as a particular type of second-order phase transition. The thermodynamic limit gives a factor exp( pTV), where p is the ground state energy-density. This factor is clearly cancelled by Jf. All the remaining divergences are associated with the continuum limit a -> 0. These are the divergences that form the subject of renormalization. They are —
called the ultra-violet (UV) divergences. One notational change needs to be made now. In more complicated theories, there will be several fields, and the functional-integral solution of
theory involves an integral over the values of a classical field for each quantum field. It is convenient to have a symbol for each classical field that is clearly related to the corresponding quantum field. The standard notation is to use the same symbol. Thus we change the integration variable such
a
in (2.2.2) from A(x) to 0(x), with the result that
<O|T(/>(xJ...0(x^)|O> This is somewhat of whether
is
one
or to mean
an
using (/>
=
^|1[d0]eiS^(xO..>(%)-
(2.2.11)
abuse of notation. However, it is usually obvious quantum field, as on the left-hand side,
to mean the
the corresponding classical field,
as on
the
right-hand
side.
2.3 Renormalization
divergences in this limit; this has been known from the earliest days of quantum elec-
The difficult limit is the continuum limit a
-?
0. There
are
10
Quantum field theory
trodynamics (e.g., Oppenheimer (1930)). It is possible to say that the UV divergences mean that the theory makes no physical sense, and that the subject of interacting quantum field theories is full of nonsense (Dirac (1981)). Luckily we can do better, for our ultimate aim need not be to construct a field theory literally satisfying (2.1.2)—(2.1.5). Rather, our aim is to construct a relativistic quantum theory with a local field as its basic observable. These requirements are satisfied if we construct a collection of Green's functions satisfying sensible physical properties (for example, as formulated in the Osterwalder-Schrader axioms (1981)). We may further ask that
we
find
a
-
see
Glimm & Jaffe
theory that is close in some sense
satisfying the defining equations (2.1.2)—(2.1.5). Combining the integral with suitable renormalizations of the parameters of theory satisfies these requirements. to
functional
The basic idea of renormalization
one-loop
comes
the
from the observation that in
graphs divergences example, they change the mass of the particles described by cj)(x) the
amount to shifts in the parameters of the
action. For
from the value
m to some
other effective value, which is infinite if m is finite.
Renormalization is then the procedure of cancelling the divergences by the action. To be precise, let us consider the c/>4
adjusting the parameters in
theory
with <£
=
(3A0)2/2
-
m20A2/2
-
gA*/41
+
A0.
(2.3.1)
subscript zero is here used to indicate so-called bare quantities, i.e., those that appear in the Lagrangian when the (3A0)2/2 term has unit coefficient. (We also introduce a constant term. It will be used to cancel a UV divergence in the energy density of the vacuum.) Then we rescale the
The
field by writing A0 so
that, in
=
=
ZdA2/2 ZdA2/2
dropped A0 from
-
-
(2.3.2)
Zil2A,
terms of the 'renormalized field'
<£
We have
=
A, the Lagrangian is
m2ZA2/2 g0Z2A*/4! m2A2/2 gBA4/4\. -
-
SC since it has
no
effect
The Green's functions of the quantum field <f> (2.3.3) as the Lagrangian in the functional integral
on
(2.3.3)
the Green's functions.
obtained by using (2.2.2). We let Z, m0, and
are now
g0 be functions of the lattice spacing a, and we choose these functions (if possible) so that the Green's functions of </> are finite as a-> 0. If this can be
constructing a continuum field theory, and it is termed'renormalizable'. The theory may be considered close to solving
done, then
we
have succeeded in
11
2.3 Renormalization
(2.1.2)—(2.1.5). This is because the theory is obtained by taking a discrete (i.e., lattice) version of the equations and then taking a somewhat odd continuum limit. We will call m0 the bare mass, and g0 the bare coupling, and we will call Z the wave-function, or field-strength, renormalization. It is also common to call mB and gB the bare mass and we will not do this in this book.
viewing the
Another way of c£
=
coupling;
but for the sake of consistency
renormalization is to write (2.3.3)
as
dA2/2 m2A2/2 gA*/4! + 6Z3A2/2 6m2A2/2 SgA4/4!. -
-
-
(2.3.4)
-
We will call the first three terms the basic
Lagrangian and the last three the
mass m and the renormalized Lagrangian. coupling g are finite quantities held fixed as a->0. The counterterms Z SZ 1, Sm2 m^ m2, and Sg gB g are adjusted to cancel the divergences as a-^0. This form of the Lagrangian is useful in doing perturbation theory; we treat 3A2/2 m2A2/2 as the free Lagrangian and
The renormalized
counterterm
=
—
=
=
—
—
—
interaction. The expansion is in powers of the renormalized coupling g. The counterterms are expanded in infinite series, the remainder
as
each term cancelling the divergences of one specific graph. The form (2.3.4) for S£ also exhibits the fact that the theory has
independent parameters, and of
m
and g. The counterterms
are
two
functions of m, g,
a.
We will discuss these issues in much greater depth in the succeeding chapters. For the moment it is important to grasp the basic ideas:
(1) The self-interactions of the field create, among other things, dynamical contributions to the mass of the particle, to the potential between
particles, and to the coupling of the field to the single particle state. Thus the measured values of these parameters are renormalized relative to the values
(2)
appearing
These contributions,
in the or
Lagrangian.
renormalizations,
are
infinite, in many
cases.
The most important theorem of renormalization theory is that they the only infinities, in the class of theories called 'renormalizable'. (3) The infinities
counterterms,
are
cancelled by wave-function, mass, and coupling that the net effect of the interactions is finite.
are so
quantitative the sizes of the infinities, the theory is constructed a lattice theory. The infinities appear as the lattice when divergences spacing goes to zero.
(4) To make as
the continuum limit of
12
Quantum field theory 2.4 Ultra-violet regulators
In the last sections
we
showed how to construct field theories
the functional integral
Ultraviolet zero,
as
the continuum limit of
by defining
lattice theory.
a
divergences appear as divergences when the lattice spacing, a, goes to and are removed by renormalization counterterms. The lattice
therefore is a
To be able
regulator,
or
cut-off, for the UV divergences.
divergences quantitively and to construct a theory involving infinite renormalizations, it is necessary to use some kind of UV cut-off. Then the theory is obtained as an appropriate limit when the cut-offis removed. There are many possible ways of introducing a cut-off, of which going to a lattice is only one example. The lattice appears to be very natural when
to discuss the
working with the functional integral.
But it is cumbersome to
within perturbation theory, especially because of the loss of Poincare invariance. There are two other very standard methods of making an use
ultraviolet cut-off: the Pauli-Villars method, and dimensional regularization. The Pauli-Villars (1949) method is very traditional. In its simplest version it consists of replacing the free propagator field theory by
SF(p,m;M)
=
^
i/(p2
—
m2)
in
a
scalar
p2-M2
(m2 M2) (p2-m2)(p2-M2)' -
(2A1)
approaches the original propagator. The behavior for large been clearly improved. Thus the degree of divergence of the Feynman in the graphs theory has been reduced. All graphs in the t/>4 theory, except for the one-loop self-energy are in fact made finite. In the </>4 theory it is necessary to use a more general form in order to make all graphs finite: As M
->
oo, this
p has
SJp,m;MuM7) FKy i9 V
=—
\- (p2-M*) (M\-M¥)
—-^
(p2-m2)
(m2
i
=
-
=
M2)
 ̄(p2-m22)(m2-mJ) i  ̄ ̄
(p2 It is
usually convenient
Now the
and
p2
=
-
(Ml-m2)(M22-m2) m2) (7-M2) (p2 M22)' -
to set Mx M2. regulated propagator has extra poles at p2
M2. Since
to the pole at
p2
=
one
(2.4.2)
=
of the extra poles has
a
=
M2, or at p2
=
M\
residue of the opposite sign
m2, the regulated theory cannot
be
completely physical.
2.5 Equations It is normally true unphysical features.
that a
of motion for Green's functions
theory with
an
13
ultra-violet cut-off has
some
Perhaps the most convenient regulator for practical calculations is dimensional regularization. There it is observed that the UV divergences are removed by going to a low enough space-time dimension d, so d is treated
as a
continuous variable. In perturbation theory this
can
be done
consistently (Wilson (1973)), as we will see when we give a full treatment of dimensional regularization in Chapter 4. However it has not been possible to make it work non-perturbatively, so it cannot at present be regarded as a fundamental method. Since it is only the renormalized theory with no cut-off that is of true interest, the precise method of cut-off is irrelevant. In fact, all methods of ultra-violet cut-off differences
are
equivalent,
are
mainly
a matter
of
at least in perturbation theory. The practical convenience (or of personal
taste). Thus dimensional regularization is very useful for perturbation theory. But the lattice method is maybe most powerful when working
beyond perturbation theory; it is possible, for example, to compute the functional integral numerically by Monte-Carlo methods (Creutz (1980, 1983), and Creutz & Moriarty (1982)). Within
perturbation
theory
one
need
not
even
use
a
cut-off.
Zimmermann (1970, 1973a) has shown how to
apply the renormalization procedure to the integrands rather than to the integrals for Feynman graphs. The lack of fundamental dependence on the procedure of cut-off is thereby made manifest. The application of this procedure to gauge theories, especially, is regarded by most people as cumbersome. 2.5 Equations of motion for Green's functions We have defined
a
collection of Green's functions by the functional integral the definition are a certain number of limiting below (2.2.6).) This definition we will take as the basis
(Implicit in procedures, as listed
(2.2.2).
for the rest of our work. First
we must
check that it in fact gives a solution of that we are to derive the equations of
the theory. This means, in particular, motion (2.1.10) for the Green's functions, thus ensuring that both the
operator equation of motion (2.1.2) and the commutation relations (2.1.5) hold. (For the remainder of this chapter we will not specify the details of how renormalization affects these results.) It is convenient to work with the generating
functional (2.2.5). We make where e is a small number, and A(x) A(x) ef(x), of x". an Since the is function integration measure is invariant f(x) arbitrary the change of variable
->
+
14
Quantum field theory
under this shift, the value of the integral is unchanged:
|[d^]expjiS|>4+e/]
M+e/)j[= |[d^]expji5[^]+
+
IA
A. (2.5.1)
Picking
out the terms of order e
dVW where,
as
usual,
gives
[cL4]exp^iS|X|
we
f
+
8S =
SA(y)
0,
define the functional derivative SS
dP
(2.5.3)
SA(y) ̄ ̄nA ̄dA' _
Since f(y) is arbitrary,
we
get
[di4]exp|iS[^]+ J
(2.5.2)
\aj\
SS
.
=
8A(y)
0.
(2.5.4)
Functionally differentiating JV times with respect to J, followed by setting 0, gives the equation of motion (2.1.10). For example,
=
(5
0
=
,/Ttttt [left-hand side of (2.5.4) ]J=0 SJ(x)
=
=
=
=
jr
JT
-
i[dA]eiSlA]
A(x)i
SS
SA(y)
+
Sw(x-y)
[dA]eiSW{iA(x)[- UA(y)- P'(A(y))] + 3w(x-y)}
J^iDy\[dA']e[SiA]A(x)A(y)
-in/O|70(x)</>(y)|O>-i<O^^ (2.5.5)
which is equivalent to (2.1.9).
Note that in the fourth line we have exchanged
the order of integration and of differentiation for the □ y term. We have also used the normalization condition (2.2.4). It is important that the derivative of the quantum field (next-to-last line) is outside the time ordering, and SS/S(j)(y) in the last line is defined to be a shorthand for the combination of
operators in the previous line.
15
2.6 Symmetries This is somewhat
paradoxical since
we
have the operator equation of
motion:
from
<0|
which
it
is
tempting to deduce that the Green's function Tcj)(x)SS/d(j)(y)\Oy should be zero. However, in view of the work above
it is convenient
by the functional-integral
to define this Green's function
formula
<0lT^|0>=^j[d^eiS[^(x)^ Then,
have seen, the Dy is implicitly outside the time-ordering. inside the time-ordering gives a commutator, so that we get the
as we
Bringing it
(5-function term in (2.1.9)
or (2.5.5). The momentum-space version of the equation of motion (2.1.10) is often useful. We define the momentum-space Green's functions
GN(pt,
?
? ?,
PN)
=
GN(Pi,..., pN){2n)*8?{Pl
=
The momenta pj
d4xt.. .d% exp {i(pl-xl
be regarded
+
+
?
?
?
■
?
■
+
+
pN-xN)}GN(xu..., xN) (2.5.6)
pN).
flowing theory implies the (5-function for momentum conservation that is explicitly factored out in the last line of (2.5.6). A convenient notation (which we will use often) is to write are to
out of the Green's functions.
as
Translation invariance of the
GN(p1,...,pJV)
=
<O|r0(p1)...^(pN)|O>.
(2.5.7)
Implicit in this formula is the definition that the integrals over x defining the momentum-space field </>(p) are all taken outside the time-ordering, as stated in (2.5.6). We will use a tilde over the symbol for any function to indicate the Fourier-transformed function. Fourier transformation of the
equation
of motion
(2.1.10) gives
-q\0\U(q)¥(Pl)...4>(pN)\0y + <0\TP\¥)(q)¥(pd...¥(pN)\0> =
-i
I <O|T<£(pJ...0(p._J^
7=1
2.6
We
Symmetries
the consequences of symmetries. As we will see, interesting problems in renormalization theory that stem
now turn to
many
(2.5.8)
there
are
from the
16
Quantum
field theory
following question: If
a classical field theory has certain symmetries, does the symmetry survive after quantization? Generally, it is the need for renormalization of the theory that makes this a non-trivial question. The symmetry properties are expressed in terms of Green's functions by
the Ward identities. (Historically the earliest example was found by Ward (1950) in QED.) If the symmetry is not preserved by quantization there are extra terms called anomalies. In many cases there are no anomalies, so we will derive the Ward identities in this section ignoring the subtleties that in some cases
Chapter
lead to anomalies. Discussion of anomalous
cases
is given in
13.
theory of N fields which we collectively denote by a vector 0 (0,,..., </>N). Our discussion is general enough to include the case of fields with spin. We consider a symmetry group of the action S[0]. This is a Consider
a
=
group of transformations
on
the classical fields
0-+F[>;ctf]
=
(2.6.1)
0',
which leaves the action invariant:
S[*'] a
(2.6.2)
S|>].
w (w<z) is a set of parameters of the group, which we assume here to 0 Lie group, i.e., the a/s take on a continuous set of values. We let w
Here
be
=
be the
=
=
identity: F[0;O]
=
0.
It is easiest to work with infinitesimal
transformations:
dF- =
o
(A summation convention
on a
(2.6.3)
aA5a0,
is understood.)
In the quantum theory the symmetry is implemented as a unitary representation U(a)) of the group on the Hilbert space of states such that
U(a>)jU{a>)-! Since the representation is unitary, U(o) where the generators Qa algebra of the group:
The normalizations
are
are
=
we
=
(2.6.4)
F[0;o>].
may
parametrize
exp(i(D"Q,\
the group
so
that
(2.6.5)
hermitian operators which represent the Lie
such that the structure constants cafiy are totally are then given by:
antisymmetric. The infinitesimal transformations <5^iW
=
i[a^,W]-
(2-6.7)
2.6 Symmetries There
(1)
are a
17
number of special cases, each with its
special features:
own
Global internal symmetry: A finite-dimensional Lie group acts on the point of space-time, with the same transformation at each
fields at each
point. Thus
yf=-iWi% where the ta form
a
(2-6.8)
hermitian matrix representation of the Lie algebra:
Single-particle states carrying this representation Lagrangian is invariant.
are
annihilated by </>,..
The
(2) Global space-time symmetry: The group effectively is on
space-time; the Poincare
a
transformation
group and its extensions are the usual
cases. For a Poincare transformation xM->
corresponding transformation
A£xv
+
aM,
have the
we
of the fields:
(2.6.10)
(/>;(*)-+(/>.(Ax +a)jR(A)V
Here R is a finite-dimensional matrix representation of the Lorentz group (never unitary if non-trivial), acting on the spin indices of 0. The Lagrangian is not invariant. It transforms
as
J^[0,x]-+J^|>,Ax + a], so
that the action S
=
Jd4xif is invariant. Infinitesimal transformations
of (j> involve the derivative of (/>. (3) Global chiral symmetry: This looks like
global internal symmetry but differently right-handed parts of Dirac fields (which we have yet to discuss). Anomalies are often present see Chapter 13. (4) Supersymmetry: This is a generalized type of symmetry where Bose and Fermi fields are related (Fayet & Ferrara (1977)). The only case that we acts
on
a
the left- and
-
will discuss is the BRS-invariance (Becchi, Rouet & Stora (1975)) of gauge theory. (5) Gauge, or local, symmetry: Any of the above symmetries
symmetry whose parameters depend
a
may be
o(x). In quantum theories, these are not really implemented by unitary transformations. Their treatment is rather special. The elementary examples are general coordinate in variance in General Relativity, and extended to
a
gauge invariance in The basic tool for
on x.w
electromagnetism.
discussing symmetries
is Noether's theorem, which
relates them to conservation laws. This theorem in its most
form
applies only
to
=
symmetries
of the first three types.
straightforward
18
Quantum For
Noether's theorem asserts that
global symmetry,
a
field theory a
conserved
current^ exists for each generator of a symmetry. Let the Lagrangian have
the infinitesimal transformation S£-+S£ + ofb^ so
=
&+
(2.6.11)
co*d^ Y£,
that the action is invariant. Define
Then the
equations
of motion
imply
dj£/dx? The generators of the symmetry
conservation of y£, i.e., =
group
(2.6.13)
0.
are
Q^Wxjl
(2.6.14)
The canonical commutation relations imply that
tiMx)#3\x [ft?^)]=-i^M
D,°W, </?,(>-)]
as
-
=
-
y),
(if *°
=
A (2-6.15)
required by (2.6.7). only transformations that are symmetries of the 'broken symmetries'. There are several cases (not
We need to consider not
quantum theory, but also mutually exclusive). Let us define them, since there is
a
certain amount of
confusion in the literature about the terminology: (1) Explicit breaking: The classical action has a non-invariant term. If 6aJ? d^Y£ + Aa then the Noether currents are not conserved: =
Aa. An important case is where this term is small, so that it can be as a perturbation. (2) Anomalous breaking: Even though the classical action is invariant, the quantum theory is not, and there is no conserved current. The classical
dj*
=
treated
action is important for the quantum theory, since it appears in the functional integral defining the theory. The cause of anomalous breaking is generally an ultra-violet problem: dj% ^ 0 in the UV cut-off
theory, and the non-conservation does not disappear when the cut-off is removed. (Cases
are
conformal transformations and
some
chiral
theories.) (3) Spontaneous
breaking:
The action is invariant and the currents
conserved (in the quantum
theory),
but the
vacuum
are
is not invariant
under the transformations. Whether
or not a
symmetry is broken either spontaneously
anomalously is a dynamical question. That is,
one must
or
solve the theory, at least
19
2.7 Ward identities
partially, to find the answer. Frequently, perturbation theory is adequate to do this and
lowest
order or next-to-lowest order calculations suffice.
integral part of treating anomalous breaking (see renormalization-group methods are sometimes
Renormalization is an
Chapter 13),
while
necessary in treating spontaneously broken symmetry (Coleman & Weinberg (1973)). The case of spontaneous symmetry breaking that is not visible in perturbation theory is often termed dynamical (Jackiw & Johnson (1973), Cornwall & Norton (1973), and Gross (1976)). Anomalous breaking is sometimes called spontaneous, but this is a bad terminology, because it
gives
two very different
phenomena the
same name.
2.7 Ward identities Ward identities express in terms of Green's functions the consequences of a
it is broken). One derivation applies the equation of motion (2.1.10) to the divergence of a Green's function of the current jj. There are two terms: one in which the current is differentiated, and one in
symmetry (whether
or not
which the ^-functions
defining the time-ordered product are differentiated. identity expresses not only conservation of its current but also commutation relation (2.6.15), which is equivalent to the
Thus the
Ward
a
transformation
The
law.
Ward
renormalization of a
identities
are central
to
a discussion
of the
with symmetries, expecially if spontaneously
theory
broken. Our derivation of Ward identities begins by making the following change of variable:
Ai(x)-+Ai(x)+f(x)8aAi(x) in the functional integral for the generating functional Z[J]. before, the variation of the field A{ under fa(x) is x -?
oo.
a set
we
variables
0=
=
HereyJ,
Here
SaAt is, as
symmetry transformation, and
a
of arbitrary complex-valued functions that vanish rapidly
as
We get
Z[J] (Here
(2.7.1)
=
j[dA]
exp
{iS[A +/*<5aA]
assumed that the
(2.7.1).) The
measure
terms in
+
J^. +f'8gcAi)}.
is invariant under the
(2.7.2) that
are
+
exp
(iS
+
jJ-
A
W/^OO
+
change
of
linear in f" give
f[dA]expMS fj-AV-5S[A+/^aA]/5/^)
f[dA]
(2.7.2)
i^My)}-
is the Noether current in the classical theory.
+
iJ'M.-()')} <2-7-3)
20
Quantum
field theory
The Ward identities follow by functionally differentiating with respect to J(x). Thus one differentiation gives
the sources
^7<0| r£0#,(x)|0>
-
=
oy
while a double differentiation
i<5<4>(*
-
yK0\SMy)\0>,
(2.7'.4)
gives
^<O|77^)0f(w)^(x)|O> -iS^w-yK0\T5My)<t>jM\°>
=
-
i<5<4>(*
-
><)<0| T0,(w)^(/>.(y)|O>.
(2.7.5)
Note that, just as in our derivation of the equation of motion for Green's functions in Section 2.5, the derivative d/dy" is outside the time-ordering. The general case is:
A<oiTOn*j*,)io> °y
i
=
"
=
i
X <5(%
i
"
*/)<0| T^GO J] ^,.W|0>.
(2.7.6)
Important consequences of these Ward identities are obtained by integrating over all y (with y° fixed). The spatial derivatives give a surface term, which vanishes, so that we have, for example,
jd3>'^o <0| 7£0#,(x)|0> The spatial
-
i<5(*°
-
/)<0|^f(x)|0>.
integral of;'0 is just the charge Q°. The time derivative acts either
the charge that on
=
or on
the ^-functions defining the time-ordering;
<0|rdea/dt^(x)|0> + <0|[ea,^(x)]|0>?5(x0-y0) -i5(x°-y°K0\SMy)\0y. =
In this
equation
find
(2-7.7)
(2.7.6), we may choose the y°. Therefore an arbitrary operator dga/dt is zero. The
and its generalizations from
times of the fields
</>f(xf)
not to coincide with
Green's function of dga/dr is zero, remaining part of (2.7.7) therefore
so
that the
gives:
<O|[ea,0,-M]|O>= -i<0|^.(y)|0>. From
so we
(2.7.8) and its generalizations with
more
fields,
we
(2.7.8) find that the
ga's
have the correct commutation relations with the elementary fields 0f to be the generators of the symmetry group. Finally, another specialization of (2.7.6) is to integrate it
over
all y* and to
21
2.8 Perturbation theory
drop
the
resulting surface
term. The result is that
I<o|r5A/*;)n6,((*.)|o>
o=
<5a<0|7n^,(xi)|0>.
=
(2.7.9)
i= 1
All the above
equations
are true
for the
case
of
a
completely
unbroken
symmetry. The derivation breaks down at the first step if we have anomalous breaking. (In Chapter 13 we will discuss the anomaly terms that
must then be inserted in the Ward identities to make them correct.) For
explicitly
broken symmetry, where d-j
=
Aj=Q,
we must
add
an
a term
<0\TAx(y)Y\4>n,(XiP>
(2-7.10)
* = 1
to the
side of (2.7.6).
right-hand
In the case of a spontaneously broken theory the basic Ward identities (2.7.4)-(2.7.6) remain true-we still have an exact symmetry. But the integrated Ward identities (2.7.7) and (2.7.9) are no longer true. Equation (2.7.9) must be false if the vacuum is not invariant, and the derivation fails because the surface term is not zero. This is caused by the existence of zero-
particles. These Weinberg (1962)) are
mass
Nambu-Goldstone bosons (Goldstone, Salam & characteristic of theories with a spontaneously
broken symmetry.
2.8 Perturbation theory As
an
example, consider again (/>4 theory, with classical Lagrangian S£
We will
=
Z(dA)2/2
-
m2BA2/2
-
(2.8.1)
gBA*/4L
the Green's functions in powers of the renormalized
expand coupling g, for small g. powers of g,
we
To
expand the functional-integral formula (2.2.2) in
write i?
=
J5?0
+
(2-8-2)
^i>
where if0 is the free Lagrangian: (2.8.3)
&0=(dA)2/2-m2A2/2, and <£x is the interaction Lagrangian: Se,
=
-
We will
gA*/4!
expand
+
(Z
-
l)(dA)2/2
-
(m2
-
m2)A2/2
-
the renormalization counterterms, Z
(gB —
-
1,
g)A*/41 (2.8.4)
m|
—
m2,
and
22
Quantum
gB
—
field theory
0,in powers of #,so that all of the terms in if, have at least one power of expansion of the Green's functions is then obtained from (2.2.2)
g. The series as:
GN(x1,x2,... ,xN)
£
(i"/n!)
f
[[d^x,)- ̄A(xJ fd4y^iW
exp(iS0M)
(2.8.5)
f[d^]|" fdV^wTexp^o^])
(i-/n!)
Here
[d4y^o= [d4y(^2/2-m2/(2/2)
SoW= is the free action.
Each of the terms in the series is (aside from
a
Green's function in the free-field theory
normalization), (1951) formula:
a common
Mann-Low
so
(2.8.5) is equivalent
to the Gell-
uN(xl9... x;V) ,
_
£ (iWfl U*yJ)<0\T<t>F(x1)...ct>F(xN) f[ J^(yi)|0> \j=l J
n^O
/
_
jfj
.
tr/ni)(f\ Uyj]<o\Tf\ ^(yj)\oy (2.8.6) Here t/>F is a free quantum field of mass m. It is the field generated from the free Lagrangian if0 (d(j)F)2/2 m2t/>F/2. Then i^ is the quantum interaction Lagrangian, if j£f0, which is a function of the free field 0F. =
-
—
To compute the integrals in (2.8.5) it suffices to compute the functional of free-field Green's functions:
Z0[ J]
=
J-
|[<U]expl iS0IXI+ yA J- ^ ^ -r
generating
(2.8.7)
[d^]exp(iS0[^]) This is done by of variable:
completing the
A(x)
Here, GF(x), is
-
square, i.e.,
A(x)
+
by making the following change
fd*yGF(x
-
y)J(y).
(2.8.8)
the Feynman propagator satisfying (□ +
m2)GF(x)
=
-
i<5(4)(x),
(2.8.9)
23
2.8 Perturbation theory and
x°
boundary condition that,
a
=
—
it, GF(x)
-?
0
as x -?
oo.
after rotating to Euclidean space by
Thus
The result is that
Z0[J] Green's functions
=
exp
j± fd4xd V(*)GF(*-y)J(y)\.
of free fields
(2.8.11)
obtained by differentiating with respect
are
to J; for example
(0\T<t>(x)<t>(y)\0}=- SJ(x)SJ(y)\J
=
0
(2.8.12)
GF(x-y).
=
derive the well-known Feynman rules for the interacting from (2.8.6). These can be given either in momentum or coordinate theory space. In either case the Green's function GN is written as a sum over all We
can now
possible
topologically distinct Feynman graphs. Each graph T consists of a by lines. It has N 'external vertices', one for each
number of vertices joined
0(xf),
with
one
line attached, and
number,
some
n, of interaction vertices.
corresponding to the terms in Lagrangian (2.8.4). The vertex for the A* interaction has four
The interaction vertices are of several types, the interaction
lines attached and the vertices for the dA2 and A2 interactions have two lines attached. The value of the
position yj of the
n
graph, denoted
interaction vertices. The
integral over the integrand is a product of
/(T), is the
factors: (1)
GF(w
z) for each line, where
—
w
and
z are
the positions of the vertices at
its end. (2) A combinatorial factor 1/S(r). (3) i#B for each A4 interaction. —
(4)
(5)
—
—
i(m| i(Z
—
—
m2)
for each A2 interaction.
l)d2/dw"dwM
respect to
for each
w act on one
(dA)2
interaction: the derivatives with
of the propagators attached to the vertex.
For each Feynman graph a number of equal expanding (2.8.5). If T has no symmetries and if
contributions arise in it has no counterterm
vertices, then this number is n !(4!)" so that the explicit n! in (2.8.5) and the 4! in each interaction
are
cancelled.
Graphs with symmetries have a number of
factor of the symmetry number 5(r). (For by 6.) The combinatorial example, the self-energy graph Fig. 2.8.1 has 5
contributions smaller
a
=
factor is then the inverse of S(T).
24
Quantum field
theory
CO Fig. 2.8.1. A graph with symmetry fac- tor 5
Fig. 2.8.2. A graph with
6.
=
a
vacuum
bubble.
is the sum of all graphs with no external lines. graphs in the numerator that have disconnected
The denominator of (2.8.5)
The result is to cancel all vacuum bubbles
(like Fig. 2.8.2.).
In momentum space each line is
Feynman rules
(1)
A factor
(2) A factor
i/[(27r)4(/c2 (2n)4'
or
m2
—
times
vertex (external
(3) An integral
assigned
a
(directed)
momentum k. The
are:
+
ia)]
for
a
a momentum
line with momentum k.
conservation ^-function for each
interaction).
the momentum of every line. (4) A combinatorial factor 1/S(T). (5) i#B f°r eacri ^4 interaction. over
—
(6) (7)
\{ml m2) for each A2 interaction. i(Z \)p2 for each (dA)2 interaction, where p is the momentum flowing —
—
—
of the propagators attached to the vertex.
on one
The perturbation series in (2.8.5) need not be convergent, but only asymptotic. Let GN be the sum up to order gn of the perturbation series for ?
the Green's function GN. Then it is asymptotic to GN if for any satisfies
\GN ̄GNJ as g
=
n
the
error
(2.8.13)
0(gn+')
general, perturbation theory is asymptotic but not convergent. rigorously known (Glimm & Jaffe (1981)) for the </>4 theory in the
->0. In
This is cases
that the space-time dimension is d
=
0,1, 2, 3. (d
=
0 is the
case
of the
ordinary integral dx exp ( while d
=
-
m2x2/2
-
gx*/4!),
1 is the quantum mechanics of the anharmonic
Physically, the reason for non-convergence is that when g unbounded below and
so
oscillator.)
the energy is
the vacuum-state continued from g>0 is phenomenon in quantum
unstable. (Dyson (1952) first observed this
electrodynamics.)
< 0
Spontaneously broken symmetry
2.9 In later
chapters
we
will
assume
(2.8.13).
momentum behavior, it will be important
When
25
compute large-
we
to understand the maximum
possible validity and accuracy of the calculations if the perturbation theory is asymptotic but not convergent. 2.9
Spontaneously
broken symmetry
(/>4 interaction. If m2 is positive and g is small, we have a theory
Consider the
of particles of mass
slightly perturbed by the interaction. This interaction is basically a repulsive (5-function potential, as can be seen by examining the Hamiltonian in the non-relativistic approximation. There is a symmetry m
But if m2 is negative this interpretation is incorrect. The true situation can
by noticing that the functional integral (in Euclidean space) is dominated by classical fields with the lowest Euclidean action, which is be discovered
SEuciM (Remember
=
(dA)2
that
[d4x[ =
A
=
are constant
A_
=
(2.9.1)
(dA)2/2 + m2A2/2 + gA*/4i].
is negative.) If m2 > 0, then 0. But, if m2 < 0, then there are two minima;
-(dA/dz)2-VA2
the minimum action field is A
these
-
=
fields with P'(A)
=
0, i.e., A=A+
-J(-6m2/g).
=y/(
—
6m2jg)
and
in the impose the boundary condition A(x) ->A integral. (The condition A^A_ gives equivalent physics, A symmetry of the action.) Then field configurations because of the A with A close to A will dominate. We may understand this by observing A that field configurations with large regions where A is not close to A will give small contributions to the Euclidean functional integral (2.2.8) because their action SEucl is so big. Indeed, a constant field with A not equal to A A and its A has infinitely more action than one with A A contribution to the integral is zero. One's first inclination then is that the only configurations that contribute have A^A+ or A^A_ as x->oo. We choose to
+
as x ->oc
functional
->
—
+
+
+
or
=
_
+
However, other configurations contribute, because there
or
are
or
_
_,
many of
integrate over all possible fluctuations. However, one can that in general A will be argue even rigorously (Glimm & JafTe (1981) A typical configuration of the classical field ^(x) will to A close to A be close to one of these values over almost all of space-time. them
one
-
has to
-
-
+
_.
choice of boundary condition A^>A + , even more is true: a configuration is close to A A + almost everywhere, rather than to
Given
typical
or
either A
our
=
_
or A +.
The reason is that if it had
a
large region with A(x) close to
26
Quantum field theory A ±A
+
+
-
+
-
+
-
-
d=\
d>\
Fig. 2.9.1. Illustrating transitions between regions with fields close minima of the potential
A_ (Fig. 2.9.1), then there would be
to different
contribution to the action
a
proportional to the size of the boundary between the regions and of negative A(x). Only if the space-time dimension is d
of positive A(x) —
1 will
have
we
finite contribution from the boundary. This special case is quantum mechanics of a particle in a potential with two wells. The particle can tunnel a
between the two wells. In the
have discussed, of a discrete rather than of a continuous the argument that A is close to A + almost everywhere for the symmetry, case we
important configurations is than
one.
correct in all
The quantum field therefore has
space-time dimensions greater expectation value close
a vacuum
to A+:
<0|<£(x)|0>
=
^
\&A ̄]A{x)e-S{A]^A
+
.
In the case of a continuous symmetry, there is a continuous series of minima of the potential. A field configuration can interpolate between different minima without going over a big hump in the potential. The only
from the gradient terms in the action. This suppresses that do not stay close to one minimum, but only in more configurations than two space-time dimensions. In one space-time dimension there is no penalty
comes
spontaneous breaking of
a continuous symmetry (Mermin & Wagner (1966), Hohenberg (1967), and Coleman (1973)). Perturbation theory can be considered as a saddle point expansion about
the minimum of the action. We write A(x) A\x) + v where v we treat A'{x) as the independent variable. We have =
if Here C
have
a
=
-
=
m
(3Af/2
V/2
theory
of
-
-
M2A'2/2
gv*/4!,
particles
-
and M2 of
mass
gvA'3/3\- gA'*/4\ =
gv2/2 + m2
M
=
with both
-
an
=
A
>
Now
(2.9.2)
+ C.
2m2
+.
0. We
A'3 and
an
now
A'4
interaction. The symmetry is hidden; its only obvious manifestation is in the relation between the A'3 coupling and the mass and Af4r terms: gv
=
M(3gyt2.
(2.9.3)
27
2.10 Fermions
We will show later that renormalization counterterms are correctly given by continuing in m2 from positive m2. The vacuum expectation value of cf> has corrections which can be computed in perturbation theory
<0|</>|0>
=
? +
<0|</>'|0>
=
v +
0(g1/2).
(2.9 A)
Exactly similar methods can be applied if there is a continuous symmetry. Then the Goldstone theorem tells us that there will be a massless scalar particle for each broken generator. 2.10 Fermions The field theories obtained by functional integration as in Section 2.2 all theories of bosons. This follows from the symmetry of the Green's
are
functions
under
exchange
<0|7\/>(y)<£(x)|0>).
of
fields
<O|!T0(x)0()O|O>
(e.g.,
=
In turn, this symmetry property follows from the
functional-integral formula (2.2.2)
because the integration variables (the values of the classical field A(x)) commute with each other. To get a theory with quantized fields, it is necessary to define something like an integral over anticommuting variables. A rather small number of
properties of integration are needed to derive the equations of motion for Green's functions. Requiring these properties determines the integration operation uniquely (Itzykson & Zuber (1980)). As an example, consider the following Lagrangian £e Here
0
=
^ is
7*d^
a
generating
The fields and the a
a
free Dirac field:
(2.10.1)
¥(\¥-M)\l,.
¥ a row vector, while functional of Green's functions is written as:
four-dimensional column vector and
The
Z[rj,rj]=jr sector of
=
for
[[d^d^expji
J2?+
#+
UA
(2.10.2)
rj(x) and fj(x) take their values in the fermionic algebra. In the lattice approximation the definition (2.10.2) is really algebraic (Itzykson & Zuber (1980)).
sources
Grassmann
of the integration in
Green's functions
defined by differentiating with respect to the sources. One important difference between ordinary integration and Grassmann integration will be important in treating gauge theories. The simplest case are
{?
of this difference is in the integral over two variables x and where a is a real number. For ordinary real variables the dxdxeiiflx
=
27t
dx<5(xa)
=
2n/a.
x
of exp (ixax),
integral
is
(2.10.3)
28
Quantum field theory
For Grassmann variables
get
we
\dxdx&*ax
ia.
=
The overall normalization is irrelevant for
our
(2.10.4)
applications, for it is always
cancelled by the overall normalization factor in the functional integral. What matters is that the a-dependence of (2.10.4) is inverse to that in (2.10.3). 2.11 Gauge theories A gauge symmetry is an invariance under a group G where the group transformation is different at each space-time point. The earliest examples
General Relativity (where G is GL(4), the group of linear transformations of the coordinate system), and electrodynamics (where G is the were
rotations). Yang & Mills (1954) and Shaw (1955) generalized general group. Beg & Sirlin (1982) and Buras (1981) explain some of the uses of gauge theories as theories of physics. Let G be a simple group and let a matter field \\/ transform as group of phase
the idea to
a
Mx)-exp(
-
i^(x)t>(x)
=
(2.11.1)
l/(ai(x))"V(4
The field ^ is a column vector of components, and the hermitian matrices ta form a representation of the group, with structure constants caPy defined by
[W/J=i<Wr The matrices U(a)) form
a
(2-1L2)
representation of the group.
In order that the action be gauge invariant, we need
a
covariant
derivative:
(2.11.3)
D^^id^igA^
we have introduced the gauge potential A^. It is a vector under Lorentz transformation. As far as its gauge symmetry properties are or in terms of components concerned, it can be written as a matrix A^:
Here
AM
(2.11.4)
A,= I^r.. 3
It transforms under the gauge group
as:
\(x)-> UHxr'Wixi-ig-'d^UHx)). To build
an
action,
we
need the
which transforms as F-*U ̄lFU.
field-strength
tensor
(2.11.5)
2.12 A
gauge-invariant Lagrangian "
=
where
we
assumed In
tjp <5a/J/2. fields. If there are tr
Quantizing
=
with
+
spin \
nP
 ̄
29
matter fields is
(2.11.7)
MW,
the conventional normalization
of the
ta's, viz.,
exactly similar way, an action can be set up using scalar matter fields in several irreducible representations a term
an
for each is needed in
coupling
F?VF"72
tr
gauge theories
<£. The transformation (2.11.5)
ensures
that the
g is the same for all matter fields if the group is non-abelian.
The form of the infinitesimal transformations is needed:
Ji 8,0*1 S
^F^ If the group is
not
e.g., SU(2)?SU(2).
=
=
=
simple,
igco^t^ ^co' + gc^caPAl,
dcapyFlv.
(2.11.8)
then it is the product of several simple groups, a gauge field and an independent
For each there is
coupling. 2.12 A gauge
theory
such
as
solved by the functional fermion propagator.
<o|rwx)0oo|o>
Quantizing
gauge theories
defined by the Lagrangian (2.11.7) can be integral. Thus, as an example, we can write for the
the
=
one
^^^
(2.12.1)
In fact,
a lattice approximation to the functional integral forms the basis of Monte-Carlo calculations (Creutz (1983) and Creutz & Moriarty (1982)). The only trouble with (2.12.1) is that it is exactly zero. To see this we observe
that, given any field configuration, we can make a gauge transformation on it, as in (2.11.1) and (2.11.5). The new field configuration has the same action as the old field. Thus the only dependence on the gauge transformation is in
explicit \^{x)^{y). Now the gauge transformation is independent at each space-time point. So (2.12.1) contains a factor
the
n
joints z
=
\dU(z))u-\x)?U(y)
J
/
(number))
\<iU(x)U(xyl V
dI7(y)l%)\
(2.12.2)
30
Quantum
which is
field theory
(Note that the propagator is
zero.
a
matrix in the representation
space of the gauge group.) The vanishing of (2.12.1) is not a fundamental problem, for we may choose only to work with Green's functions of gauge-invariant operators (e.g., tfty, F?vFa"v, the Wilson loop (Wilson (1974) and Kogut (1983))). But the
vanishing is
a
disaster for
formulating perturbation theory; for
the basic objects needed to write the Feynman rules the elementary fields. An elegant solution to this
Faddeev
are
among
the propagators for
problem
was
given by
& Popov (1967). The integral over all gauge fields is written as the of the integral over fields satisfying some given gauge condition
product (such as d'Aa 0) and of the integral over all gauge transformations. Any field configuration can be obtained by gauge transforming some configuration that satisfies the gauge condition. For a gauge-invariant Green's =
function, the integral over gauge
over gauge
transformations amounts to
an
overall
inverse factor in the normalization. So the integral transformations can be consistently omitted.
factor which cancels
an
The new integral over fields with the gauge-fixing condition imposed also provides a solution to the theory. But the gauge-variant Green's functions like (2.12.1) no longer vanish. It is necessary, moreover, to find the correct measure for the integral; this was the key point of the work of Faddeev and Popov. These authors also constructed
a
slightly
formulation that is most often used, and that
different formulation; it is this we
will review
treatment and further references are to be found in
Itzykson
A detailed
& Zuber (1980).
summarize the argument and derive the Ward identities in the form that we will use them. Here
we
will
now.
merely
We will consider gauge conditions of the form Fa[A,x] =/a(x). There is condition at each point of space-time and for each generator of the
one
group. The functional Fa might be d Aa, for any real valued functions of x. Faddeev and
<0|TX\0}
Popov write =
JfGl
an
example. The functions/a(x) are
arbitrary Green's function
as
f[cU][#][diAJXexp(iSlnv)A[^]\\S(Fa-/J.
J
x'"
(2.12.3) is the gauge invariant action, and X is any product of fields. The 5inv factor A|\4] is a Jacobian that arises in transforming variables to the set of fields that satisfy the gauge condition plus the set of gauge transformations. Here
The key result is that
A[A ̄\ A=
is
a
determinant,
so
that it
f[dcj[dcjexp(ii?gc).
can
be written
as
(2.12.4)
2.12 Here ca and ca are
ghosts. The
Quantizing
anticommuting scalar fields, called the Faddeev-Popov gauge-compensating Lagrangian is
so-called
^gc where
ScFa
For the
31
gauge theories
=
(2.12.5)
caScFa[A,xl
is the infinitesimal transformation of
case
Fa
=
Fa
with
a)
replaced by
c.
dA*
Se%c
=
d?c(d?ca
+
(2.12.6)
gcafi7cfiAl) + divergence.
independent fields. They are not genuine physical fields, as they do not obey the usual spin-statistics theorem. A convenient form of solution to the theory is obtained by averaging over  ̄1 ally's, with weight exp (— £ \fl/2). This leaves gauge-invariant Green's functions unaltered, and gives the following formula: We treat
c
and
c as
<0| TX\0} with
=
Jf
different normalization. The
a
The action
integral
is
over
+
ifgc).
(2.12.7)
all fields (A, \j/9
=
|d4x(i?inv
+
i?gf
The
gauge-fixing
a
c).
an
chosen parameter.
arbitrarily Fa.)
and
term is
J?gf=-iF,2/(2£), where £ is
c,
(2.12.8)
already defined the gauge-invariant Lagrangian by (2.11.7)
JSfgc by (2.12.5).
into
\J/,
S contains three terms: S
We have
I[d fields]Xeis
(If desired,
(2.12.9) it may be absorbed
redefinition of
The advantage of the form (2.12.8) is that Green's functions of the elementary fields
are
defined
invariant observable X the
objects
as
<0|T*|0> If
as in a simple non-gauge theory. For a gauge- equations (2.12.3) and (2.12.7) define the same
=
JTGI [d^d^di?]A:exp(i5inv).
(2.12.10)
variant, then all the definitions give different results, and (2.12.7) depends on £. Quantities that depend on the choice of a gauge fixing X is gauge
are
called gauge dependent, of course. We see that gauge in variance of the a Green's function implies gauge independence.
operators in
important to distinguish the concepts of gauge in variance and gauge independence. Gauge in variance is a property of a classical quantity and is in variance under gauge transformations. Gauge independence is a property It is
of a quantum quantity when quantization is done by fixing the gauge. It is
32
Quantum field theory
independence of the method of gauge fixing. Gauge invariance implies gauge independence, but only if the gauge fixing is done properly. Gauge theories such as (2.11.7) have a dimensionless coupling if space- time is four dimensional. General results, which we will treat in later chapters, imply that the theories need renormalization. However these same
results
available by
imply that many more renormalizing (2.11.7).
extra couplings
are
counterterms may
Chapter
In
absent. The tools needed
are
12
we
be needed than
are
will prove that the
the Ward identities for the
will prove in the next section. It is also necessary to prove that the unphysical degrees of freedom represented, for example, by the ghost fields ca and ca do not enter unitarity relations. This proof also gauge symmetry. These
needs the gauge Zuber (1980)).
we
properties exhibited in
the Ward identities (see Itzykson &
2.13 BRS invariance and Slavnov-Taylor identities
fixing, the gauge invariance of a gauge theory is no longer manifest in the functional-integral solution. Slavnov (1972) and Taylor (1971) were the first to derive the generalized Ward identities that carry the After gauge
consequences of gauge invariance. Their derivation
simplified by Becchi,
Rouet & Stora
was
very much
(1975) through the discovery of what is
called the BRS symmetry of the action (2.12.8). BRS symmetry is in fact a supersymmetry, that is, its transformations
now
involve parameters that take their values in a Grassmann algebra. Let 8X be a
fermionic Grassmann variable. Then the BRS transformation of a
or a
gauge field is defined to be
^brs^A
<Ws*A
=
=
a
-
gauge transformation with of
ig(c"SX)tail/
=
=
matter
caSL Thus
igt^ij/SX,
ig¥tac*SA9
Observe that 8k is fermionic, so it anticommutes with fermion fields (c, c, ^, \ji). The ghost fields transform as
<W'a
W.
=
=
-igcafiycpcy5l9 (2-13-2)
W?-
(Note that ca and ca are not related by hermitian conjugation, contrary to appearances.) Since ^inv is invariant under gauge transformations, it is BRS invariant. Hence
^BRS^
=
=
=
<W^gf + ^gc) (l/£)FaSMFalA;x] -
0.
-
(l/S)Fa5McFa
 ̄
*Ars(W (2.13.3)
2.13 BRS invariance and
In the second line
we
used
anticommuted 8k and
we
addition
8
33
identities
to prove the last line zero in the first two terms of the second line. In
J^gc c
=
cx8cFx, while
—
nilpotence of
used the
we
Slavnov-Taylor
the BRS transformation:
\2
^fY*.
=
0,
(2.13.4)
which follows from anticommutativity of the c's. By applying the Noether theorem we find a conserved current: ih*
=
gfrtjc
F*">Dv<r
-
-
(l/w-A'iyd' (2.13.5)
-ig(d"ca)cl'cycalh.
the BRS transformations involve Grassmann-valued
Although
given in Section 2.7 goes through need the integrated Ward identity only
parameters, the derivation of Ward identities
unchanged. (2.7.9). A case
For
our
we
purposes, of (2.7.9) applied to BRS invariance is called a Slavnov-
Taylor identity. A simple example is 0 &H*s<0\TAl(x)c,(y)\0y/8A. =
-<0\T(dflcx + gc,Syc*A?)cl!(y)\Oy + {VZ)<0\TAHx)d-A'(y)\0>.
=
We have defined the notation
<5BRS (quantity)/<5A to
(2.13.6) mean that the 8). in the
BRS variation is commuted or anticommuted to the
The most used o
=
=
Here
X is a
right and then deleted.
of the Slavnov-Taylor identities
cases
are:
WO|rxc.M|o>/<5;. -<0\T(SBRSX/81)ca\0> + (l/ZK0\TXd-A°\0y.
product of fields with total ghost number equations of motion. Let:
(2.13.7)
zero.
We will also need
#*=-Zr-d?JT7r=W-MW, ^
Se^
<ei(t
=
?A(-i^-M),
=
-
=
-
DVF">"
-
gh'%^
+
i\/Wd-A' +
0C??(3"c%
^d"c",
Se',= -Uc.- gc^id^U". Then each of these is
theory
we
zero.
Furthermore, for <j> equal
(2.13.8) to any field in the
have; <o|rjs^(x)x|o>
=
i<o|Ta*/^(x)|o>,
(2.13.9)
34
Quantum field
theory
where time derivatives in ^ are taken outside the time-ordering (as see
usual
-
Section 2.5).
2.14 Feynman rules for gauge theories Feynman rules are given in Fig. 2.14.1 for the Lagrangian of (2.11.7), with the gauge-fixing term Fa d-Aa. Note that these agree with the figures but =
not the
equations of Marciano & Pagels (1978). They are the rules for quantum chromodynamics (QCD), the theory of strong interactions, if the
gauge group is SU(3). The fermions
are
the quarks and consist of several
own mass term. (In the conventional the field is called the gluon field and the gauge terminology, gauge
triplets of SU(3), each with its
symmetry is called the color symmetry of strong interactions. Each
representation in the quark field is called
irreducible
a
flavor, and has
a
label: u, d, s, c, b, etc.) The same Lagrangian
also describes quantum electrodynamics (QED) if the change gauge group to C/(l). In that case there is but one gauge field (the photon) and, since the group is abelian, the three- and four-point self- we
p
ij/
p
—
M + \E
■wX 'a, n 4" p
\J^
ApX
fAi Af
H
ca
->-r> ̄ J
P
=
"
=
n-^v
S P'
9caPy[(qK
-
-
rK)gXu
+
(rx
-
px)g,K
+
(/>?- q,)gK J
ig2[caPecyd {g^gkv gKVg?J -
+
ca8tcfiyt(gKkg^
-
gKftgXv)]
=-gcafiyP'>
At 2.14.1. Fig. Feynman rules for
the gauge theory defined by the
Lagrangian (2.11.7).
2.15 Other symmetries
35
of (2.11.7)
interactions of the gauge field vanish. Moreover, with the gauge fixing term F\A\ 8-A there is no coupling to the ghost fields, so we may drop them =
from consideration. The transformations
phase
rotations:
\j/-+e ̄lq<°\l/ where
the matter fields
are simple field of the (negative for charge
on
q is the
the electron field). The transformation of the gauge field is
A^A^-V d^co.
2.15 Other symmetries of (2.11.7) The
Lagrangian (2.11.7)
is gauge invariant. After gauge
fixing
we
get
invariant, but which has BRS symmetry. The action (2.12.8) also is invariant under global gauge transformations those
(2.12.8), which is
not gauge
-
with constant
a>
-
because
chose the gauge fixing not to break this
we
symmetry. There are also what in strong interactions are called flavor symmetries. These are transformations that act identically on every member of an irreducible representation of the gauge group. In this case they give
conservation
of the number of each of the different flavors of quark. Other symmetries of parity and time-
flavor symmetries include the discrete reversal invariance.
Charge-conjugation is also an invariance of (2.12.8), and its action on the ghost fields is rather interesting. Let us define ea by the parity of the representation matrices under transposition: ta In this and the
=
Ejl(%).
(2.15.1)
following equations, the symbol (J) means that the on repeated indices is suspended. The fermion and
summation convention gauge fields transform
as
usual:
(2.15.2)
^->?c(iyV)(<A)T. where rjc is
a
real matrix such that
tl The
ghosts transform
=
(2.15.3)
nXnc.
as: ca-+
-
eaca,
c*->
-
eaca.
(2.15.4)
AI Green's functions. They so only two couplings of the gauge indices are possible: c^y which is antisymmetric, and a symmetric coupling which we can call dxPy. Charge-conjugation invariance prohibits Consider the ca
-
cp
-
A\ and the A\
-
A*
-
are
invariant under global gauge transformations
the
symmetric,coupling.
36
field theory
Quantum
2.16 Model
field theories
Although the concepts of quantum field theory are very general, we have reviewed them by examining mainly two specific models. The first was the theory of a real scalar field (2.1.1), mostly with the (/>4 interaction (2.2.1). The second was a gauge theory (2.11.7) with matter described by some Dirac fields. It should be clear that the general principles apply to any Lagrangian ££. A field theory is specified by listing its elementary fields and giving a formula for ¥£\ It is solved by a functional-integral representation of its Green's functions.
is to describe the real world. To the extent that a field- is description the correct one, the fundamental problem in physics
The aim of physics theoretic
is to find
theory. In fact the Lagrangian (2.11.7) appears to electromagnetic interactions if the gauge group and matter fields are correctly chosen. Weak interactions can be included by the Weinberg-Salam theory, and many speculations have been made about extensions (see the proceedings of most recent conferences on high-energy physics). Our aim in this book would be badly served by only treating real theories. One reason is that we wish to develop techniques and concepts applicable to any field theory, for example not only to the many Grand Unified Theories currently under discussion (see Langacker (1981) and the correct field
do this for strong and
Ross (1981) for reviews), but also to the theories to be invented in the future. As is usual in the subject, we will make use of field theories that are more properly called models. The (/>4 theory is an obvious case. Another
important reason for using models is to be able to discuss particular aspects of the methods without having other complications to clutter up the presentation. Particular models will be introduced
as needed. Some will recur often, such as the t/>4 theory and the simple gauge theory (2.11.7). Another frequently used model is the (/>3 interaction of a real scalar field:
<£ This is much
=
{d(t>)2ll
-
-
003/3!.
(2.16.1)
than the other models. It is not
unphysical completely consistent. Because of more
m2(j)2/2
even
the (/>3 interaction, the energy is not bounded below. This is manifest in the classical theory and true in the quantum theory (Baym (I960)). Hence any state must catastrophically
decay. But the perturbation theory is well-defined, and somewhat simpler than for the t/>4 theory. So it proves very convenient to use the cj)3 model in treating the elements of the theory of renormalization within perturbation theory.
2.16 Model
field theories
37
constructing models is to change the dimension, d, of physical value 4. One motivation for this is that the renormalization problem becomes easier as d is reduced; the degree of divergence of a Feynman graph decreases. As we will see in Chapter 4, it is Another way of
space-time from
its
both useful and possible to treat d
as a
continuous variable, for the purpose
of computing the values of terms in the
perturbation expansion.
3 Basic
examples
chapters we will develop methods to treat large-momentum behavior. The complete treatment becomes rather intricate at times, so this In the later
chapter is devoted to
exposing in their simplest form the issues we will be discussing. We will do this by examining the self-energy graph in t/>3 theory. This will exhibit the basic phenomena which we will later be treating in detail. We will
see
that (in four-dimensional
space-time)
the
graph is
re-
normalized by a mass counterterm. Then the concept of 'degree of divergence' will be introduced by varying d, the dimensionality of space- time. This device will enable us to see how simple power-counting methods
determine what
counterterms are needed. It will also introduce us to the
regularization. The renormalization group will be introduced by examining the behavior
method of dimensional
of the graph
as its external momentum, p, is made large. By exploiting the arbitrariness in the renormalization procedure, we can reduce the size of
higher-order contributions when p" is large. 3.1
One-loop self-energy
in
<£3 theory
Consider the graph shown in Fig. 3.1.1 in the (j)3 theory of (2.16.1). We define its contribution to the self-energy to be i times the value of the graph with the external propagators removed:
^i{p2)
=
li^rk[k^-m2
The overall factor \ is
a
+
u-\i{p + kf-m'+^
symmetry factor. k
-O-
Fig. 3.1.1. One-loop self-energy graph in <f>3 theory. 38
(3U)
3.1
One-loop self-energy
in
<f>3 theory
39
When all components of k* get
large, this integral diverges of an ultra-violet divergence. As we will see, logarithmically. simplest example the divergence can be cancelled by a mass counterterm. But to explain the renormalization properly, we must discuss a number of other issues as well: It is the
(1) The fact that if | k° |
^
| k | the divergence as k goes to infinity appears to
be much worse.
(2) A precise
way of
cancelled
formulating
the statement that the divergence is
by (3) The arbitrariness inherent in the renormalization. (4) The interpretation in coordinate space. a mass counterterm.
3.1.1 The first of these
Wick rotation
problems is handled by recalling
the Wick rotation into
Euclidean space that was used to define the functional integral. This rotation determined the sign of the ie in the free propagator. The Wick
it, then performing the integral, and finally analytically continuing back to real time. In momentum space, this forces us to work with k° + ica, the opposite sign rotation involved starting with
imaginary
time t
=
—
=
appearing
so
that in the Fourier transformation &kx is always
In the Euclidean formulation let us
perform
the
a
phase.
/c°-integral first. The pole
/c°-plane is shown in Fig. 3.1.2, when p° is imaginary. In this and (p + k)2 are both negative, so that the integrand is positive
structure in the situation k2
definite. Observe that the factor i coming from the Wick rotation combines with the overall factor of i in (3.1.1) to make Z, real. (We have d4/c =
idaxl3fc.) Now rotate
shown in
Fig.
p°
back to
real value. If
a
3.1.3: there is
no
| p° | < m then we have the situation
obstruction to
rotating the k° contour to run a problem from the
along the real axis. It is only at this last step that there is
L p°  ̄EP+k
+
P
^p
+ fc
-X- E*
Fig. 3.1.2. The
fc°'-plane
when
imaginary.
p°
is
Fig. 3.1.3. The fc°-plane when p° is real, but \p°\ is less than m.
Basic examples
40
IN
m-
Fig. 3.1.4. The /c°-plane when p° is real, but
region of |fc°| |fc|. integral by rotating  ̄
we
pick
pole
up a
run
m.
we merely have to define along the imaginary axis.
the
region
to the
illustrated in Fig. 3.1.4. Again
(p + k)2
problem
the /c°-contour to
is greater than
|p°|>m. The case of negative p° is Wick-rotate to imaginary k°, but this time term. Now the pole term occurs only when (p0)2 > m2 + p°
Now continue
To avoid the
\p°\
we
E2+k.
Thus it contributes only in a finite region of k; the UV still from the integration over imaginary k°. comes divergence The moral of all this is that the UV divergence is essentially Euclidean,
i.e.,
=
we may
regard k°
as
imaginary 3.1.2
and k2 < 0, (p +
k)2
< 0.
Lattice
quantify the divergence. The divergence comes from the asymptotic large-/c behavior of the integrand which is l/(k2)2. Let us add We next need to
and subtract =
:{{'
z1
a term
Iff 32n
d4k
with this behavior: 1
1
l(k2
-m2
+
ie)[(p + k)2
-m2 +
ie]
(k2
-
fi2
+
1
d4fc- (k2-n2+ie)2
+
is)2 (3.1.2)
integral is manifestly finite, for we have subtracted off the leading asymptotic behavior of the integrand. To avoid introducing an extra
The first
divergence Since
we
at
k2
=
0 we have subtracted
l/(/c2
—
pi2)2
rather than
l/(/c2)2.
of/i is irrelevant; 2 i is unchanged. is independent of p. This is the fact that
add this term back on, the value
The second term, while
divergent, divergence by a counterterm. Of course we are manipulating divergent integrals, so that (3.1.2), as it stands, makes no sense. We will remedy this defect by using the fact that the theory is defined initially on a lattice. The propagators will then be different will enable
us to
cancel the
functions of momentum. However, the structure of (3.1.2) will be unchanged after
imposing
a
cut-off,
as we
will
now
show.
One-loop self-energy
3.1
To define the functional integral also had to put the
theory, limit qa
theory
on a
let the free propagator ->
0, this is just i/(q2
—
in
<f>3
41
theory
only had to Wick rotate time, but space-time lattice, of spacing a. In the lattice of a particle of mass M be SF(q;M, a). In the we not
M2 + ie). But it is
zero
if qa
>
1, since high-
is, of course, that when one makes a Fourier transformation on a discrete space, one only uses
(The
momentum states do not exist on the lattice.
modes with
momentum
self-energy
wave-lengths longer
the lattice is finite, and (3.1.2)
on
Zx(p, m\a)
-i2 10
than now
lattice
a
spacing.)
The
reads:
(r
\d*k[SF(k;m,a)SF(p + k;m,a)-SF(k;ti,a)2]
=
32n
^
+
=
reason
\d*kSF(k;ix,a)2\ (3.1.3)
Zlfin(p,m,/i,a) + Zldiv(m,/i,a).
integrals are now convergent, so (3.1.3) is a correct version of (3.1.2). As the lattice spacing goes to zero, the first integral approaches the first convergent integral in (3.1.2). The second integral diverges, but is All the
independent of p. Thus (3.1.2) is not nonsensical, provided that the propagators are implicitly replaced by lattice propagators wherever necessary.
3.1.3 Interpretation of divergence No matter how it is limit. The From
use
manipulated, the self-energy diverges in the continuum us to quantify the divergence.
of a lattice cut-off now enables
(3.1.3)
12_ Zi=T&jd4^F(^;/i,a)2 32n4
327U4 2
16tc2
=
-Q2
Jfl)2+fc2<1
+
Zlf
dcod3/c—-—=r; k2 (w2
+
r-r
+
n2)2
+ finite
fl/a
J
0
(/c2 + /i2)2
—^lnl/a +finite
+ finite
asa-+0.
(3.1.4)
interpret the divergence as follows: Let £ be (i times) the sum of self-energy graphs. (As usual, the self-energy graphs are graphs for the Thus we can
propagator that have the external lines amputated and that cannot be split
42
Basic
G*
Fig.
+
=
-#- -#-#- +
3.1.5. Summation of
into disconnected parts
examples
self-energy graphs
+
into propagator.
by cutting a single line.) Then
G2(p2)
=
i/(p2
-
m2
-
I +
-
the full propagator is
ie).
(3.1.5)
This equation is illustrated in Fig. 3.1.5; the propagator is the
sum
of
a
geometric series involving the self-energy. The actual mass mph of the particle is determined by the pole position, p2 m2h. Evidently m2h is not m2 but m2+ E(p2 m2h,m2). In other words, the self-energy represents =
=
the dynamical contribution to the mass coming from the interactions. The divergence (3.1.4) is independent of p2, so it is precisely a contribution to the mass. (We ignore higher orders for now.) Traditionally, one observes that it is convenient to parametrize the theory, not by the mass parameter m that cannot be observed directly but by the physical mass mph. One writes the mass term in <£ as -
m2ct>2/2
=--
-
m2h</>2/2
-
The first term is left in the free
i/(p2
—
that the free propagator is Lagrangian, second term called the mass counterterm is
m2h + ie). But the
-
-
put into the interaction propagator has a pole at
dynamical contribution
(3.1.6)
?5m2</>2/2.
so
Lagrangian,
p2
m2h.
=
to the
and
adjusted
so
that the full
The counterterm exactly cancels the
particle's
mass.
This is the basic idea of
renormalization. It is physically irrelevant that dm2 happens to diverge. In perturbation theory, dm2 is determined as a power series in g. To 0(g2) we have, in addition to Fig. 3.1.1, the graph of Fig. 3.1.6 corresponding to the mass counterterm in
is the
parametrization
(3.1.6). The self-energy sum
of Figs. 3.1.1
to
0(g2)
with the
new
and 3.1.6. We call it the
renormalized self-energy: ^iR
=
=
{^i(p2;mph^) + ^2}|fl-o Ilfin(p2;mp2h;/i,0)-f[<5m2 + Ildiv].
(3.1.7)
X Fig.
3.1.6. Counterterm graph to the self-energy.
adjust dm2 first of all to cancel the divergence in £ ldiv, so that the term in square brackets is finite as a->0. Then we adjust the finite part so that We
<5m2 + Zldiv=
-XUi>p2h,mp2h,/z).
(3.1.8)
One-loop self-energy
3.1
For the renormalized
*..-£K
self-energy
(*
-
value at
-
we
=
43
find
mp2h + ie)[(p + k)2 p2
<fi3 theory
in
-
m2h + ifi]
(k2
-
\i2
m2h.
+
ie>
(3.1.9)
^-dependence cancels. Equation (3.1.9) gives the value of the self-energy in the continuum theory
Note that the
correct to
0(g2). 3.1 A Computation
One way to calculate course a Lorentz
integration
Z1R
is to differentiate with respect to
scalar.) Integrating the result gives Z1R; the
is fixed by the renormalization condition that
p2. (It
is of
constant of
TlR(m2h) is zero.
We have:
asu
_
<>
f
r
'J'
 ̄'e'
U,?(k2 -m2,,.,*? ??, is) [(p k)2 32;r4p2 J This is identical to what
we
+
+
+
(3.1.1(0
..,?■
-
m2
+
i£]2
would have obtained from the unrenormalized
expression (3.1.1) without regard to the fact that it is divergent. We could have written it down directly without going through the long explanation that we used. But then there would have been no defense to the argument
that
we
are
manipulating meaningless quantities
and that therefore
quantum field theory makes no sense. Since (3.1.10) is finite it can be easily calculated by using
a
Feynman
parameter representation and then by shifting the /c-integral: <5I1R
ig2
dp2
16a V
f J
ig2
f
16kVJ 32*2
g2.
J0 o
32n2dp:
d**
dxx
dx
p-(p+k)
[Wph
,4,.
-
P2x
-
2p-kx -k2- ie]3
P-jk + pjl-x)]
a\m2fh-p2x(l-x)-k2-ieY x(l-x)
[m2h-p2x(\-x)-\
|rdxln[mp2h-p2x(l-x)]|.
(3.1.11)
44
Basic
Using the condition Z1R
=
0 at
*lR ̄32n2 This integral
can
p2
examples =
dxln
be worked out
3.2
m2h
now
mp2h(l
-
gives x +
x2)
(3.1.12) .
analytically.
Higher
order
graph of Fig. 3.1.1 is not the only divergent graph in the theory. In Chapter 5 we will discuss the general theory of renormalization and we will see how to extend the removal of divergences to all orders. In this section we will only consider a class of graphs which have divergences generated because Fig. 3.1.1 occurs as a subgraph. Examples are given in Fig. 3.2.1.
The
One property should be clear. This is that the divergences come from subgraphs all of whose lines are part of a loop. A general way of
characterizing these subgraphs is to define the concept of a one-particle- irreducible graph or subgraph. A one-particle-irreducible (1PI) graph is one which is connected and cannot be made disconnected by cutting a single line. A graph which is not 1PI is called one-particle-reducible (1PR). The in Fig. 3.2.1 are all one-particle-reducible, since they all have one or lines that when cut leave the graph in two disconnected pieces. The
graphs more
self-energy subgraph of Fig. 3.1.1 consisting of the two lines in the loop is 1 PI. This identical subgraph occurs several times in the graphs of Fig. 3.2.1. We introduced
a mass
6
counterterm
into the interaction,
so
that the
o^>
w
(c) Fig. 3.2.1. Graphs containing the one-loop self-energy
as a
subgraph.
3.3 Degree
of divergence
(a)
45
(b)
Fig. 3.2.2. Counterterm graphs to Fig. 3.2.1.
counterterm
graph Fig.
the counterterm vertex
3.1.6 cancels the
can
be used
as an
in Fig. 3.1.1. Clearly interaction anywhere in a graph.
divergence
In fact, all graphs containing it can be found as follows: (a) take a graph with the loop of Fig. 3.1.1 occurring as subgraph one or more times, but with no mass counterterm
vertices; (b) replace
one
(or more) of the
occurrences of
the loop by the counterterm. The terms generated from Figs. 3.2.1 (a)-(c) are shown in Fig. 3.2.2. sum of the original graph and its counterterm graph(s) is the just original graph with every occurrence of the loop replaced by its renormalized value iZ1R. It is sensible to keep the counterterm associated with the loop, thereby considering the loop plus the counterterm as a single
Evidently, the
—
entity. In the
case
of the graphs of Fig. 3.2.1 the result of this
graphs finite. The generalization out in Chapter 5.
make the worked
3.3 We
Degree
of
to an
procedure is arbitrary graph will
to be
divergence
how the Wick rotation ensured that the UV divergence of the one- self-energy is a purely Euclidean problem. The divergence is from the
saw
loop
region
| fc^ |
-^
oo, without any
regard
to direction.
Thus, simple power-
a logarithmic divergence. The power- counting involves merely counting the powers of k in the integral for large k.
counting determined that there is The divergence is logarithmic
as
Power-counting in this fashion
the lattice spacing, a, goes to zero. works for a general graph to determine
46
Basic examples
degree of divergence' or the 'superficial Chapter 5. There we will also see how the value of the degree of divergence determines the particular counterterm vertices needed for a theory. This will enable us to determine whether or not a theory is renormalizable by invoking arguments revolving what is called either the 'overall
degree of divergence'. This
we
will discover in
around the dimension of the
coupling. dimensionality, d, of space-time in the self-energy graph, Fig. 3.1.1. We can then explore the the degree of divergence, as determined by power-
In this section we will vary the
calculation of the
relation between
counting, and the momentum dependence of the counterterm. The integral for Fig. 3.1.1 is now ,2
^(p2,m2,d) The
space-time
1&
=
has
ddk(k2 -m2
2{2iif one
+
ie)[(p + k)2 -m2
time dimension and d
—
+
ie]'
(3'3J)
1 space dimensions. In the
Feynman rules the factors (2n)A get replaced by {2nf, since they arise from the result
f
ddxQikx
=
(3.3.2)
(2n)dS^(k).
The number of powers of k in the
divergence of the convergent. But whenever degree of
integral is now d 4; we call this the If graph. d is less than four, then the graph is d is greater than or equal to four, the graph —
diverges. Our discussion in the previous sections tells
by adding
renormalize it
Now, differentiating
try
to
a counterterm.
once
with respect to p" gives convergence if d
^1 Z2t U =
(27r)d
dp*
us we must
^±^
J V-m2)[(p + fc)2-m2]2'
=
4:
l(3 3 3)} *
degree of divergence of the integral being reduced by one to d 5. integral diverges logarithmically. However one might surmise that the divergence comes from the piece of the integrand proportional to kfl k would kill the divergence. This is in fact and that symmetrizing by k-+
with the
If d
=
—
5 the
—
true, but let us be more simple-minded. Differentiating again with respect to divergence d
9% dp?dpv ^dpv To recover
—
a
result with degree of
6:
in'
[^AHp + kUp + kl-gJip + kf -m']}
(2n)d (2n)d]J
Z1R
p gives
(k2
at d= 5 we
integration that appear
-
m2)[(p + k)2
integrate twice.
as an
-
There
m2]3
(3J4)
are more constants
additive contribution of the form: A +
of
B^p*.
3.3 Degree
of divergence
47
require Lorentz invariance of £, so the Bfi term is eliminated, and we are left with a mass term as the only counterterm. This is However,
we
must
the first and simplest example of the
use
of
a
symmetry argument
to
determine the form that we will allow for a counterterm.
(j)3
3.3.1
atd
=
6
Differentiating Zj three times gives a finite result with degree of divergence d 7. Integrating to obtain Z1R gives arbitrary integration constants of the form A Bp2, where again we have used Lorentz invariance. If we went to the lattice we would find divergences proportional to I/a2, m2ln(a) and p2ln(a). The fact that these terms have dimension 2 corresponds to the fact that the integral for Ex has degree of divergence 2. To make it finite we must not only use a mass counterterm but also a counterterm proportional to p2; the total we will call dm2 dZp2. Let
us now
go to d
—
6.
—
—
—
This is generated by
a counterterm
-
(3.3.5)
6m2(j)2l2 + 6Z{d(f))2l2
in the Lagrangian. Evidently the value of the
degree
maximum number of derivatives
or
of divergence is reflected as the powers of p in the counterterms.
Equally, it is reflected in the integration constants that appear when we recover Z1R from the differentiated T,l. These two phenomena happen for a general graph, as we will see later. The method of proof will in fact be to differentiate each graph enough times with respect to its external momenta until it is finite.
example of the wave- can interpret it the bare field for the The physically by examining propagator propagator. can be expressed in terms of the propagator of the renormalized field: The
(d(f))2
counterterm in
(3.3.5) is of
course an
function renormalization introduced in Section 2.3. We
e2(O)
<o|r#o(p)0o(O)|o>
=
Z<0|7>(p)4>(0)|0>
=
iZ/(p2
=
-
m2h
-
£1(0)
-
dm2
+
SZp2
+
ie)
iZ/(p2-mp2h-Z1R).
=
(3.3.6)
Note the distinction between bare and renormalized fields. The residue of
particle pole in unity: the
G2(0)
=
the propagator of
iR(0)/(P2
 ̄
"*ph
+
ie)
an
+
interacting field is
finite>
as
P2
->
in
general
(3-3-7)
mph-
Examination of its spectral representation demonstrates that 0
not
<
R(0)
<
1,
48
Basic examples
(j>0 has canonical equal-time commutation relations. (The proof is given, for example, in Section 16.4 of Bjorken & Drell (1966).) Now we can always change the definition of (/> Z" 1/20o by multiplying Z by a finite factor. So it is possible to adjust Z so that the renormalized propagator has a pole of unit residue at p2 mp2h: because the field
=
=
<52 In this
case we
=
i/(p2
integrating
=
ie)
+
Wph
identify R{0) Z,R
When
-
finite,
+
with Z. The renormalized
5Z1R/3p2
=
0
p2
at
(3.3.8)
p2->m2h.
as
=
self-energy
satisfies
(3.3.9)
m2h.
Z1R from the finite derivative of Zl9 this integration constants to be determined. It is called the
to obtain
condition enables the
mass-shell renormalization condition.
3.3.2
Why
may Z be
zero
and yet contain divergences!
a property of the exact theory we know that 0 < Z < 1, if we adopt the renormalization condition (3.3.9); Z is definitely finite. However its perturbation expansion starts at 1 + g2[C In (a) + finite] + \ (Here C is a constant.) This seems to be infinite as a -? 0, rather than finite. We resolve the contradiction by realizing that we should not expect higher-order terms to be small if the one-loop correction is large. For example, we could have
As
?
?
the series
Ll-02[(C/D)ln(a) + const.]J where D is
a
loop (otherwise if
we
fix
It would appear
;
through the one- positive a and let #->0 there is a region with Z > 1). impossible to derive a formula like (3.3.10) since it
positive number. infinity. In
term goes to
'
Then Z -? 0 any event
as a -? 0 even
we see
that C must be
involves summing all orders of perturbation theory. Moreover it involves an analytic continuation from within its radius of convergence | In (a) |
<
D/g2C
perturbation
to
series
| In (a) | are
=
oc.
This
in general
seems to
make
no sense at
asymptotic series rather
all since
than convergent
series. However, we will see in Chapter 7, on the renormalization group, that we can find a systematic method of calculating Z and the other
renormalizations in the limit a-?0.
It relies
theory. The behavior (3.3.10) asymptotically free theories.
freedom of the
on
the so-called
asymptotic
will turn out to be essentially
correct, in No matter what the truth is, it should be clear that the divergences in perturbatioh theory as a -? 0 need not be reflected as divergences in the exact
theory, but only
as
singularities.
3.3 Degree 3.3.3
Suppose the
Renormalizability
we now go to a
and non-renormalizability
dimension d
one-loop self-energy has
a
49
of divergence
> 6.
For example, let
quartic divergence. The
us set
d
=
8. Then
necessary counter-
terms contain up to four derivatives of the field: -
The quartic term
Sm2(j)2/2
-
SZ(d(j))2/2 + E( □ c/>)2/2.
is not of the form of any term in the
(3.3.11)
original Lagrangian,
it cannot be obtained by renormalization of the Lagrangian. When this situation occurs the theory is called non-renormalizable. Non- so
renormalizability
is
a
consideration. There
good reason possible ways to
priori are
a
for
dropping
the
avoid this, but
theory
we
from
will leave
discussion of this until later.
simple argument that helps in the determination of whether or theory is renormalizable. It links dimensional analysis and power counting. Consider a one-particle-irreducible graph T. Let its degree of divergence be S(T), and let its mass dimension be d(T). Now T is the product of a numerical factor, a set of couplings, and an integral. As we see from (3.3.1) the degree of divergence is the dimension of the integral. Therefore, if There is
a
not a
we
let A(r) be the dimension of the couplings in T,
d(r) Now consider the
3(r)
=
+
we
have
(3.3.12)
A(r).
polynomial of degree S(T) in the external momenta. For each term C in the polynomial, let S(C) be the number of derivatives and let A(C) be the dimension of the coefficient, so counterterms to T. These form a
that its dimension is the
same as T:
<5(C) + A(C)
=
(3.3.13)
d(r).
Now, the maximum number of derivatives in the
counterterms is
d(T)=d(T)-A(n If the couplings
have
large by going
to
negative dimension, then 8(T) can graph of high enough order.
a
miraculous cancellations this tells
couplings have
If the
as
arbitrarily
In the absence of
expect non-renormalizability. If the
positive dimension, we have a finite number of since A(r) is bounded above by zero and d(T)
zero or
counterterm vertices, decreases
us to
be made
—
the number of external lines increases.
couplings
never
have negative dimension,
coefficients of the counterterms
satisfy
A(C)
=
d(T)-d(C)
>d(T)-S(r) =
A(r),
we
observe that the
50
Basic examples
so that these coefficients also have non-negative dimension. This, in the simplest cases, is sufficient to ensure renormalizability. For example, in the (/>3 theory cj> has dimension d/2 1. (Recall that ¥£ has the dimension of an energy-density.) Then A(g) 3 d/2. So if d > 6 the < 6, there are only theory is non-renormalizable, as we saw by example. If d -
=
-
a finite number of possible counterterms, since these coefficients of non-negative dimension.
are
restricted to have
3.4 Renormalization group 3.4.1 Arbitrariness in The infinities of
renormalized graph
renormalizable theory amount to divergent dynamical that renormalize the parameters in the Lagrangian.
contributions Traditionally
a
a
one
thinks of renormalization
as
the
procedure
of
working
with measured
quantities instead of the corresponding bare quantities. The most obvious case is that of the mass mph of the particle corresponding to an elementary field.
However to take the traditional view is much too
restrictive. This issue can be understood
by looking at strong interactions. There we theory, QCD, particles corresponding to the do fields not to exist appear elementary (pace LaRue, Phillips & Fairbank (1981)). So arises the hypothesis of quark confinement not proved from in which free
a
have
-
so far according to which quarks are never isolated particles. Even has the so, theory quark masses, which can be measured (up to considerable uncertainties for the light quarks). But one cannot identify these masses
QCD,
-
with the directly measurable
masses
of free
quarks.
One must
only speak
parameters, measured, in this case, rather indirectly. There is no problem in taking this point of view. For example,
of
mass
the
(f>3 Lagrangian &
=
d(j)2l2
-
as a
basic
m2(/>2/2
-
Lagrangian plus
#3/3!
-
we
write
a counterterm
Lagrangian:
dm24>2/2
Sg(f)3/3!. (3.4.1)
6Zd(j)2l2
-
-
avoid identifying the renormalized mass with the mass mph of a particle. Similarly we do not identify the renormalized coupling, g, with any specific measured quantity, and we do not define Z by requiring that the But
we
residue of the
propagator's pole
be
unity.
Consider the calculation of the one-loop choose SZ
=
0 for this case; the
self-energy (3.1.7)
self-energy at d 4. We can only divergence is in dm2. The renormalized =
is
SiR
=
^ifin
+
(<5m2
+
Zldiv).
3.4 Renormalization group We must choose dm2 to have a
divergent part
51
to cancel the
divergence
at
0 of £ldiv. But the finite part of dm2 is not determined; the arbitrariness is the same as that of the integration constant when obtaining I1R by
a
=
integrating dyLl/dpfl. At first sight it might appear that the arbitrariness ruins the theory unless one pins down m to be the physical mass mph. This is in fact not so; the arbitrariness is
more like the arbitrariness in choosing a coordinate system. first Suppose computes the propagator with the mass-shell condition £ir(P2 0 0. Then one
=
=
G2
<
=
=
i/[p2
<
-
"
m2h
^idiv
-
-
Ilfin(p2) + Ilfin(p2 Slfin(mp2h) + 0(g*).
One could also compute with
G2
m20
=
=
i/[>2 m2
-
-
m2
Ildiv
a
-
-
=
m2h) + 0(a4)], (3.4.2a) (3.4.26)
different finite part to
5m2,
Ilfin(p2, m2) + g2C + 0(a4)], g2C + 0(a4),
where C is any chosen number. The
X¥(p\m2)
=
self-energy i:Uin
+
is
with
a
result
(3.4.3a) (3.4.36)
now
g2C.
Evidently the two ways of renormalizing the theory give the same results if we require that the bare mass ml is the same in both of (3.4.2) and (3.4.3). In the
ml
complete solution of the theory, say by the functional integral, it is only that matters, not the partition into a renormalized mass squared m2 and
a counterterm
—
Zldiv m2
with the
=
g2C. Clearly we have m2h + g2C Ilfin(mp2h) + 0(a4),
-
-
(3.4.4)
0(a4) terms depending on the renormalization of higher-order self-
energy graphs. We come then to the central idea of the renormalization group. The
physically irrelevant, for a change in the arbitrary constant C can be exactly compensated by a change in m2. A change in C merely gives a different parametrization of the set of theories arbitrariness in the definition of Z1R is
be obtained by varying the mass parameter m. The renormalization group is the set of transformations on the parametrizations of the
that
can
theory. The transformations are accomplished by moving parts of the terms in ¥£ from the basic Lagrangian to the counterterm Lagrangian. In the case of
m
it is
a move
from the free
Lagrangian
a
to the interaction
Lagrangian.
rearrangement of the perturbation series, which is the
gives key to the many practical applications It might be objected that
This of course
of the renormalization group.
p2-m2-Ilfin(p2,m2) + a2C
52
Basic examples
is not
equal
to
p2 since the
m2h
-
-
Ilfin(p2,m2h) + Zlfin(mp2h,mp2h),
parameters in
mass
Zlfin(p2)
parametrized in either way is the
are
same.
different, whereas the theory
But since
m2h
—
m2 is
0(g2)
the
difference in the two expressions is in fact 0(#4). Thus the rearrangement of the perturbation series does not leave the p2-dependence of the coefficients invariant. The
0(g*)
terms will cancel the difference
(up
to even
higher-
order terms), etc.
utility of the renormalization group is precisely in its ability to reorganize the perturbation series. Since one effect of the interaction is to The
induce dynamical contributions to the
good idea
mass
and couplings, it is evidently
a
to arrange that these contributions are small. The result is to
higher-order corrections and thus improve the perturbative calculation. Now the effective size of the dynamical mass or coupling must be treated as dependent on the situation under consideration. This can be seen by examining Z1R given in (3.1.12) at large p2: reduce
the
reliability of
values
of
a
L1R
 ̄
(02/327t2)[ln(
-
p2/m2h)
+ constant
+???].
(3A5)
If | p21 is large enough this can be large. Since the graph occurs as a subgraph of higher-order graphs, it is likely (and often is true) that higher-order graphs are as important as low-order graphs at large enough p2. This situation is undesirable and can be remedied by a renormalization-group transformation. We absorb the large part of Z1R into a redefinition of the renormalized mass
m2. We must examine higher-order graphs
that there are no further sources of
systematically
in
Chapter
large p2 to demonstrate large coefficients. We will do this at
7.
3.4.2 Renormalization prescriptions
resolving the ambiguity in constructing given theory, each of these ways corresponding to a particular parametrization. It is essential that, whenever a particular divergent graph occurs as a subgraph of a bigger graph, the ambiguity is There are
infinitely
many ways of
the counterterms for
resolved in the
a
same way at
each occurrence, since the corresponding
counterterm vertex is generated by
perform
concrete
calculations
a
one
ambiguity., Such a rule is called renormalization scheme.
single term in the Lagrangian. So to adopts some rule to resolve the a
renormalization prescription
or
3.5 Dimensional
53
regularization
infinitely many possible renormalization prescriptions, a few have become standard, because they are especially convenient either for practical use or for theoretical considerations. In this section we will explain two of the standard ones with the aid of the example of the one-loop graph Fig. 3.1.1. We have already encountered the mass-shell, or physical, scheme. The Of the
is defined to be the physical mass, i.e., the position of the Wave-function renormalization is fixed by requiring the propagator pole. residue of the pole to be unity (see (3.3.9)). Couplings can be defined by renormalized
specifying
mass
the value of
a
suitable S-matrix element.
A possibility that is much used in discussions of renormalization theory is the BPH or BPHZ scheme (Bogoliubov-Parasiuk-Hepp-Zimmermann), as zero-momentum subtraction. Let Y be a one-particle- irreducible (1P1) graph that is divergent, i.e., it has (5(r)>0. The prescription is that at zero external momentum its renormalized value R(T) and its first S(T) derivatives with respect to external momentum are zero.
otherwise known
this by subtracting off the first S(T) integrand (Zimmermann (1970)), that is, the renormalization is performed before the integration over loop momenta. No explicit UV cut-off is needed. In this scheme the self-energy already discussed is, at d 4, The BPHZ scheme is to
implement
terms in the Taylor expansion of the
=
£(1RPHZ) while
=
at d
=
y(BPHZ)
Jd4/C{[(fc2
32^4 6,
we
^2
"
_
_
m2}]
{k2
^2
_
j>
(3A6)
have:
fJ6,f
*M\(k2
.
m2){{p + k)2
128tu6 J
+
1
1 -
m2)[(p + k)2
-
m2]
(k2
-
m2 + i£)2
[Vfc2-(/c2-mV]|
Irk
(/c2-m2)3
3.5 Dimensional
(fc2-m2)4
J*
[X)
regularization
we used the lattice as a cut-off, or we in is the renormalized what are interested really regulator. However, theory with no cut-off. We could equally well use some other kind of
In
our
initial treatment of UV divergences
regulator. For example,
higher derivative S£
=
a
Pauli-Villars type of cut-off is achieved by
term in the
dA2/2
Lagrangian.
m2^2/2
-
-
qA 3/3!
-
[(Q
+
For
a
example, from
m2)^]2/2(M2
+ counterterms
-
m2) (3.5.1)
54
Basic
examples
obtain the free propagator (2.4.1). When the cut-off M goes to infinity, the propagator is the ordinary one, but when p2 is much bigger than M2, it
we
is smaller by a factor p2/M2. in six
or
Thus UV divergences
fewer space-time dimensions; the
take the limit M
-?
are
cut-off for the theory
divergences
reappear when we
oc.
If we defined the theory by a functional integral the lattice would appear intermediate step, but the a -? 0 limit would give no divergences, if M is
as an
Although the Euclidean Green's functions for the cut-off theory (3.5.1) exist, the Minkowski space field theory is not physical. A symptom of this is that the pole of the free propagator at p2 M2 has the wrong sign of residue; it implies a particle with negative metric. A theory with no cut-off can be obtained by adding counterterms with
finite.
=
appropriate M-dependences to cancel the divergences and then taking the M limit. As an example we showed that counterterms cancelled the divergences of the one-loop self-energy graph. Although we assumed a lattice regulator, we used no properties of the lattice propagator that are not true for the Pauli-Villars case. We assumed only that: (1) If the cut-off is taken away (i.e., M 0) with p and m fixed, then the propagator goes to i/(p2 m2). -?
oo
-?
oo or a -?
—
(2)
If
p2
-> oo
with fixed cut-off, then the propagator is sufficiently much 1/p2 that the graph is not UV divergent.
smaller than
(3)
In the Euclidean
region there
are no
propagator poles.
In principle, any method of imposing a UV cut-off is good enough, but in
practice
methods
some
are
more
convenient than others. For most
purposes dimensional regularization is the most convenient. The method starts from the observation that U V divergences are eliminated by going to a small enough space-time dimension d. We can use the space-time
dimension
as a
that the cut-off
regulator provided can
we treat
d
as a
continuous variable (so
be removed by taking the limit d->4). This idea has
a
long history, but its popularity roughly started after the papers by Wilson (1973) in statistical mechanics and by't Hooft & Veltman (1972a), Bollini & Giambiagi (1972), Ashmore (1972), and Cicuta & Montaldi (1972) in field theory (especially non-abelian gauge theories). Since vector spaces of non-integer dimension do not exist as such, it is not obvious that the
concept has any consistency, let alone validity,
even
in
a
purely formal sense. This we will remedy in the next chapter. For the present we
will
uniqueness integral
assume
manipulations to the
and existence, and
apply
standard
3.5 Dimensional
55
regularization
until it is of a form where we can sensibly assign a value. We will (following Wilson (1973)) express (3.5.2) in terms of a standard Gaussian integral:
|ddfcexp(/c2)
=
i \dco
give this the value ind/2, which is correct if d is
It is sensible to
(3.5.3)
\dd-lkexp(-w2-k2). an
integer.
In
following calculation of the value of (3.5.2) with non-integer d, the assumed properties of the integration are italicized. All the manipulations are valid for any integer value of d for which the integral converges. the
We use the
Schwinger representation for each propagator:
rdaexp[-a(m2-fc2-ie)].
l/(m2-k2-ie)=
Observe that because of the Wick rotation
exchange
we treat
k2
as
(3.5.4)
negative. Then
we
the order of integration to obtain
2(27r)" Jo
da
db
Jo
\d"kexp[-(a + b)m2 + bp2 + 2bpk + (a + b)k2l
J
(3.5.5) We shift k" by x
=
a/z
an amount
p"b/(a + b)
change variables
and
to z
=
a +
b,
to get:
Zi=^M dxrdzz[ddfcexp{-z[m2-p2x(l-x)]+z/c2}. z(Zn> Jo Jo J After scaling k by
a
factor
z
2
we
find that
E1=J|—I dx|=°dzz1-d/2exp{-z[m2-p2x(l-x)]}
d"kexp(k2). (3.5.6)
brings in the dimensionality d. We have now reduced the ^-dimensional integral to the form (3.5.3), which we defined to be i7rd/2.
It is this stage which
The z-integral in (3.5.6) gives
a
T-function,
so we
finally obtain:
^i=^F2T(2-d/2)^dx[m2-p2x(l-x)r12-2.
This result is unique (except possibly for
an
(3.5.7)
overall normalization, which is
for every ^-dimensional integral). The divergences The now reside in the T-function which has simple poles at d 4, 6, 8, residue of each pole is a polynomial in p of degree equal to the degree of universal
-
the
same
=
—
divergence. One of the main advantages of dimensional regularization is immediately apparent. Not only was the integral unchanged from its form in a theory in
56
Basic examples
integer dimensional space-time with no cut-off, but the method of calculation was unchanged. Use of the representation (3.5.4) is an efficient way of obtaining a parametric representation like (3.5.7) for a Feynman graph. A second advantage that we will see later is that it preserves not only Poincare invariance in the regulated theory, but also gauge symmetries. This was a main motivation for its use by 't Hooft & Veltman (1972a) and many others. Most methods of introducing a cut-off fail in this respect. (For example, gauge invariance is preserved invariance is lost.)
on
the lattice but full Poincare
importance in practice is that continuous space-time dimension is also a gauge-invariant cut-off for A third
advantage
-
also of great
-
a
infrared divergences in theories with massless fields (Gastmans & Meuldermans (1973), Gastmans, Verwaest & Meuldermans (1976), and Marciano & Sirlin (1975)). A trivial example, but without any gauge invariance, is by the (/>3 self-energy with m 0 at d 2. It is =
8tt2J which is divergent at k
increasing
d. Care is
present in the
same
=
(fc2
given
=
i£)[(p + k)2 + i£]'
+
0 and at p
=
—k. The divergence is regulated
required in using this method if UV divergences graph, for they are regulated by reducing d.
by are
3.6 Minimal subtraction 3.6.1
Definition
self-energy (3.5.7) we compute the renormalized 4 by adding a mass counterterm Sm2(g,m2,d) and d Suppose we choose m to be the physical mass. Then
From the unrenormalized
self-energy Z1R
at
then letting d->4.
=
=^r(2-j/2)|^dx{K-rti-x)f-2
-[mp2h(l-x + x2)r2"2}. (3.6.1)
Now
T(z)
has
a
pole
at z
=
0:
r(z)
where
yE
0.5772
=
'
...
^h'
in agreement with
=
l/z-yF
+
is Euler's constant. So at d =
our
(3.6.2)
0(z), =
4
#,|"dxlnH-p2x(1-2Xn earlier calculation.
(3.6.3)
57
3.6 Minimal subtraction
divergence in Z, amounts to a simple pole at d 4, a rather obvious way of renormalizing it is to define dm2 to cancel just the singularity, i.e., the pole ('t Hooft (1973)). This, of course, means that we are Since the
changing see
=
renormalization
our
prescription. By referring back
to (3.5.7),
we
that dm2 in this scheme is: Sm2
from which
we
=
(g2/32n2)/(2
get I1R by expanding Zj in '
f
ilR ̄327t2
-
a
d/2),
(3.6.4)
power series in d
\ ̄m2-p2x(l-x) ̄\
-
4. We find
)
Unfortunately
this contains the logarithm of a dimensional quantity. The is that in the expansion in powers of d 4 we did not allow for the fact that g has a dimension dependent on d. Therefore we implicitly introduced a mass scale. reason
—
To make this scale
explicit,
we
rewrite the coupling
(3.6.6)
g^tx2-d/2g,
we have introduced a parameter fi with the dimensions of mass, called the unit of mass ft Hooft (1973)). The redefined coupling g now has
where
fixed dimension
equal
to
0
y(MS)_ ^1R
1, and the renormalized self-energy becomes
f
dxiln
"32tt2J0 jln
We derived this
by observing
 ̄m2 -p2x(l -x)
47i/r
+ yE
(3.6.7)
?
that
?2-d/2 =e(2-d/2)lnM =
1 + (2
-
d/2)\n fi +i(2
-
d/2)2 In V +
''' ?
This renormalization prescription, where counterterms are pure poles at physical value of d, is called minimal subtraction (MS). The unit of mass fi is entirely arbitrary. Thus the self-energy (3.6.7) now
the
depends
on
amounts to a
three parameters instead of two. However a change of fi change of renormalization prescription, so the change can be
compensated, in this case, by a change in m. a
In effect minimal subtraction is
one-parameter family of renormalization prescriptions. 3.6.2 d
We can also
There
we
=
6
minimal subtraction to the six-dimensional
apply
theory.
define Sm2
=
Sz
=
¥0
=
poles
at d
=
6,
poles
at d
=
6,
jx3-d'2(polesati
^
I
=
6)J
(3.6.8)
58
Basic examples
Note that the renormalized
coupling
is
dimensionless. Since
now
r(z-l)=-l/z + yE-l as z ->
0,
we
+
0(z)
(3.6.9)
find that i2
_
y(MS). 128tt
(m2-p/6)(yE-l)
dx[m2-p2x(l-x)]ln
+
m2
—
p2x(l
—
x)
(3.6.10)
471/i
and
w-s?^+0^- 1
a2 6
x
(3.6.11) (3.6.12)
647T d-6
The counterterm Sg for the coupling can also be calculated. From the graph of
Fig. 3.6.1,
we
find (Macfarlane & Woo (1974)) 7
Sg
Fig.
_
=
we
3
64n3(d-6)
+
oto5).
3.6.1. One-loop vertex graph in
3.6.3 Renormalization When
9
,.3-d/2_ n
(3.6.13)
</>3 theory.
group and minimal subtraction
discuss the renormalization group in Chapter 7,
we
will focus
on
particular subgroup. The transformations in this subgroup consist of multiplying /i by a factor and making compensating changes in the renormalized coupling and mass. As a group it is trivial being a representation of the group of positive real numbers under multiplication. What is non-trivial is the way in which it is represented in relation to the parametrization of the theory by a renormalized coupling and mass. The renormalization group can be exploited in calculating high-energy one
-
full treatment will be made in Chapter 7, the basic idea can be seen by examining the one-loop self-energy. Let p2 get large (with m and \i fixed), and consider the propagator defined using minimal 6: subtraction at d behavior. While
a
=
p2
i-
7687t3
In
+ const.
+
O(04)
(3.6.14)
3.7 Coordinate space To be able to make use of
a
perturbation expansion
order corrections small. But this is not
so
59
keep higher- large. The The theory \p2\.
we must
in (3.6.14) if
p2
is too
large correction can be avoided by setting fi2 to be of order is unchanged if we make suitable changes in g, in m, and in
the scale of the
field. We will learn how to do this in Chapter 7, with the result that the
corrections
are
effectively
moved from
perturbation series to the lowest-order
higher-order graphs.
large
terms in the
3.6.4 Massless theories
(for simplicity) to the self-energy of the four-dimensional the limit m -> 0. If we use mass-shell subtraction, we have Consider theory. Let
us return
(3.6.3), which diverges as m2h-+0. The divergence is an artifact of the mass-shell scheme, for which ,2
(*i
Sm*=2(!^r(2" d/2)mdp^Ldx(1"x+x2)dl2 =
^r^^-ln(mp\)
In addition to the
ln(m*h)
pole needed
+
72-
finite(asmph-0,^4)l.
to cancel the UV
(3.6.15)
divergence, there is
a
term.
Physically what happens is that in a massless theory there are long-range forces. These mean that separated particles are never completely free of each others' influence. Thus, for example, the singularity in the propagator is not a simple pole, for the self-energy (with MS subtraction) is, from (3.6.5),
s-Ks?)**-2}
(3616>
The mass-shell renormalization prescription relies on the assumption of a simple propagator pole to generate counterterms, so it must fail. However, the nature of the propagator's singularity is an infra-red problem, so it is irrelevant to the question of whether
an
ultra-violet divergence
can
be
renormalized. Some other renormalization scheme, like minimal subtraction, must be used in the massless theory.
3.7 Coordinate space A good way to understand the infinite renormalizations is to work in coordinate space, as was emphasized by Bogoliubov & Shirkov (1980). This
point of view is especially useful in treating field theories
at
finite
60
Basic examples
temperature or on a curved space-time background, as we will see in Chapter 11. There we will see why the counterterms are the same as at zero
space-time. ordinary calculations, it is cumbersome to work in coordinate space, because the propagator SF(x) for a free massive field is a Bessel function. In momentum space the propagator is simple: i/(p2 m2 +ie). However the asymptotic behavior of SF(x) as x-?0 is simple. temperature in flat
For most
—
We have
Sf
=
4^/2(_x2)d/2-i
The one-loop correction
G2,z
=
=
where E^z
—
 ̄
(92/2)
\ddz
+less singular
to the propagator
Udz \d"wST(x
\ddwSF(x z)!^ -
-
W)2SF(w
-
w)SF{w
0.
(3.7.1)
is
G2{x,y)
z)SF(z
-
as x -+
-
-
y) (3.7.2)
y),
w) is the self-energy in coordinate space:
±1(z-w)=-(g2/2)SF(z-w)2.
(3.7.3)
This is singular at z w, and causes a logarithmic divergence in (3.7.2), where we integrate over all z and w. The fact that the divergence is from the =
region
z ̄w means
that it is in fact
a
<5-function:
G2a=\ddzSF(x-z)SF(z-y)
By Wick rotating the w°-integral and using the following result (next chapter) for the ^-dimensional integral of a Lorentz-invariant function
\ddwf(w2) we
-
i[2nd/2/r(d/2)]
pdvyw^-Y(w2),
(3.7.5)
find G22
as
=
d
-?
=
tddzSF(x
-
z)SF(z
-
y)l6Jg{4_d)
+
finite,
(3.7.6)
4.
Evidently, the divergence is cancelled by adding
a
(5-function to
t.l:
61
3.7 Coordinate space which is
exactly
the
mass counterterm
computed earlier by
momentum-
space methods. The
important point, which
is in fact true for
arbitrary graph,
an
is that
any UV divergence comes from a region in coordinate space where several interactions occur very close to each other. The divergence can then be
cancelled by
a counterterm
interactions. If the
which is
divergence
is
a
worse
(5-function in the positions of these logarithmic, then the counter-
than
term will include derivatives of the (5-function. In any event the fact that it is a (5-function means that the counterterm can be included as a local
locality means that it is a product of fields at point. Since the singularity at x 0 of the free propagator SF(x) is independent of the boundary conditions used to define it, we should expect, for example,
interaction in the action. The
the
same
=
that the counterterms used in thermal field theory
temperature. At state, and the
temperature, the
are
the
same as at zero
replaced by a mixed The conditions for momentum-space boundary SF(x) change. non-zero
vacuum
is
proof that the counterterms are temperature-independent is therefore made coordinate-space proof is unchanged.
difficult, but the
4 Dimensional
We have
seen
how convenient it is to
perturbation theory by date,
no-one
regularization regulate the
UV
divergences
of
continuation in the dimension of space-time. To use the method in the complete theory. But
has shown how to
in perturbation theory, as we will now demonstrate, it is consistent and well-defined. Now all results obtained by this method can be obtained by
other, more physical methods (say, a lattice regulator). But frequently much labor is involved. This is not a triviality, for in complicated situations, especially in gauge theories, it enables us to handle the technicalities of renormalization in a simple way. The idea of dimensional continuation has been used for a long time in statistical mechanics (see, for example, Fisher & Gaunt (1964)). It became more
very prominent when Wilson & Fisher (1972) discovered the
a-expansion and applied it to field-theoretic methods in statistical mechanics (Wilson (1973), Mack (1972), and Wilson & Kogut (1974)). In the a-expansion one e spatial dimensions, and expands in powers of e. At the same works in 4 a in time, purely field-theoretic context, a need arose to find a way of non-abelian regulating gauge theories that preserved gauge invariance and —
Poincare invariance. This led to dimensional
regularization ('t
Hooft &
Veltman (1972a), Bollini & Giambiagi (1972), Cicuta & Montaldi (1972), and Ashmore (1972)). Speer & Westwater (1971) had actually discovered the method earlier, but their paper is considerably more abstract, and had not attracted much attention.
integer non-integer integration dimension, d, cannot be taken completely literally. Either it is a set of purely formal rules for obtaining answers or it is an operation that is not literally integration in d dimensions, but only behaves in many respects as if it were Now vector spaces either have infinite dimension or a finite
dimension.
So the concept of
integration
in d dimensions. It is not sufficient to treat it
on a
space of finite
only
as a set
of
formal rules (even though that is what it becomes in practice), because one must know that the rules are consistent with one another and with the carries out
integrals. To show that explicit definition.
algebraic manipulations
one
inconsistencies
we must construct an
can
arise,
62
on
no
Dimensional regularization There
63
three issues to address: (1) uniqueness, (2) existence, and (3) the possibility of constructing
are
properties. Uniqueness is necessary, to avoid
two definitions, each definition being self-consistent but giving different results from the other definition. Existence, shown by construction of an explicit definition, is necessary to prove that no inconsistencies arise. Once having
seen
cannot
just
that integration in non-integer dimension can be defined, we that all properties associated with ordinary integration
assume
true; indeed they need not be. So we also have to prove those properties which
are
true. We
is
also must prove that the results agree with
we
need and which
are
ordinary integration if d
an integer. These considerations are quite non-trivial, as can be seen by considering,
for example, the
anomaly in the Ward identity for the axial current t'ie 8au8e model (2.11.7). If the fermion masses are zero, naive application of the fermion equations of motion shows that the
j(5)= ipy^ys1!' then
a
m
current is conserved: d
jf5)
=
0. In fact, the current is not conserved,
as
by Adler (1969,1970) and Bell & Jackiw (1969). A counter term can be added to jf5) to make it conserved, but only at the expense of removing its gauge invariance. shown
Among the objects to be extended to d dimensions are the Dirac matrices (y" and y5). If we assumed the obvious generalization of their anticom- mutation relations, then for all values of d
{yV/v}
=
But then
we
=
would have
2<TU
{y5,y"}=0,
y52
we
i.
}
J
(4.0.1)
would be able to derive the false result that the anomaly for the
gauge-invariant axial current is zero. So there has to be an inconsistency ('t Hooft & Veltman (1972a)). More complicated problems in a similar vein arise when treating supersymmetric theories (Jones & LeVeille (1982)). In this chapter we will start by stating the axioms for d-dimensional integration given by Wilson (1973). These are sufficient to prove uniqueness. Our calculation of a one-loop graph in Section 3.5 was in fact a
uniqueness proof for one particular integral. Then we will explicit definition of ^-dimensional integration. The vector
realization of the construct
space
on
an
which
we
work is in fact infinite dimensional.
Unfortunately, the definition gives a divergent result in most cases, so we a powerful enough extension (Section 4.2). We then
will next have to find
some standard properties (Section 4.3). One particular result involves finding a definition of the metric tensor on an infinite-dimensional
prove
Dimensional regularization
64
space such that its trace is d rather than infinity. Then we will be in a position to derive some useful formulae
(Sections 4.4 graph calculations. Finally we will show how to define Dirac matrices; this is obviously important if we are to be able to calculate consistently graphs containing the Adler-Bell-Jackiw anomaly. The utility of a precise definition such as we give is that if inconsistencies and
4.5) for use in
arise at
some
Feynman
stage, then
always
one can
go back to first
principles
to
discover the error.
4.1 Let d be a
complex number. We wish to define an operation integration over a ^-dimensional space:
that
J-
regard
as
Definition and axioms
/'(p) is
any
given function of a
may
(4.1.1)
d'p/(p).
Here
we
vector p,
which is in the d-dimensional
space. We will suppose that the space is Euclidean. (Minkowski space is l)-dimensional regarded as a one-dimensional time together with a (d —
Euclidean space.) Following Wilson (1973) we definition in which the space is actually infinite
will
give
an
explicit
dimensional; it is the
integration operation that gives the dimensionality. Making d a positive integer n will effectively insert a (5-function in the integration that will force all vectors involved in defining the function /(p) to lie in some n- dimensional subspace. What properties must we impose on a functional of/ in order to regard it as d-dimensional integration? The following properties or axioms (due to Wilson (1973))
are
natural and
necessary in
are
applications
to
Feynman
graphs: (1) Linearity: For
any
complex
Jddp[a/(P) (2) Scaling:
+
numbers
MP)]
For any number
a
and b
jd V(P) jddpflr(p). + b
(4.1.2)
s
fd-p/(sp) (3)
=
a
=
s"-|<lV(P)-
(4.1.3)
Translation in variance: For any vector q
(ddp/(P We will also
require
+
q)
=
fddp/(P).
rotational covariance of
our
results.
(4-1-4)
65
4.1 Definition and axioms
Linearity is invariance
are
true of any
integration,
basic properties of
while translation and rotation
Euclidean space, and the scaling
a
property embodies the ^-dimensionality. Not only are the above three axioms necessary, but they also ensure that integration is unique, aside from an overall normalization (Wilson (1973)). In fact, they determine the usual integration measure in an integer- dimensional space (again up to normalization). The proof is simple: Use linearity to expand/(p) in terms of a set of basis functions. Choose a basis such as the functions
/s,q(P)
(4-1-5)
exp[-52(p + q)2].
=
Then the integral of a basis function can be written in terms of the integral of one
single function:
The integral of this
[ddP/M one
(P)
=
s ̄d
(4-1.6)
function sets the normalization. It is natural to
require that the value be the usual write
one
I<
can
|dVxp(-p2).
ddlpd',2qexp(-p2-q2)
in integer dimensions and that
=
we
(4.1.7)
d^'^kexpt-k2).
J'
Thus the normalization is given by
|ddpexp(-p2) An abstract uniqueness theorem is
=
7r''/2.
(4.1.8)
not sufficient for us. We also need an
explicit formula so that a ^-dimensional integral can be written as a sequence of ordinary integrals. This will be important in allowing us to prove standard properties of the integration. In addition it ensures that there exists
a
self-consistent definition. It is
consistent definition exists; the
a
priori possible that
uniqueness theorem only applies
no
if the
integration operation exists. A function /(p) that we integrate could in principle be any function of the components of its vector argument. However,
meaning of the components of, say, soon see
that there
will work with
are
in fact
a vector
infinitely
do not, a priori, know the in 3.99 dimensions. We will
we
many components. In
rotationally covariant functions.
So
we
will
practice, we that/is
assume
function of a finite set of vectors: p, q2,..., q^ say. For example, scalar function is a function only of scalar products a tensor
/
=
/(f>2,p-qi,4,2,.--)-
a
(4.1.9)
66
Dimensional regularization an ordinary function of scalar numbers, rather than some complicated kind of function. Of course the values of the scalar
Thus / is in fact more
products lie in restricted ranges. Thus:
q2>0,
k-q*|<qX2. A
tensor function is obtained
(4.1.10)
by writing explicit 8ij, with scalar coefficients.
tensors in terms of the
vectors p, qi,..., q^ and of the metric tensor
example,
we
might
/I'iP,q) Such functions
are
=
^y/fl(P^p?q,q2)
vectors as we
in
a
an
+
the most general that
later how to handle the
matrices.) To give
antisymmetric
ordinary
(4.1.11)
^y6(p2,p?q,q2).
we
need to consider. (We will
tensor
realization of the objects p,q1?...,
will show in
For
have
eKA/iV
and
we assume
that they
vector space. The space must be infinite
So
a moment.
are
dimensional,
define the vectors each to be
we
see
the Dirac y-
infinite
an
sequence of components, p (p\p2,...)> just as we can define dimensional vector V as a sequence of three components (V1, V2, metric is given by: =
a
three-
V3).
The
an integral with, say, regulator for a physical theory in a space- time of dimension d0 4, but also as a regulator for a model theory in any 5 or 6 or The vectors qt,q2,... in (4.1.9) can higher dimension, e.g., d0 be thought of as momenta of external particles, and our vector space must be large enough to accommodate d0 linearly independent momenta. Since d0 is arbitrary, we are forced to infinite dimension.
The d
=
reason
3.99
can
for the infinite dimensionality is that
be used not
only as
a
=
=
To define the d-dimensional integral of a scalar function, we find a finite- dimensional subspace containing all the q^'s. Then we write p as a
component Pn in this space and P
=
=
orthogonal component
an
pT:
P|| +Pt
i>je,.
(4.1.12)
+ pT.
The 'parallel space', in which lie the q^s, is spanned by an orthonormal basis
1,..., J). We define the integral over p to be the ordinary J- e,. (with j J dimensional integral over Pu performed after integration in d =
—
dimensions
over pT:
Jddp/(P) [dp1 =
?
?
dpJ
(dd-W(p).
(4-1.13)
4.1 Since
f(p) does
not
depend
Definition and
the direction of pT
on
d*-J?Tf(p)
we now can
define
(4.1.14)
dPjPdT-J-lf(V\
Kd
=
67
axioms
0
Kv (with v d J) is effectively the area of the surface of a hypersphere in v dimensions. The value of Kv is obtained by considering the special case where/is chosen to be a Gaussian see (4.1.8) with the result Here
=
-
-
-
Hence
we
have
a
definition of d-dimensional integration in terms of
ordinary integration:
l"dpm-m^ikh"J>* ̄J ̄,/,p)
(4U6)
We must check that the result is independent of the choice of the subspace of
the P||. We must extend the definition to handle the divergences at pT= 0 when d is small, which we will do in Section 4.2. Then in Section 4.3 we will prove
important properties of our definition. But first there
are a
couple
of
details to clear up. The J-dimensional subspace of p,|'s is chosen subject only to the requirement that it include all q .'s. So it is possible to extend the space to include extra dimensions. To show this has no effect on the value of the
integral
we must prove roo
KA JO for any
r
dpp-lg{p2)=\J
function,
true since the
g, which
right-hand
27r(v-D/2
where
AkKy_A dpjpr'gipf + k2) -
depends
=
(4.1.17)
JO
X
on a
scalar argument. This
equation
is
side is
iJ>'"-w,!o dxx(v-3)/2(l-x)-1/2,
r((v-i)/2), p* xp2 and k2 =
r qo
oo
(1
—
(4.1.18)
x)p2.
'parallel' subspace have no effect on both spaces to a common larger extend integral, merely is that The there sole problem may be a divergence in (4.1.18) at space. x 0; this we will cover by Section 4.2. To show that different choices of the
the value of the
we
=
Up till
fij"(v\
now we we
a
scalar function. If it is
component-by-component.
J-
component
have supposed /(p) is
work
d"p/12(p),
To
define,
a tensor
say,
the
Dimensional regularization
68
take the parallel space to include the 1- and 2-directions and any vectors which/17 depends. Then we proceed as before. q^ For example, suppose/°(p) pip*g{¥2\ where g is a scalar function. Then we
on
=
j/12(p)d<p JdpMp2 [d'-Wp2^1)2 =
while
dV n(p2)
=
|
+
(p2)2 + p2]
=
0,
d'-Wi^Pjj+P2)
dp oo
cir(d/2)
^yppd+it9(P2)
-h
"W29(P2l
Generalizing this result,
we see
that
JdVp^(p2) ^[dV2^(p2). =
More
general
cases are
treated in Section 4.3.
4.2 Continuation to small d The convergence of the definition (4.1.16) is ^-dependent at pT 0 and oo. It improves at pT oo when d at 0 it but pT smaller, improves pT gets when d gets bigger. Even for a function that decreases exponentially at large =
=
=
=
p, and that is analytic for finite p, the defining integral has a divergence if the transverse space has a dimension d J < 0; this is forced to happen if d is —
negative or zero. So our first task in this section is to find an explicit formula for the continuation of (4.1.16) to arbitrarily negative d. We will see that the pT-integral has poles whenever (d J)/2 is zero or a negative integer, but that these are cancelled by the zeros in 1/T((d J)/2). We will then be able to adopt the resulting formula as a definition of the —
-
^-dimensional integral of a function for which (4.1.16) converges for value of d. An example of such a function is /(P) The
q2,
=
(qi+P)2 + (q2
+
P)2
+ ?2'
and
parallel space must be at least two-dimensional to accommodate q l
so we may set J
only if d > if d < 2.
J
=
=
no
2. Then the transverse
integral
converges at pT
2, while the complete integral converges
at p
=
oo
=
0
only
69
4.2 Continuation to small d
ddp/(p)
We will have a definition that defines
for all small
enough
d. For larger values of d
we define the integral by analytic continuation. In will there be ultra-violet general poles at certain values of d just as in the we in Sections 3.5 and 3.6. Feynman graph computed -
explicitly define the continuation to small d, it is sufficient to consider function/(p2). Let us assume that/->0 rapidly enough as p-> oo that To
a
dV(P2) converges at p
-?
oo
for
some
2nd
(4.2.1)
dpp'-W
=
T(d/2).
positive value of d.
We also assume that/(p2) is
0. Then (4.2.1) converges and is analytic in d for some range analytic at p 0 < Re d < dmax. We define the integral for all other values of d by analytic =
continuation in d. Explicit formulae for the continuation to smaller d's are constructed by adding and subtracting the leading behavior at p 0. For -?
example, the following formula gives the integral in the range <dm
2 < Red
?
2^ ddp/(p2)
27Td/2
(
dPpd-\f(p2)
'fW){,
=
dpp--1[/(p2)-/(0)]+/(0)C-/d^
+
This is independent of the arbitrary When
—
2 < Re d < 0
we
d'p/(P2) at d
=
2nil2 =
0 the zero in
rw2)
-?
oc
to obtain
(4.2.3)
dp/-1[/(P2)-/(0)],
l/r(d/2) is cancelled by the pole
term to
fd°p/(p2)=/(0). We extend this procedure
positive integer
(4.2.2)
constant C.
may let C
!-
while
—
to continue to
—
give (4.2.4)
2/
—
2 < Re d <
—
2/ for any
/:
ddp/(P2)
2nd'2
f00
=
r(d/2).
dppd-l{f(p2)-m-p2m- ̄ ...-(P2)r\o)/i\},
f|d-2ip/(p2)
=
This equation gives
(4.2.5)
(-7r)-'/<'?(0).
us
the
integral
when
—
2/
—
2 < Re d < —21
on
the
70
Dimensional regularization
assumption that the original formula (4.2.1) converges when d is just greater than zero. Suppose now that (4.2.1) diverges at p oo for all positive values of d, but that/is power behaved as p -? oo. Then it is sensible to adopt (4.2.5) as the definition of the integral. This particular definition is very important =
since that
we
are
will
dimensional continuation to regulate Feynman graphs
use
ultra-violet divergent at d
Feynman graph frequently has
=
a
4. The definition
(4.1.16) applied
number d
negative
—
to a
4 of transverse
complete pT-integral with d
dimensions in order to ensure ultra-violet convergence of the
integral. Then replaced by d
we —
may
apply
the definition (4.2.5) to the
J.
Another obstacle to continuation in d is sometimes that
/(p2)
is not
0 but has a power-law singularity there. We may generalize analytic at p2 of the derivation (4.2.5) to write down a formula for the continuation of the =
integral. An example of the
use
of (4.2.5)
f(p2) where A /
=
The
(p2
+
definition is
A)/(p2
and B are numbers. The definition
1, (4.2.5) gives
integral
can
us a
be
jddp(p2 which
=
as a
can
+
A)/(p2
+
B)
=
B\
+
(4.2.1) diverges for all d, but with
definition valid for
explicitly computed
given by choosing
-
to
2 < Re d < 0:
give:
(7iBf2(A/B
-
1)T(1
-
d/2),
be continued to all d.
Suppose /has
a
power-law singularity, J
(P
+
q)2(p2
+
as
for
example
m2)*
The definition (4.1.16) of the integral of this function converges if 2 < d < 4. To continue it to lower d we must subtract the power behavior at p q, we can Then in 0 or at for at did as we (4.2.2). p singularities p1 0, just =
=
-
=
define the integral of say
(p
+
p2 q)2(p2 + m2)'
One result of all these definitions is that the
integral
of
a
power of p is
71
4.2 Continuation to small d zero:
(4.2.6)
d?p(P2r=o
for any value of a (integer or not). It should not be thought that there is any (4.2.6). It follows from (a) the explicit continuation of (4.1.16) to
choice in small
d, and (b) application of the continued formula
lddP(P2r.
Consistency of the formalism
=
(P2)7(P2
+
?2)-
Then when
ddp/(p2) If
linearity is
also
-
=
2a
requires (4.2.6). 2 <d <
-
-
2a
as a
definition of
For suppose we
that/(p)
have
d'p[/(p2)-(p2)7m2].
(4.2.6). singularity differently, by a function that is not But then, for example, the simplification obtained in (4.2.2) would no longer occur.
to be true then we have
We could subtract out the
just
a
power of p.
by taking
C
-?
oo
Observe that if in the first of
positive-definite function,
our
definitions (4.1.16), we take / to be a positive. But when the integral
then the integral is
is continued away from the
region where this definition converges, then the subtraction terms mean that the integrand is no longer positive definite, so that the integral need not be positive. At the end of Section 4.1, we proved that the value of a ^-dimensional integral does not depend on how we split the integral into an ordinary integral over some integer-dimensional 'parallel space' and a spherically symmetric integral over the remaining dimensions. We let J be the dimension of the parallel space. Then the proof consists of examining what happens when J is increased by one. Ultimately we had to prove (4.1.17), which is were no
not
a
property of ordinary integrals. We assumed d
subtraction terms. To generalize the result to the
positive,
we must
>
J,
case
so
that there
that d
—
J is
prove that [J/2-d/2]
K.d-j
A-J- 1
dpp1
-K'-'-'\
f(p2)- d/c
Z dpxPi
f 0
/w(0)p2-/n!
f(k2+P?)
[(J+l-d)/2] -
z
/■(?), (k2)p2n/n\
(4.2.7)
n = 0
symbol [d\ denotes the largest integer smaller than a. To prove the equation we change variables on the right-hand side to x and p, where Here the
72
p2-
Dimensional =
xp2
and k2
(1
=
-
x)p2.
regularization
For the
right-hand side
get
dPP>J-J-\
dxx(J-J-3)2(1_x)-l/2
K-d-j-i
we
[{J+l ̄d)!2)
f(p2)- =
^-,-1
dxX?-J-^2(l-X)-^2\
0
dP2(Pn2\di2-J/2-
1
JO
n
[j/2-d/2]
f(p2) ̄
fn\(l-x)p2)xnp2nln\
Z
/(n)(0)p27?!
Z
n = 0
J
[(J-c/)/2]
nn2n
[(J+l-d)/2]
I
n = 0
J
=
/i _
\j-n
2j
n
"
(4.2.8) Here we have added and subtracted l(J-d)/2] n = 0
so
that the
p2 of pd ̄J ̄2 times each scaling p2 by (1 x), we get:
integral
convergent. After
2iVd-J-
over
dxx(d-J-3)/2(1 _x)-l/2
1
square bracket term is
—
0
dp2(p2)d/2-J/2-1
x
JO [J/2-d/2]
f(p2)-
Z n
=
/(W)(0)P2^!
0
[(J+l-d)/2]xn/j _xjJ/2-d/2-n
2n
r.2j
[(J-d)/2]
/("V)-
/0)(°)
Z
(n-j)l
Integration by parts in the p-integral gives Kd_J_l
J
dxx^-^l-xT^x [(J+l-<i)/2]
£
1-
/I
x"(l-x)J'2-d/2-"
?=0
[J/2-d/2] A-J-l
dpp
n = 0
.
1/2-
nl(l+J/2
-rf/2-n)lJ
4.3 Properties
73
x-integral is used to show that it equals r(l/2)r[(d-J-l)/2]/r[(d-J)/2], from which the required result
An
integration by parts
on
the
follows.
4.3 Properties Property 1. Axioms: The definitions (4.1.16) and (4.2.5) satisfy Wilson's axioms (4.1.2), etc., for ^-dimensional integration.
Proof. We reduced ^-dimensional integration to ordinary integration so linearity follows from linearity of ordinary integration. We must choose the p|| space to be large enough that it is the same for both functions / and g in (4.1.2). Our explicit continuation (4.2.5) to arbitrary negative d ensures that reducing the dimension of the transverse space is no problem. Scaling and rotation covariance are explicit properties of all our definitions. Translation in variance is valid for ordinary integration, so it follows from definition (4.1.16) provided the pu space is big enough to include the vector q used in the axiom
(4.1.4).
Property 2.
U (p2)a =n.2Md+2*-2Pn?+d/mp-*-d/2) r(<*/2)r(jB) J V + M2/ Proof. Immediate from (4.2.5). Note that this implies that the power
of
p2
is zero, since
T(f})
^
l/p
as
'
[
,J
integral of a
/?-?().
Property la.
|dJp(p2)*
=
0.
(4.3.1a)
Proof. Already done. Property 3.
f
dV(P,q,-)=
Proof. Contract with
kp^/(p,q,..).
(4.3.2)
<5q which projects out the derivative with respect to the component of q in the <5q direction. Then make the parallel space (of p|| 's) big enough to include dq and use (4.3.2) on ordinary integrals. This is true for all Sq. a vector
74
Dimensional
Property
regularization
4.
&% Proof and definition
(4.3.3)
ofdtj. Now 3ij is defined to be the component form of
a contravariant tensor with 8iJ
=
1 if / =j and
definition of the covariant tensor
thing. This gives
d.
=
3^5^
=
co.
5tj is
zero
otherwise. The obvious
the inverse matrix, i.e., the
as
However in
an
same
infinite-dimensional space,
there is space for a different definition. A contravariant tensor may be defined by specifying its components. But a covariant tensor co is fundamentally a linear function acting on covariant tensors: co(T).We
can
write co(T)=
only if the sum converges. symbolize by 8tj) to be rotation
(DtjTij
We need the covariant 5 (which
we
invariant, and to give S(T)= Tli whenever the sum exists. We would also like contraction with
5tj
to commute with
*u ddVPiPif(p2)
=
integration.
Sij "
and
^
5ij d>y/(P2) Since
we
have
us
infinite sum,
dW/(P2)/<*j
(4.3.4)
d>2/(P2)-
we cannot
immediately apply linearity
to
define am
dT{d/2)
ddplTijP,^(p2-l).
71
£T"
Whenever
But if the
example
equation.
prove this
Let
an
=
For
sum
converges, this definition
diverges, then it is possible
(43.5)
gives
to
get
a
finite value for 3(1). In
particular,
mJJwvj^n'-i)-*,
as
required.
rotationally invariant. Commutation of will now be a consequence of integration dtj two integrals which we will prove later.
The definition is
contraction with
commutativity
of
and
-
Property 5. Integration by parts:
|dV/(p)/^
=
0.
(4-3.6)
4.3
75
Properties
Proof. Work component-by-component. Contract with
an
arbitrary
vector k:
integral, we must put k in the parallel space, and we can of (4.3.6) for ordinary integration in the space parallel to k. proof
Then, to use
the
define this
{'
Property 6. To define integration
over two
(or more) variables:
|d-pdV(p,k;qi,-q?) we must
rules
choose to calculate
one
already stated.
integral
For this definition to be sensible
the order of
then the other,
need the result to be
we
to the
independent
of
integration: ddP dV
=
JddkjdV.
(4.3.7)
We could also allow the dimensions of the p- and different.
according
Then
of order of
exchange
integration
k-integrals
to
be
j'dcipjdd'k->j'dd'k{ddp.
is allowed only if d d\ or if / is independent of pk. Proof. It is sufficient to consider the case that there =
/=/(p2,p-k,k2). (If
there
q/s,
are
then
we
q/s,
so
that
a
finite-dimensional
we
apply the theorem
take out
integral for both k and p which spans all q/s and then
are no
remaining dimensions.) The left-hand side of (4.3.7) is
to the
4^-1/2
roc
dp
r(d/2)r((d-i)/2)
foe
/*oo
dkA
dkTp-,-lk"T-2f(p\kip,k2l+k2T), (4.3.7L)
while the right-hand side is /■ oo
f oo
4nd ̄l!2
r(d/2)r((d-i)/2).Jo
d/c
J
f*
dp, -
oo
| dp^-^-'/fPi+pip,*,*2).
J
(4.3.7R)
Here kl is the component of k parallel to p, while pt is the component of p parallel to k. Change variables to, say, p2, k2, and z p-k/(pk) Pi/ \/(P2 + P?) kJ \/(k2 + *r X with the result that both (4.3.7L) and =
=
=
(4.3.7R)
are
equal
4*d
1/2
(
to /??
'-1)/2)J0 dpp"
WI2)T({d
dkk*-
dz(l-z2f-3v2f(p\pkz,k2). (4.3.7C)
76
Dimensional regularization
The theorem is thus
proved in
the case that
(4.3.7L) and (4.3.7R)
are
both
convergent. Note that it is not a trivial consequence of the corresponding result for integer-dimensional integration. If the dimensions of the integrations are not the same, then let the k integral have dimension d'. The left-hand side gives
47C<d
+ d'-l)/2
T(d/2)r((d'
-
l)/2)
pd^-1 pdfc^-1 r
d2(i-z2)<d'-3>'2/,
which in general is not the same as the corresponding expression for the right-hand side. But if / is independent of z, then the z-integral can be
!■
computed
The result is
explicitly.
A-(d+d')!2
foe
&kk*-lf(p2,k2). 0
(4.3.8)
problem is
that if d is not
positive, we must make subtractions as in clearly asymmetric between the two orders (4.3.7L) and (4.3.7R) of performing the original integral (with now d' d);in practice, d will be the number of dimensions transverse to the external vectors A
(4.2.5). These
are
=
applications to Feynman graphs d will therefore be negative in regulate UV divergences. So we must use (4.2.5) to define the integrals. Then (4.3.7) does not give (4.3.7L), (4.3.7R) and (4.3.7C). We solve this problem by defining an auxiliary integral with a
q!,..., qN. In
order
to
convergence
factor,
say
I(a9d)- Assume
i
/ is power-behaved
d?V(p,k)exp [ at
-
a(p2
+
(4.3.9)
k2)].
or R) is UV and (4.3.7R) are IR (4.3.7L) The function I(a, d) is analytic in a and d.
infinity.
Then for all d, (4.3.7L
convergent. Moreover, if d > 1 then both convergent without subtractions.
Continue down to small enough d that (4.3.7) is UV convergent. Then I{a,d) is given both by (4.3.7L) and (4.3.7R) with / replaced by a(p2 + k2)], and with subtractions made. Now set a 0 to prove / exp [ =
-
the theorem (4.3.7). Property 7. ddk
d'p/(p2
+
k2)
=
dV(q2).
(4.3.10)
Proof. Since / is independent of pk, the previous theorem shows that the left-hand side is
independent
of the order of
integration,
even
if
the
11
4.3 Properties dimensions of the p- and
change variables
Property
to q
k-integrals
(p2
=
+
k2)1'2
are
and
x
different. Then =
use
(4.3.9) and
p2/q2.
8. w v sap ) dV-pW)
=
■
with
T1-1'
f'. \Tli-i<At[g], ^iS°dd'
[5ili25i3i4'--5i-*-it +
=
+ all
if t
rn
(4-3.11)
is even,
'?-
permutations
of the
(4.3.12)
fs]/t!,
and
,W
T(d/2)r(t/2 + l/2) ddp(p2)'/2<7(P2) T(\fl)T(dl2 +1/2) 2nd/2r(t/2 +1/2) [*
-r(i/2)rw2 +
,/2)J0d^""'"(^ lx
,
,
<43'13)
Proo^. If r is odd, antisymmetry of the integrand under p-> p makes integral over the 'parallel' space zero. Antisymmetry under reversal of one component of p, symmetry under —
the
permutations of the fs, and rotation invariance give the general form (4.3.11) and (4.3.12). Computation of one component (say, i{ i2 =
it
=
=
?
?
?
=
1) then gives (4.3.13).
Examples.
IdVpWt-aW U'ppW).
Property
9. Consider an
integral
/(Pi,...,P,)= which is
UV convergent
(4.3.14)
fdV(k,Pi,...,Pj)
by power-counting at
^
=
4; that is /
(4-3-16) =
0(l/k4+a) as
infinity in any direction, for some positive number a. Then the is analytic in d and in the parameters p., when d is close to four, if the
k goes to
integral integrand
is
analytic. If
integral at d
integral of/.
=
the
pf's
lie in the first four dimensions, then the
4 has the same value as the
ordinary four-dimensional
78
Dimensional regularization
Example Suppose / has the form
/=/(R5p1,...5pJ)exp(-Ak2 where for
any vector
let 7 be the
we
let
v
v
be its projection onto the first four
be its
definitions of ^-dimensional integration,
d£i"4kexp(-Ak2
/
=
=
have
+
2Ap-k)
I(7i/A)di2-2exp(Av2).
manifestly analytic
in d and p. If
7 Notice that there is
we
(d'k/
/=
This is
(4.3.17)
2Ak-p),
projection onto the remaining dimensions. Then ordinary four-dimensional integral of/. By use of our
dimensions and let we
v
+
no
dimensions. However, if
=
(4.3.18)
we set
d
=
4, the integral becomes
7exp(^p2).
restriction
on
p, even
though
p
=
0 in four
let d->4 and p->0, the limit is smooth.
we
Proof of Property 9. The proof is easily made by examining the definition of the ^-dimensional integral in terms of ordinary integrals. As usual we divide the space into a finite-dimensional parallel space big enough to contain P!,...,Pj, and into a transverse space containing the remaining
dimensions.
It is convenient to choose the
parallel
space to have
an
odd
number 2N + 1 of dimensions. Then: {d-l):2-N
lT((d-l)/2-N)
x{F(kl9k2,...9k2N+l9k*)-
£
dk2T(k2Tyd ̄
Fi?\kl9...9k2N+lMlV*d
(4.3.19)
Here we have used F to denote / considered as a function of the first 2N + 1 components of k and of k2. Since / is an analytic function of k, it can be expanded in powers of k2. As required by the definition, we have subtracted offa power series in k\, to
give convergence at fc7 respect to
=
0. We use F(n) to denote the nth derivative of F with
k\. The integrand
convergence at kT
=
of the k2 integral behaves
0 if d > 3. Since
/ is analytic,
as
kd- ̄5,
have
so we
there are
no
other
singularities at finite k, and the only other possible source of a divergence is from large k. The subtractions do not introduce a divergence provided that
d<
5. Moreover,
there
are no
we
have assumed that
other large k
divergences
/= 0(l/fc4+fl)
as
fc-> oo,
so
that
when d is close to four. Hence (4.3.19)
79
4.3 Properties
analytic in a neighborhood of d 4. Now the integral will depend on the p.'s only through the Lorentz scalars Va'Vb (with 1 < a < b < J). To determine this dependence, it is sufficient to keep the p£'s within some fixed J-dimensional subspace. Since (4.3.19) is a perfectly finite integral, it is an analytic function of the p-'s. converges and is
=
To determine the value of the
the first four dimensions,
integral when d
=
4 and when the
p^'s are in
the freedom to vary the dimension of the in the definition (4.3.19). Let us now make it four 'parallel' space dimensional. We will obtain an integral of the form: /=
T
we use
d/c1...d/c4T^-^ pdfcfe''-5/(/c1,/c2,/c3,/c4,P)
if d>4. When
we
let d->4, the
resulting divergence cancels the
I(d as
=
4)=
required. alternatively continue from f
=
The
2nd/2 ̄2
over
=
Idk.dk^k.dkJik^k^k^k^Ol
We may /
k is singular at fc of the inverse T-function to
integral zero
f00
Jd/c,...d^Y^7^^2) J
d<4
0; the
give (4.3.21)
using
dfc^"5C/(fci>- -,^,^2) /(^i, -
-
o
(4.3.20)
-
^4,0)]. (4.3.22)
singularity at k 0 is cancelled, but as(i->4we gives the same result (4.3.21). =
get
a
divergence
at
k= oc which
Comment In this proof we used the freedom to alter the dimension of the 4, it was parallel space. To show that the integral is well-behaved at d =
convenient to choose the parallel space to have an odd dimension. But to 4, it was convenient to choose the parallel compute the actual value at d =
space to have an equivalence in a
dimension, specifically, four. It is instructive to see the simple non-trivial case. (The general case was summarized even
at the end of Section 4.2.) Suppose 3 < d < 4. Then define nm-2 7? =run n
r{d/2 foo
/2
=
-
2) J
0
dk\B)d^f{k^k2^k^\
-(<*-5)/2
(4.3.23)
foo
J d/C5r(^-5))J0dfcT2(fc')(d"7)/2[/(/Cl'---'fe4'fc'
+
fcT2)
-f(ki,...,k4,k25)l
(4.3.24)
80
Dimensional regularization
Our definition of the ^-dimensional integral of
dfc1...dfc4/1
/= J so we must prove
To do this
(1
—
x)k2
we
=
/
tells
that
us
dfcj ...d/c4/2,
(4.3.25)
00
-
that lx= I2. change variables in /2, by setting k2
=
xk2 and
k\
=
to obtain:
wW-5)/2
r(d/2-5/2) Jo
Jo
1/2^/2-7/2 dk2(k2)dl2-3(l-x)-1!2x'
xlf(kl,...,k4,k2)-f(k1,...,k4,K2(l-xm
rr(d-5)/2
r(d/2-5/2)JO
dx\ dH2kd-6(l-x)-ll2xdl2-7/2x J0
x{[f(ku...,kA,K2)-f(k1,...,kM -U'(k1,...,k4,H2(l-x))-f(kl,...,k4,0)]}. In the last line we subtracted and added
integrate seperately each
(4.3.26)
f(kt,...,k4,0), so that we can particular, we have
term in square brackets. In
dfc2^-6[/(/c1,...,/c4,P(l-x))-/(/c1,...,/c4)0)] =
(!-*) 2-d/2
Comparison with
the definition
i2_7i-1/2r(d/2-2) f1 I,
dk2kd-6lf(kl,...,k^2)-f(kl,...ik^0)].
T(d/2-5/2)
(4.3.23) of /x
shows that
dx[(l-x)-1/2xd/2"7/2 -x^2-7/2(l-x)3/2"d/2].
The integral is in fact the analytic continuation from d > so
that it
10.
Multiple integrals integral
function, result lx =/2
Property
equals F(d/2-5/2)r(l/2)/r(d/2-2). follows. are correct at
d
=
5
(4.3.27)
of a
The
beta-
required
4.
Consider the
/(P15...,PN)=
\d%...ddkLf(kl9...9kL^l9...9vN).
(4.3.28)
This might represent a Feynman graph with N external lines and L loops. Then the p-'s and k.'s represent momentum vectors. Suppose that at d 4 the integral is completely convergent in particular that there are no ultraviolet (fc-> oo) divergences or subdivergences. If we restrict the pf's to the =
-
first four dimensions and set d
integral of /.
=
4, then / is the ordinary four-dimensional
4.4 Formulae
for Minkowski
81
space
Proof. For each vector v, we define the projections v and v, onto the physical and unphysical dimensions, as before. The result to be proved is that if Pj
=
pf for each of the p.'s then / defined
as
the limit
as
d -? 4 of the
dimensionally regulated integral ordinary integral. We each over a into an four-dimensional split integral integral ordinary kz over kt and a (d 4)-dimensional integral over kf. The result to be proved is is identical to the
can
—
then that
limjd'-4^...^^^ as
(4.3.29)
d->4. This formula is
proved by doing all but the integral over ft,.
Let the result
be/(1): /(l)(k1)= its only dependence (4.3.29) is
on
kt
idd-%...dd ̄*kLf;
is via its
length.
(4.3.30)
We then have that the left-hand
side of
by use of the Property 9. dimensions. We
can
dependence on kj
is
on
its first four
|d-4k3...d--4kL/(kl,k2,k3,...).
/(1)(0)= Another L—2
Notice that the
then repeat this process to show that
repetitions give (4.3.29),
from which the desired property
follows.
4.4 Formulae for Minkowski space In this section
we
derive
a
collection of results that
are
useful for Feynman
graph calculations.
4.4.1 To convert an
Schwinger parameters
arbitrary graph in d dimensions to using
a
parametric integral,
we
first rewrite each propagator
Then
we
perform the momentum integrals.
Since all Feynman
graphs are of
82
regularization
Dimensional
polynomial in momenta times a product of simple scalar propagators, we only have to calculate d-dimensional integrals of the form: the form of
a
I^"?(A,B)
ddkk>"
=
/c"-exp [
...
-
(
-
Ak2
2Bk)l
-
(4.4.2)
on the parameters introduced by (4.4.1), while B" these depends parameters and also linearly on the other momenta (both external and loop momenta).
Here A
depends only on
By linearity
we can
'1
uses
3
■=n 2dBUl J=1 v_??/ljV
It
(This
find ln by differentiating /0:
linearity
translation fc"-?fc"
ddkexp{Ak2
=
i(n/Af2 exp(
\(n/A)d/2exp(
4\ddkk>-k>'kvexp(Ak2
1 A
=
=
the
i(n/Af2e-B2::A
+
2B-k)
B2/A)(
-
r
-
-
+
(4.4.4)
2B-k)
+
d"/c/c"/cvexp(^2
Ix3"v=
=
70 by using
i(n/A)dl2exp(-B2/A).
/?= \ddkk"cxp(Ak2 =
(4.4.3)
B"/A, the scaling /c->/c A ̄i/2, and Wick rotation:
Thus
=
2B-k).
of rf-dimensional integration.) We find —
I0(A,B)= \ddkexp(Ak2
I"2V=
+
j
+
-
B?/A),
(4.4.5)
2B-fc)
B2/A)(B"BV/A2 -ig"v/A),
(4.4.6)
2B-k)
BXB"BV
(B' g"v +
B"gXv + Bvgx") 2A2
(4.4.7)
\ddkk",kxk"kvcxp( ̄B2/A)
i(it/A)d'2exp(-B2/A)
VBKBXB?BV
(BKBxg"v + five similar) 2A3
A*
}■
+
{gKXg'lv + two similar)  ̄| 4A2
(4.4.8)
Each of the loop-momentum integrals is performed in this way. At each stage the momenta only appear quadratically and linearly in the exponent.
4.5 Dirac matrices
83
4.4.2 Feynman parameters It is also
common to use
'-£
l/(AB) and its
=
dx/[Ax + B(\
-
(4.4.9)
x)]2
generalizations: 1
I\* +
A*&—E'
p+ ?■?&)
x
r(a)r(^)---r(e) x
Jo
dxdy-dzd{\—x
—
y
z)
x
xa-y-l...ze-l (Ax + By + ---Ezf+fi+-+e'
(4.4.10)
Here A, B,...,E represent the denominators of the propagators of a Feynman graph. The resulting momentum integrals have the form
k'"---k''"
C JHf-lh,— lAdh ?v*
]a\-k2_2P.k + cy
Application of (4.4.1) and J0
\d"k/(
=
-
k2
-
the results
2p-k
ind/2(C + p2)"'2 "Ha
J\
=
Jf
indl2(C + p2Y'2-'(
=
=
-
\ddk k"/( -k2-2p-k
=
-
(4.4.4)-(4.4.8), etc., gives
Cf
+
"
=
{(4.411)'
+
rf/2)/r(a), Q*
p")r(a
d"kk?kv/(-k2-2p-k
(4.4.12)
-
d/2)/T(a),
(4.4.13)
C)a
+
ind,2(C + p2f2-xx x
[r(a
-
d/2)p"p"
-
T(a
-
1
-
d/2)g?v(C
+
p2)/2]/r(a).
(4.4.14)
4.5 Dirac matrices The Dirac matrices satisfy the following
properties:
(1) Anticommutation relation: {/,yv}=20*vl.
(4.5.1)
84
Dimensional regularization
Hermiticity:
(2)
/w
*-/ if/x > 1.
When we use dimensional regularization, the Lorentz indices range over an infinite set of values, so we need infinite-dimensional matrices to represent the algebra (4.5.1). We will also need a trace operation:
trl
=/(<*),
representation behaves as if its dimension were f{d). We must require f(d0) to be the usual value at the physical space-time dimension, d d0. Usually this means /(4) 4. that the
so
=
=
linear operation on the matrices which we will define later. integer dimension d 2co, the standard representation of the /'s
The trace is
In
an even
a
=
has dimension 2°\ However, it is not necessary to choose f(d) 2d/2. The variation / (d) f (d0) is only relevant for a divergent graph, so, by Chapter =
—
7, any change in f(d) amounts to a renormalization-group transformation. usually convenient to set f(d) f(d0) for all d.
It is
=
To set up a formalism for dimensionally regularized y-matrices, treat the following issues:
(1)
we must
representation of the anticommutation relations; consistency. for the trace of an arbitrary product of yM's must be
We must exhibit
a
this will ensure
(2)
The formulae
derived. (3) While a knowledge of the /'s alone is sufficient for QCD and QED, we must show how to define a y5 so that we can treat chiral symmetries.
This will also give
us a
definition of the
antisymmetric
tensor
eKA/iV.
following construction gives a representation: Let co be a positive integer, and suppose inductively that we have defined a 2W dimensional representation yfw) of the algebra (4.5.1) for 1 by 1. We define the infinite dimensional y* for 0 < \i < 2co 0 < fi < 2oj having a sequence of y^'s down the diagonal, and zeros everywhere else: The
—
—
VM
y
<
\i
<
2a> +
\
0 vy
o vC.. no?
|
?
(4-5-3)
+
higher representation yf(0+1} of dimension 2<° *, 1. In order that (4.5.3) apply independently of co, we must
We will construct the next
with 0
bu —  ̄
85
4.5 Dirac matrices choose
This satisfies the anticommutation relations (4.5.1) and the hermiticity relation (4.5.2), provided that 0 < fi, v < 2co 1. Our task then is to find —
f°r ^ 2w and 2co + 1. Notice that the induction starts with
yfa>+1)
=
1
*°?=
0\
,
co
=
1. We can define
_/
0 1
-i)> y^-\-\
Given the 2a)-dimensional
?0)-
?
n_i o
representation yfa
H-5-4)
■
_,n
o
we
define another matrix
Observe that
&> Also, when at co at d
=
=
y<?>
?<?>,
2, we have f
=
=
=
{iwX>}=o.
1>
(4-5-6)
y5, in the usual notation for Dirac matrices
4. We define
/,w+1,"V-y?.,
o
yivi=(-° ifr)- It is easy
to
check
that
(4-5-7>
(4.5.1) and (4.5.2)
are
satisfied for 0</i,
v£ 2(o+l.
explicit representation of the Dirac matrices for any u>, (4.5.3). Standard manipulations involving the anticommutation relations are valid independently of d. Two useful results are: We
now have an
and for the infinite-dimensional case, because of
rtv=T{y",y?}i=<i /W?
=
=
rfi,
(4-5.8)
20?vy"-/y?yv (4.5.9)
(2-d)yv.
We also need traces of y-matrices, in a
=
graphs
with fermion loops. The trace of
atv(A)
+ b
matrix is linear :
ti(aA
+
bB)
=
(4.5.10)
tr(fl),
and is cyclic: tr(AB) Here A and B
are
any
=
(4.5.11)
tr(BA).
product of y-matrices, and
a
and b
are any
numbers.
86
Dimensional regularization These
with the value of tr 1, define the trace of any
properties, together
linear combination of products of For example, tr
(y"yv)
=
tr
(//)
(cyclicity)
=
tr
(
+
=
so we
y-matrices.
-
/y
-
tr
(y"yv)
2¥T 1)
+
(anticommutation)
Vvtr
(linearity),
1
have the usual result tr(yY)
=
^vtrl.
(4.5.12)
0KVV + 0KV")tr 1.
(4.5.13)
Similarly tr
The trace of the
(yKyVyv) product
(0KVV
=
of
an
-
odd number of
y-matrices
is
zero.
For
example dtryA
=tr =
=
so
tryA
=
[yKyKyk)
-tr(yY7j + 2tryA
-tr(yKyV)-h2tryA,
0.
possible to make a more constructive definition of the trace, along the lines of (4.3.5). It is necessary to check consistency. We can find a formula for the trace of any number of y-matrices generalizing (4.5.13). It is true for any finite-dimensional representation, yfa, so it agrees with the algebraic properties. Linearity defines the trace of more general products. with the trace. This We must also check that contracting with g^ commutes can be checked directly. A possible explicit definition of the trace of a matrix with components It should be
-
Mijis trM,. This definition
of
a
(trl)limi N-ooiV
£ Mjy
(4.5.14)
j=l
exploits the fact that each matrix y" is an infinite set of copies
finite-dimensional
independent
=
of d, the
yfa strung along the diagonal. Since the y^'s
only possible rf-dependence is in the choice
are
of the
value of tr 1.
4.6
y5and
ekx^v
dimensions, y5 iy°y1y2y3 and eK>fiX is Lorentz-invariant tensor with e0123 1. We need
In four
=
=
a
totally antisymmetric example, to define
y5, for
87
4.6 y5 and eK^v the
current
axial
ie^^yVV/^!,
and
we
=
1
=
i£*^v tr 1
appropriate definition changes
Instead of y5 £01
£-tensor
=
—
we
have
£10, £00
To continue
f(1) £u
=
in
comes
because
y5
=
have the trace formula:
tr y5yKyYyv The
The
i/sy^y^.
=
=
y°y19 and
—
=
when
ieKXflv tr 1. we go
instead of eKX/lv
to
we
two
dimensions:
have £MV, for which
U.
dimensionally,
might expect
we
y5 to
satisfy
as in four dimensions. But then, as we will see in Chapter 13, the only consistent result for y5 is that it has zero trace when multiplied by any string of y"'s. Thus we do not have a regularization involving the usual y5.
just
writing
A consistent definition is obtained by
ys
!1 -
=
if
1 if
iyoyy y3
(K??fiv) is (KAfiv) is
=
iryrvw4!>
an even an
odd
permutation of (0123), permutation of (0123),
(4-6-1)
(4.6.2)
0 otherwise.
This definition is not Lorentz invariant
on
the full space, but only
on
the
first four dimensions. We have
{y5,/}=0, |j5> V"]
(7s)2
=
0,1,2,3,
=
0,
otherwise,
=
1,
/5=y5-
(4-6.3)
a nuisance, but it does give the correct Hooft & Veltman anomaly ft (1972a)).
The lack of full Lorentz in variance is axial
if ^
Renormalization
chapter we come to the general theory of renormalization. The basic difficulty is that a graph may not only possess an overall divergence. It may have in addition many subdivergences which can be nested or can overlap in very complicated ways. Most of our effort must go to disentangling these complications. We will begin by investigating some simple graphs. These will show us In this
how to set up the formalism in the general case. The ultimate result is the forest formula of Zimmermann (1969). Contrary to its reputation, this is not
procedure, designed for pedantically rigorous treatments. Rather, the forest formula is merely a general way of writing down what is in an
esoteric
fact the natural and obvious way of extracting the divergences from any integral. Its power is demonstrated by the ease of treating overlapping
the handling of which is normally considered the bete noire of renormalization theory.
divergences,
The forest formula is applied to individual Feynman graphs. It extracts graph by subtracting its overall divergence and its
the finite part of a
subdivergences. We will, of course, need to show that the subtractions can be implemented as actual counterterms in the Lagrangian. We will also show that the counterterms are local, i.e., polynomial in momentum. An important advantage of using the forest formula to obtain the finite part of each graph, rather than working directly with counterterms in the Lagrangian, is that the procedure applies to more general situations. As we will
see
more
in Chapter 6, it will enable
important
momenta of
a
case
is the
us to
renormalize composite operators. A
computation of asymptotic behavior as external
Green's function get large. For this case, the forest formula
good derivation of Wilson's operator-product expansion, which we will discuss in Chapter 10. Let the value of a Feynman graph be written as:
permits
a
/?
U(G)(Pl,...,pN)= d"fe,...d'ifeL/(p1(...,plv;/c1,...fct)- Here pl9...,pN are the external momenta, and
k{,...,kL
are
(5-0.1) the loop
5.7
Divergences
89
and subdivergences
large-fc behavior divergences. Moreover, the very same techniques can be used to
momenta. Renormalization is removal of that part of the
that
causes
extract the behavior for large p as product expansion in Chapter 10. -
we
will
see
when
we treat
the operator
Although Weinberg's (1960) theorem tells us the power-counting applicable to either kind of asymptotic behavior, it does not tell us how to organize it. In particular it was only much later that Wilson (1969)
expansion, which is the important tool in computing asymptotic behavior, for example in deep-inelastic scattering see Chapter 14. Many generalizations have been made see Mueller (1981) for a review. These are phenomenologically very important, and the method by which they are proved is close to that for Wilson's expansion. formulated his operator product
-
-
5.1
Divergences
The idea of renormalization
and
subdivergences
theory is that ultra-violet divergences
of
a
field theory are to be cancelled by renormalizations of the parameters of the theory. We propose to prove this in perturbation theory. The use of
perturbation theory implies that we expand the counterterms in the action in powers of the renormalized coupling, g, thereby generating extra graphs with these counterterms as some of the vertices. To avoid superfluous technicalities, space-time.
we
will consider the
case
of
</>3 theory
in six-dimensional
was discovered by Shirkov & and (1955, 1956, 1980) Bogoliubov Bogoliubov & Parasiuk (1957), and we shall follow their approach. The first step is to decompose the
A very efficient way to understand renormalization
Lagrangian
as
follows: ^
Here
i/(p2
S£0 -
=
^0
+
^b
+
(5.1.1)
^ct.
is the free Lagrangian used to generate the free propagator
m2 + ie) in perturbation theory:
(5.1.2)
J?0=(d<t>)2/2-m2(f)2/2, with if b
+
being the renormalized mass. The rest of the Lagrangian, <£, JS?ct, is the interaction, and consists of two terms. The first, which we will
m
=
call the basic interaction, is
(5.1.3)
J?b=-g<P3/3\, where g is the renormalized coupling. The second term, J^ct, counterterm Lagrangian.
Consider graphs generated by
we
will call the
the basic interaction. These have UV
90
Renormalization
divergences
which
are
be cancelled
to
by graphs with some Lagrangian
of their
interaction vertices taken from the counterterm
JS?rt (The
=
(5Z(<30)2/2
-
5m2(j)2/2
-
5gcj)3/3!
-
cj> is needed to cancel tadpole graphs
(5.1.4)
dhcj).
Figs. below.) In order to give meaning to 5Z, dm2, 5g, and 5h, we must impose an ultra-violet cut-off. We will use dimensional regularization in the term linear in
-
see
5.1.4 and
5.1.5
following sections. The key to the method that we use is to realize that each of the three terms
Lagrangian should not be considered as a single quantity. Rather it is to be considered as a sum of terms, each of them cancelling the overall divergence in one particular graph generated by the basic interaction. For example, the self-energy graph, Fig. 3.1.1, gives a contribution SXZ to SZ, and a term d^m2 to dm2. Our calculation of this in the counterterm
graph led
to
(3.5.7),
so
with minimal subtraction
51Z
=
^m2
^/[3847r3(rf-6)],
=
Then c5Z=
we
have
J
l
32m2/[647r3(d-6)].j £ 5GZ
graphs G
=
'
'
'
81Z+--;
with similar formulae for the other counterterms.
utility of this idea by examining graphs like those in Fig. 3.2.1 Graphs like Fig. 3.2.2 contain vertices corresponding to the counterterm bxm2 (and SlZ). Such graphs are all generated by taking graphs like Fig. 3.2.1 with no counterterms and finding where Fig. 3.1.1 occurs as a subgraph. Substitution of the counterterm for one or more of We saw the
and Fig. 3.2.2.
these subgraphs gives the graphs with counterterm vertices. This leads to the idea that we consider by itself the renormalization of
single graph generated from
the basic
Lagrangian.
We add to it
a set
a
of
counterterm graphs to give a finite result. Only as a separate step do we recognize that the counterterm vertices are, in fact, generated from a piece of an interaction
Lagrangian. graph-by-graph method is probably the most powerful approach to understanding not only the problem of ultra-violet divergences but also many other problems in asymptotic behavior. Even so, it is not at all trivial The
to
ensure
steps
that the renormalization program can be carried out. The essential
are:
(1) To find the regions in the ultra-violet divergences.
space of
loop
momenta of a
graph that give
5.7
Divergences and subdivergences
(2) To show how to generate
a
series of counterterm
91
graphs
for
a
given
basic graph. (3)
(i.e., polynomial in
To show that the counterterm vertices are local
momenta). (4)
To find the conditions under which the counterterm vertices amount
only
to renormalizations of the parameters of the
Lagrangian.
complications in carrying out this program arise when one treats the divergence of a graph which has a divergent subgraph. To understand why there is a difficulty, we will examine the graphs of order g* for the full propagator Figs. 5.1.1-5.1.3. The
case of the
-
—O—O—+—O—*-
+
-*—0-+
x
11
(a)
(b)
x
11
id)
(c)
Fig. 5.1.1. A two-loop graph for the propagator in counterterm graphs.
(j>3 theory, together
with its
1
p
l^^^
v
(?)
*2
J^^ (?
(c)
Fig. 5.1.2. A two-loop graph for the propagator
in
counterterm graphs.
<D-
<j>3 theory, together
with its
V
(a)
(c)
(b)
(d)
Fig. 5.1.3. A two-loop graph for the propagator in (j>3 theory, together with its counterterm graphs.
We ignore the graphs with tadpoles, such
divergent
and need
condition that zero
<O|0|O>
(e.g., Fig. 5.1.5),
Let
a counterterm
us return to
there is one basic
Sh(p.
We
as
can
use a
vanishes. Then the total of the
so we
5.1.4. These
Fig.
are
renormalization
tadpole graphs
is
omit any graphs containing them.
graphs listed in Figs. 5.1.1-5.1.3. In each set graph and a set of counterterm graphs. Ultra-violet
the sets of
o
¥5
Fig. 5.1.4. A tadpole graph, together with its counterterm.
92
Renormalization <o|0|o>=
Q
+
+
—<D
+
Fig.
*
+
—CV—£>+—*
5.1.5. Graphs to
0(g3)
for
6
<O|0|O>.
divergences
involve a loop momentum that gets large, so the divergences confined to one-particle-irreducible subgraphs. The simplest always case is Fig. 5.1.1, where the basic graph has two insertions of the one-loop self-energy. It is made finite by adding the graphs with one or both of the self- are
energy subgraphs replaced by a counterterm. (We use a cross to denote a counterter m in a graph, and we use the label' 1' for the counterterm of the one-
loop self-energy.) In Fig. 5.1.2, the basic graph is more complicated. We will treat it in detail in Section 5.2, and we merely summarize the results here. It has two UV divergences. The first comes from letting both loop momenta k and / go to infinity; we call this the overall divergence. But there is also a divergence where the momentum in the outer loop stays finite. This is an example of a subdivergence. Its existence, as we will see, implies that there is a term
proportional
to
p2 In (p2)
in the
divergence
of the basic
self-energy graph.
This cannot be cancelled by any local counterterm. However there is also a graph with a counterterm to the subgraph. This graph is Fig. 5.1.2(h). After we
add the two
graphs, the non-local divergence cancels. The
overall
divergence in we use
the result is then cancelled by a local counterterm, for which the label '2\ We implement this as a counterterm in the action by
inserting terms S2Z and 52m2 into the complete counterterms 5Z and dm2. The pattern is simple. We consider as a single finite entity one basic graph together with counterterms for its subdivergences and for the overall divergence. Another case is shown in Fig. 5.1.3. There are two subdivergences, each corresponding to a vertex subgraph. The one-loop vertex is logarithmically divergent and is made finite by renormalizing the coupling (Fig. 3.6.1). Since the two divergent subgraphs overlap, the counterterm graphs are generated by replacing one but not both of the vertex graphs by its
counterterm. The overall divergence is then local and is cancelled by 34Z and S4m2.
counterterms
5.2
Two-loop self-energy
Our first task in Section 5.2 will be to
generalize the argument
we
in
verify
(</>3)6
93
the above statements. To
will then observe that
power-counting
as
in
Section 3.3 determines the strength of the overall divergence. To prove that the presence of subdivergences does not affect the form of the overall counterterm, remove
we
will differentiate with respect to external momenta to divergence. Then we will be able to construct an
the overall
inductive proof that if subdivergences have been cancelled by counterterms then the overall divergence is local and its strength is determined by simple
power-counting. We will also show how to
disentangle the combinatoric problems when subdivergences Finally, we will discuss Weinberg's theorem. This theorem tells us exactly which regions of momentum we must consider. In practice one is very simple-minded about locating UV divergences. For example, we stated that the regions giving divergences for Fig. 5.1.2are: (a) k and / going to infinity together, and (b) I going to infinity, with k fixed. In each region, all the momenta get large in a particular 1PI subgraph that is divergent by power-counting. Weinberg's theorem tells us that these are the only regions we have to consider explicitly. In the case of Fig. 5.1.2 there is another region that is important, where / goes to infinity with k also going to infinity, but much more slowly. This region interpolates between the other two, but in fact does not need to be treated as a separate are
nested.
case.
5.2
In this section
Two-loop self-energy
in
(¥3)6
will
explain the properties of overall divergences and subdivergences by computing the two-loop self-energy graphs, Figs 5.1.2 and 5.1.3, in 03 theory at space-time dimension d 6. We will again use we
=
dimensional regularization, and will need the values of the one-loop counterterms in order to cancel subdivergences. The one-loop self-energy was considered in Section 3.6.2, where we found
that the counterterms needed
one-loop (cf., (3.6.13)) the
vertex
given by (5.1.5). We can also compute graph, Fig. 3.6.1, with the resulting counterterm being ?3ff
It is worth
=
were
M3"-/V/[647c3(d-6)].
noting that this implies a
(5.2.1)
value for the one-loop term in the bare
coupling: 0o
=
=
iy-d/20+<530 + 0(<75)]Z-3/2 gn3-"'2{l +ig2/[64n3(d-6)-\
+
0{g*)}.
(5.2.2)
94
Renormalization 5.2.1 Fig. 5.1.2
In order to be able to compute Fig. 5.1.2 in closed form massless theory. The value of the
**2a"
graph
we
work with the
is then
(2n)2d 1
x
'[(p k)2 ie](fc2 ifiT2(/2 is)[(fc 02 2Jdd/cdJ/r^_L^2_L.o-|^2_L.^2^72_L.^r^_ +
+
+
+
-
, A2 +
ie]
.nl.
(5.2.3)
The inner loop is easily computed in terms of T-functions: 1
K2 dd/7277  ̄l2{k-l)2
=
now
-
d/2)
dx[
fc2x(l
-
-
x)]d/2
-2
0
=
We
i^/2r(2
i^2r(2
-
*A d/2)r{?/.2 r(d 2)
-
-
*2)?2"2.
have
1
ld'^iY-?[-(p+*)■]?
(5.2.5)
■
The denominators
can
1
be combined by
Feynman parameter:
r(4-rf/2)J0dX[^x
after which the /c-integral L2°
a
r(5-d/2)r,
^4-<,/2B
v
(5.2.4)
can
\64n3) 2\4nn2J -
d/2)T(5
T(d
-
+
3-d/2
B(l-x)]5-*2'
-
2)r(4
*
d)r(d/2 -
-
l)3r(rf
rf/2)r(3rf/2
-
-
4)
5)
^)2y(4^)"6r(2-rf/2)r(5-^w The overall ultra-violet
(5"2"6)
be performed. The result is
-(jLYeL(zpLY ̄6 T(2
x
divergence is contained
<5-2-7)
in the factor T(5
d). Observe that the argument of this T-function is exactly minus half times the degree of divergence. The subdivergence is contained in the factor T(2 d/2); this is the same as we calculated in Chapter 3. -
-
Before there
are
we
discuss further the UV divergences we should observe that divergences. These come from the existence of long-
also infra-red
range forces in
a
theory with massless fields.
In momentum space,
they
5.2
Two-loop self-energy
in
(^3)6
95
divergences when some momenta go to zero. For example, if integral over the momentum through any propagator has a divergence at zero momentum:
appear as d < 2 the
ddq/q2
 ̄
constant/^
This accounts for the factor
divergence problems,
at k
so we
=
its
T(^/2-l)3.
ignore the IR divergence.
expand Z2a in dependence on p2: us
1,2(2
2)
at d close to 2.
When d<4 there is also
0 with / and p fixed. Our
divergences would go away, but Now let
-
g2 \Vf
\64ttV
we
would not have
powers of d
i
i
only
If we used
36((<*-6)2+<*-6
-
concern
-—=-
Aw2
is with UV
a
massive field, the IR
an
explicit formula for
6 to exhibit its
In I
a
divergences,
I -|- constant
+iln2( t-P ̄2 ) + constant In ( —^ j + constant + 0(d
-
6)
and
+
1. (5.2.8)
The double pole at d
logarithm in the finite part are both reflections of the fact of having a subdivergence. The p-dependence is a power of p2 times a polynomial in In( p2). This is a characteristic feature =
6 and the double
—
of massless theories. The simple pole has
in p. Consequently, it cannot be cancelled by any local counterterm. It is easy to see that this is caused by the presence of the subdivergence. The a
coefficient that is
not
polynomial
comes from the region where the loop momentum of the inner loop goes to infinity while the momentum k in the outer loop remains finite. Integrating over finite k gives a logarithm of p times the divergent part of the inner loop. We have already introduced into the Lagrangian a
subdivergence
counterterm
for the inner loop,
which this
counterterm
so
appears
that there is
in
such
a
a
graph, Fig. 5.1.2(h),
way
as
to
cancel
in
the
subdivergence. Therefore the sum of Figs. 5.1.2(a) and (b) should have no subdivergence, but only an overall divergence. This can be cancelled by a local counterterm We will prove this in Section 5.2.2 by differentiating three times with respect to the external momentum pfi; this gives a result which has negative degree of divergence, i.e., there is no overall divergence.
(i.e., a polynomial in p).
Since the subdivergence is cancelled, there is no subdivergence whatever, so the counterterm must be quadratic in p. We represent this by Fig. 5.1.2(c).
96
Renormalization
In Section 5.2.2
we
will make explicit this proof of locality of the
counterterms. Here
we
case that
will
m
=
verify
the above statements
by explicit calculations.
In our
0, the value of Fig. 5.1.2(h) is ffV-' |\?. d"k
Z?=-i8lzZ-Z-r- (2nf e2 V
64n3)
J
k2
(k2)2(p + k)2
(-p2\r(2-d/2)r(d/2-i)2/-P2y2-3 \ 6 J (d-6)T(d-2) \4nfx2 )
g2 VP2f
-2
1
647i3J 36{(d-6)2 —£ln2(
-—f
—
d-6
In (
-—j
) -|- constant
j + constant In ( —^ ) + constant + 0(d- 6) (5.2.9)
The non-local divergence disappears when
we
add this
graph
to
Z2fl,
with
the result v
+v
-
( g2 Vp2 S
+iln2i2 ( —^-
 ̄
l
,
constant
) + constant In
\4V/
f —V-^ ) + constant + 0(d \4V/
-
6) }.
J
(5.2.10) The non-local divergence has cancelled, as promised. However, the double pole and, in the finite part, the double logarithm have not cancelled, even though it is evident from our calculation that they are associated with the subdivergence nested inside the overall divergence. This is a general phenomenon. Indeed we will see in Chapter 7, where we discuss the renormalization group, that the coefficients of the double pole and of the double logarithm could have been predicted from the one-loop counter- terms without any explicit two-loop calculations. Since the non-local divergences have now cancelled, the overall
divergence
Then
can
we
be cancelled by
obtain at d
=
6
a
choosing
a
wave-function counterterm
finite result, which
we term
the renormalized
5.2
value of
Fig.
Two-loop self-energy
(^3)6
97
5.1.2:
E<2MRS)=S2fl + Z26 + (E2c =
in
=
-p2<52Z)
{<£?] fe {>2(j¥)
+ constant ln
{i¥) constant[ +
(5.2.12) 5.2.2 Differentiation
with respect to external momenta
graph Fig. 5.1.2 has an overall divergence which is local, only after we have subtracted the subdivergence. In we will need to show that the counterterm of a 1PI graph is a general in its external momenta with degree equal to the degree of polynomial divergence. Our argument (following Caswell & Kennedy (1982)) depends on differentiating with respect to external momenta. In this subsection we will apply the argument to Fig. 5.1.2, emphasizing its generality. Then in the next subsection we will apply it to Fig. 5.1.3. Even though that graph has an overlapping divergence, traditionally considered We
saw
that the
but that it is local
a
hard
problem, we will see that our method works as easily for this graph as
for Fig. 5.1.2.
Fig. 5.1.2(a) three times with respect to p", to make divergence negative. We represent the result pictorially by
We first differentiate its
of
degree Fig. 5.2.1, where each dot indicates one differentiation with respect to p. Similarly, differentiating Fig. 5.1.2(h) three times gives Fig. 5.2.2. Now Fig. 5.2.2 cancels the subdivergence in Fig. 5.2.1, and there is no overall divergence, so their sum is finite. Thus the third derivative of the sum of Figs. 5.1.2(a) and (b) is finite. So the overall counterterm is quadratic in p. Lorentz in variance forces it to be of the form A(d)p2 + B(d). We glibly asserted that Fig. 5.2.1 plus Fig. 5.2.2 is finite. This statement is not as obvious as it seems. Let us prove it. We Wick-rotate the integrations over k and /in Fig. 5.2.1, and consider regions of the integral that might give a UV divergence. If k and / go to infinity at the same rate, then there is no
<2>
O
of differentiating Fig. 5.2.1. Result Fig. 5.1.2(a) three times with respect to
Fig. 5.2.2. Result of differentiating Fig. 5.1.2(6) three times with respect to
its external momentum.
its external momentum.
98
Renormalization
divergence, because the degree of divergence is negative. If / goes to infinity with k fixed, there is a divergence, but it is cancelled by the counterterm graph, Fig. 5.2.2. The remaining significant possibility is that both k and / go to infinity, but that k is much less than /. The ratios of different components of either one of k
or
/
are
finite,
so
we
contribution from this
may summarize the order of
region
of the
magnitude
as:
finite
AIL
dfcfe"
(5.2.13)
divergent contribution, if / is of order k3!2. We must add Fig. 5.2.2, which, as we will show, cancels this new divergence. Observe that the counterterm was arranged to cancel the divergence when / goes to infinity with k fixed, rather than when k is large, as we now have. Let us expand the inner loop of Fig. 5.2.1 in powers of k when / > k, up to its degree of divergence, which is quadratic. The coefficients of these powers are integrals over all /, restricted to / P- k. The divergences in the coefficients are cancelled by Fig. 5.2.2, and we have the following estimates of the sizes This gives
a
of the coefficients: coefficient of k°
finite<
=
finite<
=
0{k\
=
All
i;
coefficient of /c1
=
All
The
divergence P*
—
divergence
> +
divergence
>
Al/l
—
0(ln(/c)).
of Figs. 5.2.1 and 5.2.2 in the region k-* oo, with / than k, is then of order
sum
bigger
finite
All
0(k\
coefficient of k2= finite < =
—
dkk ̄2\n{k).
(5.2.14)
possibly
much
(5.2.15)
The power of k is the same as is given by the overall degree of divergence, but there is an extra logarithm. We get a finite result, as claimed. The higher-
than-quadratic terms in the expansion of the loop in powers of k give no divergence at all. What has happened? The divergence for l>k>\ could only occur
5.2
Two-loop self-energy
in
((/>3)6
99
because the interior loop was itself divergent. The fact that we sent k to infinity merely suppressed this divergence somewhat. Suppose we neglect k in the integral for the inside loop. Then the counterterm is in effect the negative of the integral over / of the loop from finite / to infinity. But in the region we are considering for Fig. 5.2.1, we are restricting / to be much bigger than fc, which is itself getting large. So if we neglect k in the loop, then we are
left with finite +
d/ (integrand of loop with k neglected).
(5.2.16)
Furthermore, we expand the loop in powers of k, to uncover the sub-leading divergences. Each extra explicit power of k in the expansion compensates for the lowering of the divergence. The quadratic term multiplies jd///, so
giving an extra logarithm (but not a power). The key step in the proof is to perform the integral with momentum / first. We have shown
whether
or not
a
divergence
the
larger
that, for the purpose of determining
occurs, we need
only consider
as
distinct
regions: (1) fc, / -? oo at the same rate, and (2) / -> oc with k finite. (We might also try k -? oo with / finite, but the subgraph with the lines carrying the loop momentum k has negative degree of divergence, so we get no divergence from that region.) The region fc, /-? oo, with k ^ /, is schizoid: it can be considered as essentially part of either of the two regions (1) and (2) that we
have just defined. As region (2), the divergence is cancelled by a counterterm when /-? oo, with k large but fixed. As region (1), the final integral over k is finite, and the only sign of this intermediate
case
is the extra
logarithm in the
integrand. 5.2.3 Fig. 5.1.3
by considering the example of Fig. 5.1.3. kid —6 the graph (a) quadratic divergence. It also has a logarithmic subdivergence when either of the loop momenta k or / gets large. The subdivergences are cancelled by vertex corrections, which are shown in graphs (b) and (c). We must prove that the overall counterterm (d) is quadratic in p. Conventionally, this graph is regarded as a difficulty in the theory of renormalization, for it contains an overlapping divergence. That is to say, one of the lines is common to both subdivergences. This is seen as a problem (Bjorken & Dr^ll (1966)) if one tries to write the graph as an insertion of a We conclude this section has
an
overall
renormalized vertex, Fig. 5.2.3, in the one-loop self-energy. The graph (b)
100
Renormalization p+
k
Fig. 5.2.3. Illustrating the problem of overlapping divergences, as
seen
in Fig. 5.1.3.
?■ ra>-oo: -
<i>-cixi> (*)
scek-jcd-
Fig. 5.2.4. Result of differentiating Fig. 5.1.3 three times with respect
to its external
momentum.
for the counterterm to one of the
subdivergences is not of this happen in our first example,
form. The
Fig. 5.1.2. of trick to with three times However, respect p works differentiating as well for Fig. 5.1.3 as it did for Fig. 5.1.2. For the sum of (a), (b\ and (c), we find Fig. 5.2.4. The point is that differentiating either of the subgraphs
corresponding difficulty
does not
our
makes it convergent, while the counterterms for the subdivergences are independent of momenta. We get terms (a) and (b\ which have re- normalized subgraphs, and graphs (c) and (d), which have no sub- divergences at all. None of the graphs has calculation of the overall counterterms is left
an
overall
as an
divergence.
The
exercise for the reader.
The correct result is (Macfarlane & Woo (1974)):
'?*-(£ 3*m if
we use
i
i
6(d-6Y
3(d-6)J*
-1,64;? '-(i&H"^-^}
minimal subtraction.
,"")
5.3 Renormalization
5.3 Renormalization of
We
have
seen
that, in order
101
of Feynman graphs Feynman graphs
to construct a sensible
(i.e., local)
counterterm
Feynman graph, one must first subtract ofifits subdivergences. This is natural since the subtractions for subdivergences are automatically generated from having the counterterms be definite pieces of the interaction for a (1 PI)
Lagrangian. Without subtraction of the subdivergences, the divergence of a graph need not be local. It may even have a power of momentum greater than the degree of divergence; an obvious case of this is a graph that is finite according to naive power-counting but that has a subdivergence. It is therefore useful to devise a procedure for starting with a basic Feynman graph G, constructing a set of counterterm graphs, and thereby obtaining a finite renormalized value R(G): R(G)=U(G) + S(G).
(5.3.1)
Here (7(G) is the 'unrenormalized' value of the basic graph (which diverges as the UV cut-off is removed), and S(G) is the subtraction the sum of the -
counterterm
graphs.
we use to construct S(G) is very general. It applies to the behavior of any integral as one or more parameters approach a asymptotic
The strategy
value. In field theory it can be applied not only to the renormalization problem but also to the calculation of the asymptotic
limiting
behavior of a Green's function
as some
but not all of its external momenta
get large. (A standard example which we will treat in Chapter 10 is the operator product expansion of Wilson (1969)). The procedure that we use for renormalization was first developed by
Bogoliubov and Parasiuk (see Bogoliubov & Shirkov (1980)), with corrections due to Hepp (1966). Their construction was recursive and has the acronym BPH. Zimmermann (1969) showed how to solve the recursion the result being called the forest formula. All these authors used -
zero-momentum subtractions. Zimmermann (1970, 1973a) showed moreover that there is then no need to use an explicit UV cut-off. He applied
the algorithm for computing R(G) directly to the integrand rather than to the integral; this formulation has the title BPHZ. It is not necessary to use subtractions. For example Speer (1974), Collins (1975b), Breitenlohner & Maison (1977a, b, c) showed how to make the same ideas work using minimal subtraction.
zero-momentum
Our treatment will aim at showing the underlying simplicity of the methods and their power to demystify renormalization theory. We will see that the methods do not depend on use of a particular renormalization
prescription,
even
though
we
will often
use
minimal subtraction.
102
Renormalization
a graph G. (A graph define by specifying its set of vertices and lines, each line joining two vertices and each vertex attached to at least one line.) We write the graph's
We will examine the structure of the subtractions for
we
unrenormalized
value
?^g(Pi
as
Pn)=
\d%...ddkLIG(Pl,...,pN;ku...,kL).
(5.3.2)
Here we let L be the number of loops and N be the number of vertices. The
external momenta at the vertices function there is
fields, but at
an
an
are p.. In a
Feynman graph for
a
Green's
external momentum at the vertices for the external
interaction vertex,
we
5.3.1 One-particle-irreducible
have p.
0.
=
graph with
no
subdivergences
The simplest case is a one-particle-irreducible (1PI) graph with no subdivergences. Then the only possible divergence is an overall divergence where the momenta renormalize the
all the lines get large simultaneously. We may by subtracting an overall counterterm:
on
graph
R(G)
(5.3.3)
U(G)-ToU(G).
=
some operation that extracts the divergence of U(G). It whatever renormalization prescription that we choose to use. implements For example, we might use minimal subtraction. In that case T takes the Laurent expansion of (7(G) about d d0, and picks out the pole terms. (We
Here T denotes
=
let the physical space-time dimension be d0; i.e., d0
d0
=
6 for the
c/>3
model
we
used in the
of two notations for the action of T or
T(G). Both will We
could
mean
the
=
4 for the real world,
previous sections.)
on an
We will
use
or
either
unrenormalized object: 7°[/(G)
same.
use zero-momentum
subtractions. In that
case T
picks
out the
5(G) in the Taylor expansion of (7(G) about zero Here 3(G) is, as usual, the degree of divergence. There are many
terms up to order momentum.
possibilities. In our work, we will use the minimal subtraction scheme. Then, for example, the one-loop self energy in (c/>3)6 gives
other
To
^ 2
=
,,6-d
ddk
(27i)d (k2-m2)[(p + k)2-m2]
pole part of dx
\^-i--T(2 (1287T
m2 —p2x(l
4nfi2
g2 (p2/6-m2) 64tt3 (d 6) -
"
—
-
x)
dfl)
x
ld/2- 3
[m2-p2x(l-x)] (5.3.4)
5.3 Renormalization The
one-loop vertex, Fig. 3.6.1, gives dk
fi9-^2
■MS
)' (k2
m2) [{p + k)2
-
pole part of
=
dx o
-W"2
\ —^f-3
( Jo
dy
m2
Observe that in this last
^3-d/2
-
-
m2] [(p + 9 + fc)2
T(3
q2xy
-
-
d/2)
-
m2]
x
(p2x + (p + q)2y)(l -x-y) 4nfi2
ld/2-3
gV""
.
part
103
of Feynman graphs
Y^jg js
an
exampie
of (7(G) to have the
0f
a
same
define the
pole to come with a factor rule that one must define the pole general dimension as (7(G), for all d.
case we
5.3.2 General
case
general we not only have to handle the case of an overall divergence, but also the case that subdivergences are nested within the overall divergences.
In
is exemplified by the propagator with two self-energy (Fig. 5.1.1), where within one graph there are two subgraphs which can diverge independently. As we saw from examples, we must subtract off subdivergences before finding the overall divergence. In view of the complications when the subdivergences themselves have subdivergences, etc. {ad nauseam), we must Another
case
insertions
be extremely precise is
helpful
to have a
mathematics.
as to
what is to be done. This is what
we
will
now
do. It
specific non-trivial example in mind, to make sense of the
Such
examples
are
treated in
subsection 5.3.3
and in
Section 5.4. The reader should try to read these sections concurrently with the general treatment in this section. First, let us define a specific divergence as being the divergence occurring
when
the
loop momenta on
a certain set of lines get
on other lines and the external momenta
given
set of lines has a
divergence
staying
big, with the
momenta
finite. Whether
or not a
associated with it is determined by power-
counting. A divergence is thus associated with a certain subgraph. (At this stage, we do not require that the subgraph be connected.)
diverges when all its lines get large loop momenta, it is said to divergence. A one-particle-reducible graph (like any of have an overall divergence some lines are not a part of cannot Fig. 3.2.1)
If a graph
G
have an overall
-
any loop. All other divergences involve a smaller subset of the lines. They, of course, overall
are
called subdivergences. Every subdivergence of
divergence
of
one
of its
subgraphs.
a
graph
is the
104
Renormalization
Fig. 5.1.1(a)
Observe that
subdivergences.
These
has
no
overall divergence, but has three
from the regions in the integration
over loop the left-hand has (1) loop large momentum, (2) the right- hand loop has large momentum, and (3) both loops have large momenta. To renormalize a graph G we assume that we know how to renormalize
come
momenta where:
its
subdivergences,
and
we
then let R(G) be the unrenormalized value of G
with subtractions made to cancel the subdivergences. Then the only remaining divergence that is possible is an overall divergence. So we define an
overall counterterm: (5.3.6)
C(G)= -ToR(G)
to R{G) the same subtraction operator T as we discussed if there is no overall divergence (e.g., if G is one-particle-reducible) earlier; then C(G) is zero. In any event the renormalized value of G is defined as
by applying
R(G)
=
R(G)
+
C(G).
(5.3.7)
The definitions (5.3.6) and (5.3.7) give us R(G) provided that we know how to subtract
subdivergences.
This is essentially
a matter
of renormalizing
smaller graphs; we will construct R(G) in a moment. Once this, we will have a recursive definition of R(G): successive
we
have done
application of us to smaller and to smaller (5.3.7) graphs with subgraphs ultimately brings no subdivergences. These we know how to renormalize. Now let us define R(G\ which is to be U(G) with subdivergences subtracted. For the case of a graph with no subdivergences we must define R(G) For
a
larger graph
=
we
U{G)
(if G has
no
subdivergences).
(5.3.8a)
define R{G)
=
U(G)+
(5.3.8b)
X Cy(G).
We sum over all subgraphs y of G, other than G itself, as indicated by the notation y £ G. The other new notation Cy(G) means that we replace the subgraph y by its overall counterterm, as defined by (5.3.6), i.e., f ^
To make only
over
could of
—
[0
ToR(y),
if
y has an overall
if y has
no
divergence
overall divergence
(5.3.9)
simple formula, we write the sum as being over all y's rather than divergent y's; then the Cy(G) is zero if y is not overall divergent. We course restrict the sum only to those subgraphs that have an
a
overall divergence. One tricky point in the above equations arises in defining C(y) for a disconnected subgraph y. An example is the subgraph of Fig. 5.1.1(a) consisting of the two self-energy loops. We will discuss this next.
105
of Feynman graphs
5.3 Renormalization
Application of general formulae
5.3.3
Equations (5.3.6) to (5.3.9) give a definition of R(G). Let us see how they apply to simple examples. For a 1PI graph with no subdi vergences they just reproduce R(G) (7(G) To U(G). Next consider a graph like Fig. 3.2.1(a) or (c), whose only divergence is a subgraph with no further subdivergences. Then there is no overall divergence, so by (5.3.7) =
-
R(G) There is only
one
subdivergence,
=
(5.3.8) collapses
so
R(G)=U(G) where y is the
(5.3.10)
R(G).
divergent subgraph.
+
to
give
(5.3.11)
Cy{G)9
Here
is the full
Cy(G)
graph
with y
To U(y). We reproduce the obvious result. There is one graph like Fig. 3.2.2(a) or (c). We now look at a graph with two or more divergent subgraphs which do not intersect and which have no subdivergences. It is sufficient to consider G to be Fig. 5.1.1(a). There is no overall divergence, so again R(G) R(G). Let y1 and y2 be the self-energy bubbles. Then the subdivergences correspond to the three subgraphs yl9 y2, and yl^jy2. (Here yx uy2 means, as usual, the union of yx and y2.) So
replaced
by
-
counterterm
=
R(G)
=
R(G)
Evidently Cyi(G) —
=
U(G)
+
C7l(G) + CJG) + CluT2(G).
(5.3.12)
is just
U(G) with yx replaced by its counterterm, To [/(yj; and similarly for y2. But what is C, (G)? It corresponds to a subtraction for y} \jy2 for the region where all loop u.,
momenta are
large.
regions where only
C(y^y2)= Here
we
But
we must
subtract from it the counterterms for the
one momentum
-
is large:
To[U{y1)U{y2) + C(yl)U(y2)+ C/(yi)C(y2)]. (5.3.13)
used the fact that yx\jy2 is disconnected,
U(y1^y2) To work out (5.3.13),
we must
to act independently
on
To[U(yi)U(y2)] To[C(yi)U(y2)]
=
so
that
(5.3.14)
U(yl)U(y2).
define T when
acting on a disconnected graph
its components. Thus: =
=
[Tot/(yi)][Tol/(y2)] [ToC(yi)][To t/(y2)]
=
=
C{y,)C{y2l -
C{y,)C(y2\
(5.3.15) (5.3.16)
etc. We used the property that To
To
U(yt), i.e.,
the
pole part
C(y ■). Furthermore, To [To U(y ?)] U{y() a pole part is itself. We therefore find
of
=
-
=
106
Renormalization
that
(5.3.17)
C(y1uyJ=[ToU(y1)-]lT°V(y2)l so
that
we
reproduce the
counterterm
The above procedure generalizes to
excessively complicated, but
graph Fig. 5.1.1(d). arbitrary graph.
It may appear it allows the smoothest way of defining R(G). an
we observe that our definitions (5.3.6)—(5.3.10) exactly reproduce results for the two-loop graphs like Figs. 5.1.2 and 5.1.3.
Finally, our
5.3.4 Summary In this section
we
have proved very little. We have set up
a
series of
definitions that state exactly what we mean by the renormalization of a Feynman graph. The notation we have introduced will be important in
making proofs. What we will need to prove is that the overall counterterms are local and of a degree in momentum given by naive power-counting. We will also show how to solve the recursion to find an explicit formula due to Zimmermann (1969). 5.4 Three-loop example
three-loop self energy graph of Fig. 5.4.1 in 03 theory in six dimensions is an example of a graph with nested and multiply overlapping divergences. We call it G. Its divergent subgraphs are: The
yx
=
y2
=
y3
=
y4
=
ys
=
{lines {lines {lines {lines
carrying carrying carrying carrying
loop loop loop loop
momentum momentum
momenta k momenta q
fe}, q}, and/or /}, and/or /}, (5A1)
yivy2>
The first four of these are connected IPI vertex
two unconnected vertex
graphs;
the last is
a set
graphs. According to our definitions of Section
of
5.3
we have
R(G)
=
=
R(G)
+
C(G)
R(G)-Tc[R(G)l
Fig. 5.4.1. Three-loop self-energy graph in 03 theory.
(5.4.2)
107
Three-loop example
5.4
This
equation states that we first subtract subdivergences and then take off the overall divergence. To define
R(G)
subtract
we
R(G)
=
this
equation
G+
i Cyi(G)
subgraphs
(5.4.3)
?Cfo)
5.4.2. The notation with the vertical bar in
we take G and replace y. by the corresponding figure, the labels 1,...,4 signify which of the been replaced by its counterterm.
denotes that
C(yt).
counterterm
Fig.
as
R(G\
subdivergences:
-G+IG| We represent this
to obtain
In the
y!,..., y4 has
R(G)
=
+ '4
1
Fig. 5.4.2. Subtraction of subdivergences of Fig. 5.4.1. Only yl and y2 have
C(y J and C(y2) are the But we have still to define C(yt) for i 3,4,
further subdivergences,
no
ordinary one-loop counterterms.
so
—
5: C(y3)=
C(y4)
c(y5)
=
=
=
-^bs-ysly^ny^l To[y4 y4|y2_T(V2)], -
-
\yxy2 T{yi)y2 [-Tf(yi)][-r(ya)]. -To
-
-
yt
T(y2) ̄\ (5.4.4)
by combining (5.4.2)-(5.4.4). If we represent 1 PI graph by enclosing it in a box, we can write
The overall result is obtained
the effect of applying T to
a
as shown in Fig. 5.4.3. There are sixteen terms in all. The first eight represent (7(G) minus its subdivergences, and the last eight form the subtraction for the overall divergence. The expansion of R(G) represented in
R(G)
Fig. 5.4.3 is
an
example of
the forest formula, to be discussed in the next
section. As
we saw
by examining two-loop graphs,
the overall counterterm for
a
first subtract off subdivergences. Otherwise graph there would be divergent contributions from where some but not all is non-local unless
we
108
Renormalization
R(G)-
<n> (?)
(b)
_M-
(d)
(e)
(f)
(9)
(A)
!L
| Previous 1 18 graphsj Fig.
5.4.3. Renormalization of Fig. 5.4.1.
subgraphs have large loop momenta. There would also be divergent contributions from the region where all loop momenta get large but at Fig. 5.4.3 explicitly.
different rates. We can check from
for R{G). Let
occur
Let
us
us
do this
that
none
of these
problems
show that the overall counterterm for G is local. We differentiate
R(G) three times with respect to the external momentum p and show that the result is finite. Given the momentum routing of Fig. 5.4.1, there are three lines to differentiate: p + fc, p + /, p + q. Here we have used the momentum by the line as a label for the line. Differentiating the original graph
carried
gives
ten terms, where the three derivatives are
applied
to any combination
of the three lines. One term is where we differentiate p + k three times
5.4.4). Although
(Fig.
overall divergence, there remain sub- examine the corresponding differentiations applied
there is then
no
divergences, so we must to the counterterm graphs (b)-(h). We must regard the derivatives as acting on these graphs before divergences are computed (by the operation
symbolized by
the box).
subgraphs yx and y3 completely finite, by divergence and y3's only subdivergence. Hence
The differentiation makes the
removing
both their overall
Fig. 5.4.4. One of the terms obtained by differentiating Fig. 5.4.1 three times with respect to its external momentum.
5.5 Forest formula
109
the counterterm
graphs (b\ [d\ (e\ and (h) are zero after differentiation. This graphs (c), (/), and (g). These cancel the subdivergences of (a) coming the subgraphs y2 and y4, which are unaffected by the differentiation.
leaves from
We may examine the other nine terms in d3R(G)/dp3 find that in fact d3R(G)/dp3 is finite, as claimed.
similarly,
and
we
5.5 Forest formula 5.5.1 Formula Zimmermann (1969, 1970) gave an explicit solution of the recursive definition of the renormalized value R(G) of a graph G. The general idea can be gleaned from the example we examined in the previous section. There the recursion generated a series of sixteen terms. One was the original graph, and the others had the subtraction operation T applied one or more times.
example, in graph (e) we first replace the left-most loop y x by T(y J, with a result we can write as Tyi(G). We then take the subgraph equivalent to y3, it viz. and replace by the result of acting with T. This gives graph (e). Tyi(y3\ The sum of the two graphs (d) and (e) is used as the subtraction for the subdivergences of G associated with y3. Each of the sixteen terms is pictured as the original graph with some number of connected IPI subgraphs enclosed in boxes to indicate application of T to the subgraph. Each term can be specified by giving its set of boxed subgraphs. Each such set is called a for est. The subgraphs which form a particular forest are either disjoint or nested: they are said to be non- For
overlapping. There set
are
The set of all
called #"(G). The first
eight
are
(in
theory notation):
(a) the empty (c) (d) (?) if) to) (h)
possible forests for G is occurring in Fig. 5.4.3.
sixteen forests
set
0,
{y2}, {y3}, {Vi.Vs}. (U {^.yJ. {y?ya}.
These do
not contain the whole
graph; they
are
called the normal forests.
The other eight forests in Fig. 5.4.3 consist of one of the above eight forests to which is added as another element the complete graph G. A forest of G
110
Renormalization
containing
G is called
a.
full forest. The distinction between
normal and full
forests is that the normal forests subtract off the subdivergences, and the full forests combine to subtract the overall divergence. Not all forests U
occur
in Fig. 5.4.3; for
{subgraph consisting
=
example, the
of lines carrying loop momentum
does not appear. Such forests contain at least
subgraph
as an
forest
one
/}
overall convergent
element.
Inspection of Fig. 5.4.3 shows that
I
*(G)=
(5.5.1)
ri(-r,)G.
C7e.^(G) yeU Here
the
sum
is
over
all forests U of G. The operator
Note that for nested
Ty replaces y by T(y).
y's the 7,,'s should be applied inside
to outside.
Equation (5.5.1) is called the forest formula; it is due to Zimmermann (1969). Suppose we compute R{G) for an arbitrary graph G by using (5.5.1). Then,
will prove shortly, the result is the recursive definition of R(G) given in Section 5.3. as we
It is convenient to let the
sum over
forests be
over
if
we
used the
all forests rather than
superficially divergent; the reason for doing this is that we
of subgraphs that
those
only consisting extra forests give a zero contribution. The over
same as
are
will sometimes wish to change the definition of T so that we make subtractions for some convergent graphs, as well as for divergent graphs.
example, such
redefinition will be the key to proving the operator product expansion in Chapter 10. We now have both a recursive and a non-recursive definition for the For
a
renormalization of a Feynman graph. It will prove very useful to have both proofs will need different forms of the
definitions available. Different
definition. In particular, proofs
graph
will naturally
use
by induction
on
the number of loops of
a
the recursive definition.
5.5.2 Proof The proof of the forest formula is elementary, but somewhat involved. We first use (5.5.1), and the following equations:
C7e^(G) yeV
C-TGoR(G\ C(G) =< n[c(ft)]> (0
'
*)
if G is 1 PI,
if G is
a
disjoint union of
otherwise,
1PI
yt%\ (5.5.3)
J
5.5 Forest formula
111
R(G), and C(G). Here #(G) is the set of normal forests (i.e., those that do not contain G). These definitions are correct for the graph Fig. 5.4.1, as can be seen by inspection of Fig. 5.4.3. Since the recursive definitions uniquely give R(G\ R(G\ and C(G) in as definitions of R(G\ of G
operation Ty, it suffices to show that (5.5.1)—(5.5.3) satisfy the (5.3.6)-(5.3.9). Notice first that (5.3.6) and (5.3.9) are the same, really except for being applied to different graphs. If R{G) given by (5.5.2) is correct, and if subgraphs are correctly renormalized, then (5.5.3) is equivalent to our original definition (5.3.6) of terms of the
recursion relations
C(G). Moreover, suppose that G is connected and one-particle-irreducible. Now each forest of G is either a normal forest, that is, a forest of which G is not
element,
an
or
it is
a
adjoined G. Then the of (5.5.1)—(5.5.3) for such a
normal forest to which is
(5.3.7) for R(G) is a direct consequence graph. If G is not a union of 1PI graphs, then there is no overall divergence,
formula
and again (5.3.7) holds. So it remains to prove the
following:
(1) R(G) is correct when G is a disjoint union of more than one 1PI graph. (Note that this case occurs in renormalizing the graph of Fig. 3.2.1(h), as in Section 5.3.)
we saw
(2) R(G) is correct, i.e., it satisfies (5.3.8), for
If
G is
forests,
disjoint
a
one
general graph.
a
union of 1PI graphs y., then each forest is
for each component. Then R(G)
=
[ ̄] R(y.),
as we
a
union of
should expect.
i
The
problem
where
we
is that this is not
make
an
manifestly
definition,
overall subtraction for G. We dealt with this problem
between (5.3.11) and (5.3.17). Our proof of (5.3.8) is by induction
graph has
true in the recursive
on
the size of G. Now
a
one-loop 1PI
non-trivial subgraphs, so its only normal forest is the empty set. Then formula (5.5.2) collapses to R(G) U(G\ just as it should. This no
=
enables
us to start
the induction.
(5.3.Sb). For this, observe that each forest U has a biggest subgraphs Ml9...,Mj. Each M, is contained in no unique bigger subgraph in U, and each y e U is contained in some Mt. The existence and uniqueness of this set of M's is seen by considering pairs y., yk of It remains to prove set of
elements of U.
Since y. and yk are
non-overlapping,
there
possibilities: (1)
y.
(2) yk
(3)
c
yk, in which
a
y., in which case remove yk from further consideration.
y.nyk
=
case remove y.
0, in which
case
from further consideration.
leave both in.
are
three
112
Renormalization
Repeat until
no
further eliminations
possible; then the result is the set of
are
Mf's. The forest U is the union of a full forest,
definition (5.5.2) of R(G)
R(G)
=
.
Here the first term
Z
I
??
our
x
(-Tn)-U(-Tyj]Gl
n yi
comes
for each Mt. We can write
\f\(-TM)
I
G+
\
one
as
from the case in (5.5.2) that U
=
(5.5.4) 0, and the sum in
the second term is over non-empty sets of disjoint 1PI graphs M{ excepting G. By setting y the case that Mt M1u...uMn and using (5.5.3) to =
determine
C(M{
=
u... u
Mn\
we
find (5.3.%b).
5.6 Relation to if We have
how to renormalize
making doing this came from consideration of examples in which the subtractions were generated by counterterms in the interaction Lagrangian. We will now show that this is a
seen
series of subtractions.
The
an
individual Feynman graph by
motivation for
true to all orders. We will
assume the natural result (to be proved later) that the polynomial degree of the overall counterterm of a graph is given by its
degree of divergence, just as for low-order graphs. First, we must make precise the result that we will prove. For each graph G, we have constructed its overall counterterm C(G). Since this polynomial
in the external momenta of G, it
derived from
N(G) is
a
an
can
be written
as
1PI is
a
the vertex
interaction term D(G)/N(G) in the Lagrangian j£f. Here same sort as the 3! that appears with the 03
symmetry factor of the
interaction term in if. Each power of
corresponds point graph and
entering D(G) corresponding field. If G is an n- lines corresponds to the same type of a
momentum
to i times a derivative of the
each of its external
field, then N(G) is n\. If there number of lines of type i
are a
entering
number of different fields and
n.
is the
G then
iV(G)=n^!-
(5-61)
i
graph for a Green's function, the forest formula gives a set of counterterms. We will demonstrate that the set of counterterm with graphs is generated from the counterterm vertices in the interaction graphs For each
Lagrangian. Consider '(f)3 theory. if
=
As before, jsf 0
+
seh
we
write the
+ <? ct
=
se 0 +
Lagrangian <ev
as:
(5.6.2)
113
5.6 Relation to ¥£ The free
Lagrangian SeQ
=
(d(j))2/2
(5.6.3)
m2(j)2/2
-
generates the propagator, while the interaction !£x consists of two terms, ifb and ifct. The basic interaction is
(5.6.4)
ifb=-^3 ̄d/2(/>3/3!, and
ifct is the divergences:
counterterm
Lagrangian
used to cancel the ultra-violet
(5.6.5)
<?ct=ZD(G)/N(G). G
sum
is
all 1PI
graphs. Those that have no overall divergence for these D(G) 0. Each 1PI graph that has an counterterm; generate overall divergence generates a term in (5.6.5). The formulae (5.6.2) and Here the
over
no
=
(5.6.5) apply in any theory. Since (5.6.5) applies to any than
six
dimensions.
theory, it applies in particular to 03 in higher
Thus
it
renormalizable theory. But the
enables
sum must
us
renormalize
to
a
non-
include counterterms D(G) with
arbitrarily large number of powers of momentum and with an arbitrarily large number of external lines for G. It is only in six or fewer dimensions that
an
the counterterms have the if 0
+
same
form
as terms
in the basic
03 Lagrangian
£ch.
Now that
part of the
we
have
proof is to
developed
We will prove that the
a
convenient notation, the most difficult
that the combinatorial factors
come out right. defined and by (5.6.2) (5.6.5) gives the Lagrangian
ensure
same renormalized Green's functions as those generated by our recursive definition in Section 5.3 (and therefore the identically same Green's functions as given in Section 5.5 by the forest formula). The proof will be
given for
03 theory
in six
or
fewer dimensions, but it easily generalizes.
Consider the full AT-point Green's function GN at order gp. It is sufficient only with connected graphs. If the theory is renormalizable (as we
to work
will prove in Section 5.7), then the Seci
=
SZ(d(t>)2/2
-
sum
of counterterms has the form:
dm2(j)2/2
-
Sg(f>3/3!.
(5.6.6)
with (by (5.6.5)) -3Z=
£
[Coefficient
£
[Coefficient of ip° in C{G)\
£
C(G)/i.
2-point G
-dm2=
2-point G
-5g= We
ignore
the
3-point O*
of
tadpoles, yet again. The term
-
ip2
in
C{G)\
(5.6.7) of order
gp in the perturbation
114
Renormalization
expansion of GN has vertices generated by the different terms in if b -I- if ct. There will be graphs with all of their vertices being the basic interaction if b. Let the set of these be called B. The other graphs will contain one or more of the counterterm vertices generated by (5.6.6). Each counterterm
decomposed into
a sum
of terms
by applying (5.6.7)
vertex. Each term has each of the counterterm vertices
divergence of some graph. corresponds to a unique basic graph overall
So
we
=
replaced by
Then in the result, each term
the
T
b(T)eB.
Z(g+ G
On the other hand, G
then be
have
G?
graphs
can
at each counterterm
we
\
I
b(T)
=
G
A
(5.6.8)
/
have constructed the renormalization of each of the
by writing R(G)
Each of the terms
=
£ Cy(G).
G+
(5.6.9)
(5.6.8) is constructed by replacing each of a set of one or more disjoint 1PI subgraphs yl9...9yj by its counterterm given by On in (5.6.9) with identifying y iDy{G). ylvy2"'vyp we expect that Tin
G?=EK(G).
(5.6.10)
G
This result would be obvious,
were it not that the symmetry factors do not match manifestly up. The problem is illustrated by Fig. 5.6.1. There the basic graph is (a\ and the complete set of subtractions needed to renormalize it consists of (b)-(e).
Now the symmetry factor for (a) is 1/8: There is a factor 1/2 for each self- energy graph and an overall 1/2 for the top-bottom symmetry of the whole
graph.
Each of the subtractions
(b)
and
the remaining factor 1/2 goes into the
graphs (b) and (c)
are
(c) has
a
symmetry factor 1/4, since
counterterm for the
self-energy.
equal.
(a)
(6)
o-o
w
M)
(c)
Fig. 5.6.1. Renormalization of
-*
a
graph,
to illustrate symmetry factors.
Both
5.6 Relation to<£
115
Considered as Feynman graphs, these are the same graph, but with symmetry factor 1/2. So they give one term with factor 1/2 in (5.6.8) (derived from the Lagrangian), while in (5.6.9) (from the recursion formula) there are two equal terms with factor 1/4. The end result is the same. We must
consider
(b)
(c)
and
distinct graphs when defining R(G\ since they we must take
as
correspond to different regions of loop-momentum space each momentum variable to be distinguishable.
general proof is tedious. The symmetry factor of a graph G 1//V(G), where N(G) is the dimension of the graph's symmetry group. So To construct
is
-
we
a
write G
where the overbar indicates
Similarly
we
define C(G)
=
computation ignoring all symmetry factors.
by C(G)
=
=
G+
we
graph
G is
Y Cy(G)
N(G) In the last line
(5.6.12)
C(G)/N(G).
Now the renormalized value of a
R(G)
(5.6.11)
G/N(G\
N(G)
n*.
C,(G)
a disjoint union of 1PI graphs y,, explicitly indicated the symmetry factors 1/Nt yt which is replaced by its overall counterterm. For a u y( the symmetry groups of the y;'s are a commuting set
have observed that y is
y2,.... Moreover, we have
for each
(5.6.13)
l/N(yt) given subgraph y of subgroups of
=
=
JV(G)/J ̄[ AT,
the symmetry group of G. Therefore the quantity
must be an
integer.
i
Next, consider the Green's functions generated by the Lagrangian (5.6.2), as
in (5.6.8),
GN=Y<-^— G+Y'—-—C (G) 1.
(5.6.14)
graph containing one or more counter- terms is generated from a basic graph by replacing some 1PI subgraphs y l,..., yi by counterterms. We write y as the union of the yf's. Then we let G/y be the graph resulting from substituting counterterms for the yf's. By the Here
we
have observed that each
116
Renormalization
Lagrangian, the result
definition of the counterterm
is the
same as
Cy(G),
aside from symmetry factors. The prime on the £' indicates that only y's giving distinct Feynman graphs are considered. (Thus, for example,
Figs. 5.6.1(h)
and
K(G, y) then
(c) =
are not
[number
we must prove
counted
separately.) Thus, if
of graphs / for which G/y
=
we
define
G//],
that X(G,y)
=
M
p
[m
(5.6.15)
.
[N(G/y)l
language of group theory. The of the right-hand side of (5.6.15) is the product of dimensions denominator of commuting subgroups of the symmetry group of G. (Note that, for It is easiest to couch this final step in the
example, two (ft3 counterterms generated by different self-energy subgraphs are counted as different.) These subgraphs generate another subgroup, of which the set of cosets in the symmetry group of G has exactly the dimension of the right-hand side of (5.6.15). But, concretely, each coset corresponds to one of the graphs counted by K(G, y).
5.7
Renormalizability
5.7.1 Renormalizability and non-renormalizability
explain the properties of renormalizability, non- renormalizability, and super-renormalizability of a field theory. We do this In this section
we
first for every order of perturbation theory, and then we consider to what extent the properties are true beyond perturbation theory, for the complete
theory. The method in perturbation theory is power-counting dimensional
analysis.
Consider first
and
(ft3 theory in a space-time of integer dimension d0. We have
how to renormalize it to get finite Green's functions by adding counterterms (5.6.5) to the Lagrangian. Each counterterm is a polynomial seen
in the field
(ft
and its derivatives. The theory is called renormalizable if the are proportional to the terms (d(ft)2, (ft2, and (ft3
only counterterms needed
present in the original Lagrangian if 0 that the
Lagrangian
ifb. This is equivalent
to
saying
has the form
if
=
(d(ft0)2/2
-
m2<ft20/2
-
0o</>o/3!,
(5.7.1)
(ft0 is Z1/20. The bare mass m0, the bare coupling g0, and Z each have singular behavior as the renormalization field-strength
where the bare field the
-f
5.7
Renormalizability
117
regulator is removed. A linear term Sh(f) is needed as well. We as being present in the original Lagrangian. In any event it is a only single extra coupling. It can be ignored if we impose the renormalization condition that <0|(/>|0> =0 to determine 8h, and if we ultra-violet
may regard it
ignore tadpole graphs. We generalize to an arbitrary theory by calling a theory renormalizable if the Green's functions of its elementary fields can be made finite by rescaling the fields (in a cut-off dependent way) and by making some suitable cut-off dependent change in the couplings and masses. In perturbation theory we determine whether
or
we
not
have
renormalizability by examining possible values of the degree of IPI for the For divergence 3(G) graphs. every graph G a counterterm is needed if the
3(G) >0. As we will prove in Section 5.8 the counterterm C(G) is polynomial of degree 3(G) in the external momenta, and provided we use
a
a
scheme like dimensional regularization that preserves Poincare invariance, the counterterms are Poincare invariant. Let
us
now
determine whether
or
not
<j)3 theory
space-time dimensions momentum (in space)
dimensions is renormalizable. In d
graphs
have dimension
d(GN)
=
N+
3(GN)
d + (1
-
the
space-time N-point IPI
d-Nd/2.
Then (by (3.3.12)) the degree of divergence of a graph =
in d
d/2)N
+
(d/2
-
for
GN
at order
gp is
(5.7.2)
3)P.
Note that the minimum value of P to have a one-loop connected graph is N. Inspection of (5.7.2) shows that if d > 6 then, for any value of N, there can be made AT-point graphs with arbitrarily high degree of divergence by going to large enough order in g. The theory is therefore not renormalizable if d >
6, and the non-renormalizability is
a
direct consequence of the negative
dimension of g.
'-?-0—O—CD
'-'-0—£)--<D-' and tadpole graphs to 4 loops
Fig. 5.7.1. All the graphs with overall divergences in <j)3 theory at dimensions where it is super-renormalizable.
those
space-time
118
Renormalization
6, only the one-, two-, and three-point functions are divergent, with degree of divergence 4,2, and 0, respectively. The permissible counterterms are just terms of the form of those in j£?0 + j£?b, so the theory is renormaiizable if d 6. Moreover, there is a divergence in every order of g (except for tree graphs, of course). ltd
=
=
3, 4, or 5, then only a finite set of graphs, illustrated in Fig. 5.7.1, have overall divergences, and renormalization is needed only for the mass and for the tadpole coupling. Again we have renormalizability. If d
=
5.7.2 Cosmological term Strictly
speaking,
we
should also consider Feynman graphs with
no
bubbles. They generate the energy of and the vacuum, density normally are ignored. But in gravitational be cannot physics, they ignored. Counterterms for such graphs (present in external lines. These
are
(j)3 theory whenever d
>
the
vacuum
2) are proportional to the unit operator. They are a cosmological constant. A counterterm is even needed for free-field theory where the divergence is conventionally removed by normal-ordering (see, for example, Bjorken & Drell (1966)). We see that normal-ordering is nothing but a primitive form
renormalization of what in General Relativity is the -
of renormalization.
5.7.3 Degrees of renormalizability It is convenient to
(1) Finite:
distinguish
no counterterms
three types of renormaiizable theory:
needed at all.
(2) Super-renormalizable: only
a
finite set of graphs need overall counter-
terms. (3) Strictly renormaiizable: infinitely many graphs need overall counter- terms. (But note that they only renormalize a finite set of terms in the basic Lagrangian, since we assumed renormalizability of the theory.) or super-renormalizability normally occur when all the in the basic Lagrangian have positive dimension. couplings Note that in a super-renormalizable theory, the number of divergent basic graphs is infinite. For example, even if there is only one graph y with an overall divergence, any graph containing y as a subgraph is divergent.
Finiteness
However all such graphs become finite after adding to y its counterterm,
so
one counterterm, C(y), appears in the Lagrangian. Mathematical physicists (see Glimm & Jaffe (1981)) have investigated renormalizability beyond perturbation theory. This is important, since
only
5.7
perturbation series
119
Renormalizability
in general
asymptotic series rather than convergent the perturbation series to obtain the Even has it been proved for many super- so, complete theory. renormalizable theories that perturbation theory gives an exactly correct account of the divergences. (A much investigated case is 04 theory in two and in three space-time dimensions.) In a super-renormalizable theory, the series for a bare mass or coupling in terms of the renormalizable quantities has a finite number of terms. Therefore the series converges, and one only has to prove that (a) perturbation theory is asymptotic to the true theory, and (b) there are no terms like exp ( l/g) in the bare masses or couplings that are smaller than series. Thus
are
one cannot
simply
sum
—
any power of g. The rigorous proof amounts to showing that in summing the perturbation series to a finite order, the error is correctly estimated by the first term omitted. In particular, this
applies to the existence of any ultra-violet possible divergence. Rigorous proofs are not yet available for any strictly renormalizable
theory. One difficulty is obvious: the series for, say, the bare coupling, g, is an infinite series, each term of which diverges as the UV cut-off is removed.
presumably asymptotic rather than convergent, one obtain directly any information about renormalization in the full error obtained in using a truncated form of the series is of the theory: the Since the series is
cannot
order of the first term omitted, and that is always divergent. It might even appear that perturbation theory has no light at all to shed on the as we
of renormalizability of the full theory. This is in fact not so, when we discuss the renormalization group in Chapter 7. If
question will
see
the theory has the property called asymptotic freedom then a series of suitable redefinitions of g allows short-distance phenomena to be computed
reliably.
In
particular the
UV
divergences
can
be
computed
in terms of
weak coupling series without divergent coefficients. It is sensible to conjecture that a suitably refined analysis can be made to obtain rigorous bounds of the errors so that the perturbative results correctly give the divergences. Monte-Carlo studies of the functional integral (Creutz & Moriarty (1982)) support this conjecture. In four dimensions, only certain
non-abelian gauge theories (including QCD) are asymptotically free (Gross
(1976)). We will also
04
see
in
Chapter
7 that in
non-asymptotically
free theories,
and QED in four dimensions, perturbation theory cannot reliably short-distance phenomena. There are, in fact, indications (Symanzik (1982)) that the full c/>4 theory is not renormalizable, contrary to the situation order-by-order in perturbation theory. like
describe
120
Renormalization 5.1 A
For theories which
are not
(1) There is only
a
perturbation theory, there are following:
renormalizable in
possibilities. Among
many
Non-renormalizability them
are
the
finite set of 1PI Green's functions which have overall
divergences, A typical
dimensions when
=
in six
or
fewer
space-time
Lagrangian,
the basic
<*?
03 theory
is
case
(d(j))2/2
-
m2cj)2/2
-
(5.7.3)
#3/3!,
linear in <\>. The one-, two-, and three-point functions have divergences, but there is no term h<\> whose coupling can be re- has
no term
normalized to cancel the divergence of the tadpole graphs. However, addition of such a term generates a renormalizable theory. More
generally, suppose
we
have
functions. A renormalizable
a
overall-divergent Green's generated by adding a finite set of
finite set of
theory
is
extra interactions. (2) There is
an infinite set of Green's functions with overall divergences. However, for all but a finite set of the Green's functions, the divergences cancel after summing over all graphs of a given order. (There are no known cases of this.) case 2, except that the divergences cancel only for the S-matrix, rather than for all off-shell Green's functions. An important case is a
(3) As for
spontaneously broken
gauge
theory, when
it is
quantized
in its
unitary
gauge.
(4) The theory is made renormalizable by going beyond perturbation theory in some systematic and sensible way. One case (as in the Gross- Neveu (1974) model see Gross (1976)) is of a theory that is strictly renormalizable and asymptotically free for some dimension d d0, and -
=
that is considered in
(5)
As for case
some
dimension d slightly greater than
is the Yang-Mills theory with extra terms destroy
(6)
d0.
1, except that the extra terms make physical nonsense. A case a mass term
in Feynman gauge. Then the
unitarity ('t Hooft (1971a)).
None of the above.
Roughly speaking, there are no general rules.
Each
case must
be handled
separately. Only for the last two cases (5 and 6) should a theory be called non-renormalizable. A fundamental theory should be renormalizable, for
actually infinite or they are finite, infinite set of parameters is needed to specify the finite parts of the
otherwise either
but
an
physical quantities
are
counterterins. Nevertheless,
a statement
that
a
particular theory is non-renormalizable
5.7
121
Renormalizability
is really a statement of ignorance: nobody has found a way to construct a physically sensible version of the theory. (Cases 1 to 5 are where somebody has found a way.) In practice, when a theory is labelled non-renormalizable, what is usually meant is that the theory is not renormalizable order-by- order in perturbation theory; such a statement can be proved by calculating a
finite number of graphs. Within the usual functional-integral
only has the complete 3, but it has been
04 theory
approach (with a lattice cut-off), not 2 and been proved renormalizable for d =
proved non-renormalizable
5.7.5 Relation of renormalizability
for d
> 4
to dimension
(Aizenman (1981)).
of coupling
perturbative renormalizability of a theory of scalar fields, we the generalize argument leading to (5.7.2). The argument will apply when no has coupling negative dimension. Renormalizability will hold with possibly the addition of extra interactions (like the h<\> term in 03 theory) whose coefficients have non-negative dimension. Our proof will easily generalize to theories with fermion and gauge fields. The problems we will encounter in gauge theories will all be to do with the question of whether these extra terms are compatible with the gauge invariance. But we will leave these questions to Chapter 12. Let a general term in <£ or j£?ct be written schematically as To prove
(coupling /')(derivative)-4 (field)*.
(5.7.4)
The vertex generated by this term is one possible graph for the 1PI Green's TN with N external lines. Thus the dimension of TN satisfies
function
d(TN)
=
d(f)
(5.7.5)
+ A.
Since coupling has negative dimension, the degree of divergence of any graph for TN is at most d(TN), as we saw from examples in Section 3.3.3, and as we will prove in Section 5.8. That is, the degree of divergence d(TN) satisfies no
(5.7.6)
d(rN)<d(rNi with equality only
graphs all of whose couplings have zero dimension. N-point graphs, we add counterterms of the form S(TN) derivatives. So the possible counterterms satisfy
for
To renormalize the (5.7.4) with at most
d(f) The last
=
d(TN) -A> 5(TN)
-A>0.
inequality follows since a counterterm with A
(5.7.7)
derivatives is needed
only if the degree of divergence is at least A. From (5.7.7) it follows that
we
122
Renormalization
need
no
couplings of negative dimension, given
original Lagrangian. Some of the couplings generated the
original Lagrangian dimension. But the number
even
because only
of
none are
present in the
be present in couplings of negative
as counterterms may not
if it contains
new
that
couplings
no
needed is nevertheless finite,
finite set of counterterms satisfy (5.7.7).
a
5.7.6 Non-renormalizable theories of physics From the discussion above, it is natural to conclude that
a
theory of physics
should be renormalizable. In fact, the strong, electromagnetic, and weak interactions appear to be described by a renormalizable theory. This theory
is
a
combination of quantum chromodynamics for strong interactions and
the Weinberg-Salam theory for weak interactions. Around 1970 there was a revolution in the theory of weak interactions when it was discovered that non-abelian gauge theories are renormalizable. It is precisely one of these theories that was found to be necessary to
construct
a renormalizable theory of weak interactions in agreement with experiment. See Beg & Sirlin (1982) for a historical review.
Unfortunately, this progress has not extended to gravity. Einstein's theory of general relativity is non-renormalizable, after quantization, and alternative. (This situation exists despite many to significant attempts improve it Hawking & Israel (1979).) It is a mistake to suppose that non-renormalizable theories should be there is
no very
promising
-
banished from consideration. interactions
were
Remember that for many years weak the four-fermion' theory,
successfully calculated using
which is non-renormalizable. For most purposes, weak interactions could be adequately treated in the lowest order of perturbation theory, where no
renormalization is needed. But the non-renormalizability of higher-order calculations raised the question of consistency of the theory: is it legitimate even an approximation to a nonsensical (i.e., non-existent) the results of calculations mean anything? The same questions Will theory?
to calculate
gravity. There, the classical theory of general relativity successful, but the quantized theory is badly non-renormalizable.
arise in
is very
We must therefore understand how and why we may use non- renormalizable theories in physics. Now, to perform consistent calculations in any theory which contains ultra-violet divergences, we must impose an ultra-violet cut-off, M, of some sort.
In the
case
of
a
we can take M to infinity and insensitive to the cut-off. Another related
renormalizable theory
obtain finite results that
are
5.7
123
Renormalizability
property of
a renormalizable theory is the decoupling theorem of Appelquist and Carazzone, which we will discuss in Chapter 8. This
theorem
applies
to a renormalizable
theory which contains fields whose
bigger than the energies of the scattering processes under consideration. The theorem states that the heavy fields can be deleted with masses are much
only
a
small effect (suppressed by
sections,
a
power of the heavy mass)
etc. The hallmark of a renormalizable
on cross-
theory is in fact that it is
complete in itself. It contains no direct indications of whether it is only part of a larger and more complete theory. These statements are false for a non-renormalizable theory. Consider the old four-fermion theory of weak interactions. Its coupling is G ^10 ̄5 GeV2. We cannot take the UV cut-off arbitrarily large, for an nth order graph has a divergence of order 2n\ it behaves like (M2G)n for large cut-off M. Counterterms to make the graph finite need
a
correspondingly large
finite number of counterterms
number of derivatives, but
only
available. Hence
take the cut-off to infinity, and if
we cannot
a
are
we want
insensitivity to the cut-off we must take M <G ̄l/2. Moreover, the energy, E, of the process under consideration must be much less than M, otherwise the calculation is dominated by details of the cut-off procedure. In other words, the four-fermion interaction is a good approximation to physics only if E
^ M ^ G ̄1/2. The minimum
possible relative error of calculations
is of the order of the maximum of M2G and
E2/M2.
Now,
it is always possible in principle to do experiments at arbitrarily energy. So the applicability of four-fermion theory at low energies
high 1/2 300 GeV there is new physics. implies that at energies rather below G" That is, the four-fermion theory becomes incorrect at that energy. The last fifteen years of weak interaction physics confirms this. (See, for example, Bjorken (1982)) For gravity, the corresponding energy scale is the Planck mass, of the order of 1019GeV. This is extremely far beyond the range of normal  ̄
accelerator experiments. Evidence for phenomena on such an energy scale come from much more esoteric observations. Examples might be
must
found in certain areas of the cosmology of the early universe, or from seeing the decay of
a
proton (Langacker (1981)).
In any case,
a non-renormalizable theory contains indications that it cannot describe all phenomena. It contains the seeds of its own destruction as a viable theory of a field of physics. So, given a successful non-
renormalizable theory, one must ask the following questions: (1) 'Of which complete theory is it a part?' (2) 'How is it related to that theory?' An
more
example is given by
the relation between the
Weinberg-Salam theory,
124
Renormalization
X-X.
Fig. 5.7.2. W-boson exchange gives
an
effective four-point interaction at low
energies.
which is
renormalizable, and the four-fermion theory, which is not. The theory arises as an approximation to W-boson exchange at
four-fermion low energy
(Fig. 5.7.2).
One replaces the propagator
il{l2-m2w\ for the W-boson,
by i/(-0-
The graph is suppressed by factor of at least E2jm\y compared to photon exchange. It gives an example of the decoupling theorem: the heavy particles have small effects at low energies. The only reason we can see such effects is the high degree of symmetry of the strong and electromagnetic interactions. These interactions conserve P,
C, T, and the number of each flavor of quark and of each flavor of lepton. Weak-interaction amplitudes are much smaller than strong-interaction or for similar processes, and are therefore normally invisible. But there are many processes that are completely forbidden in the absence of weak interactions; for these, any weak-interaction cross-
electromagnetic amplitudes
section,
no matter
how small, is all there is.
So one important way in which a non-renormalizable theory arises is as a low-energy approximation to a renormalizable theory in a process that is forbidden in the absence of the heavy fields. The heavy fields effectively give a
cut-off
on
the non-renormalizable theory. Then, for example, the four-
fermion coupling G is computable in terms of the underlying theory via formula like
G
=
constant
#2/m^ + higher
a
order corrections in g.
Here g is the dimensionsless coupling of the Weinberg-Salam theory. One manifest characteristic of this non-renormalizable theory is the weakness of
higher power of G is a higher inverse power leading power of mw as mw gets large, so it is in
its interactions. Also note that of
m^r.
We are
taking
the
a
general incorrect to calculate in the non-renormalizable theory beyond lowest order. Higher-order calculations must be done in the full theory. A slightly different situation arises in gravity. There one must perform calculations beyond tree approximation, since
gravitationally bound states
5.8 Proof of locality like
the
solar
system
are
of counter terms
125
formed by multiple exchange of gravitons.
generated involving higher-derivative interactions (e.g., The R2, R*v. etc.). ambiguity in the finite parts of these counterterms gives an uncertainty in the Green's functions. However the uncertainty is a power of momentum divided by some large mass scale, and is negligible for low- momentum-transfer processes. In weak interactions, the size of the higher- Counterterms
are
order corrections is of the
the calculations, but in
same
gravity
order of magnitude
this is not
so
as
the intrinsic
because of the
error
zero mass
in
of the
graviton. Another difference is that
gravity
is actually the strongest of the four
fundamental interactions when considered contrast,
on
atomic
or
on
a
large enough scale.
In
molecular scales, it is the other three interactions
by far the strongest. However, the strong and weak interactions finite range, so that they are essentially zero outside the nucleus. Particles can have both signs of electric charge, so that bulk matter, if
that
are
have
a
charged, tends to attract charge of the opposite sign to it. Bulk matter is therefore generally neutral. But gravity couples to mass or energy, so it is always attractive. Hence gravity wins out as the strongest interaction for large enough assemblages of matter. However at nuclear and atomic scales, it is negligible by a factor of about 1040 compared to the other interactions. Let
us
between
summarize
by restating
renormalizable
and
the key conclusion about the distinction theories. A non-
non-renormalizable
renormalizable theory considered at low energy gives some indications that at high enough energies it must break down, and cannot be a complete
theory. A renormalizable theory gives
no
such indication.
5.8 Proof of locality of counterterms; Weinberg's theorem
examples, we saw that the counterterm C(G) of a graph G is a polynomial in its external momenta, of degree equal to its overall degree of divergence 3(G). This is a general property, as we will now prove. The original proof of this theorem and some related results is due to In
our
Weinberg (1960); a simpler proof was given by Hahn & Zimmermann (1968). It is useful to distinguish three results: (1) Suppose that
a
1PI
graph
G and all its 1PI
subgraphs
have
negative
degree of divergence. Then the graph is finite. That the degrees of divergence of the graph and subgraphs are negative means that there is when all or some of the loop momenta go to infinity the other momenta finite. The problem is to eliminate the with together, of a divergence from more exotic scalings. possibility no
divergence
126
Renormalization
Suppose that a 1PI graph G has negative degree of divergence, but that it might have subdivergences. Then the graph is finite if we first subtract
(2)
off subdivergences. More simply, if 3(G) < 0 then R(G) is finite. (3) If a 1PI graph G has degree of divergence 5(G), then its overall counterterm C(G) is polynomial in the external momenta of G of degree 5(G). Property (1) is a trivial case of (2). We will reduce (3) to (2) by the same differentiation method as we used in Section 5.2.2. The proofs will be by induction. This naturally suggests that renormalization R(G) of G. One
generalization
we use
the recursive definition of the
is useful. It is that the renormalization
may be chosen so that result
prescription (3) reads ft Hooft (1973), Weinberg (1973), and
Collins (1974)):
graph G has degree of divergence 5(G), then its overall C(G) is polynomial in the external momenta of G and in the massive parameters in the Lagrangian. (The parameters in question are the masses of fermions and the squared masses of bosons.) The dimensions of the terms in the polynomial are at most 3(G).
(3') If
a
1PI
counterterm
5.8.1 Degree of counterterms equals degree of divergence
Property (3), that the overall counterterm C(G) of a 1PI is graph polynomial in the external momenta of degree 3(G). We will do this assuming Property (2), that a graph with its subdivergences subtracted is finite if its degree of divergence is negative. Let G be a 1PI graph with degree of divergence 3(G) > 0. We will consider R(G\ which is G plus counterterms We first prove
for
its
subdivergences. Following
Caswell & Kennedy (1982), let
us
differentiate the graph 3(G) + 1 times with respect to external momenta. This produces a result that has negative degree of divergence. We differentiate not only the graph G, but also its various counterterm graphs
Cy(G). we
The aim is to show that the result is actually convergent. To do this graphs are the correct
will show that the differentiated counterterm
counterterm
graphs
for the differentiated
obvious, but there are
some
subtleties,
original graph. This may will give the details.
sound
so we
Let the external momenta of G be pu...,pn. Its renormalized value is R(G)=G+ =
=
R(G)
X Cy(G) +
C(G)
R(G)-TGoR(G).
(5.8.1)
5.8
Proof of locality of counterterms
127
respect to one of the external momenta, dx denote any A-fold differentiation with respect to the external
Let d denote differentiation with
and let
dxR(G) is finite if X > 5(G). (It is clear from naive power-counting that S(dxG) S(G)- X.) Suppose we ensure that differentiation commutes with the basic subtraction operator Ty. This amounts to imposing a very natural relation
momenta. The property to be shown is that
=
between the finite parts of, for example, Ty(dG) and Ty(G). (The relation is satisfied by the pole-part subtractions, but it is possible to choose exotic
prescriptions
renormalization
graph
not
satisfying
hypothesis.)
Then for any
y we have
dC(y) It follows that, for the
point
here is that
a
a
we
have
(5.8.3)
R(dxG).
differentiation when acting
number of terms, in each of which
differentiated. It is
=
(5.8.2)
C(dy).
=
original graph,
dxR(G) The
the
one
on a
of the propagators
graph gives or
a
vertices is
simple generalization of the argument given in specific graphs that the counterterms for
Sections 5.2.2 and 5.2.3 for
subgraphs Now
of G
R(dxG)
are
the correct
is the
sum
of
after differentiation in (5.8.3). graph dxG that has negative degree of
ones a
divergence and the counterterm graphs for its subdivergences. Hence by Property (2) it is finite, so that we may choose the subtraction operator T to give zero. Therefore the counterterm in (5.8.1) for the undifferentiated graph is polynomial of degree 3(G) in the external momenta. The same argument (Collins (1974)) also shows that counterterms are in mass. Here it is necessary to note that differentiation with m2 to does not automatically reduce the degree of divergence. This respect only happens if counterterms for subdivergences are polynomial. If a
polynomials
counterterm
has
a
piece proportional
to In
(m2)
then
differentiating
with
respect to m2 leaves the degree of divergence unchanged. The proof merely demonstrates that it is always possible to choose counterterms to be
polynomial
in
m2;
it is not compulsory.
5.8.2 R(G) is finite if 5{G) is negative evident in one-loop examples that a graph with degree of divergence 3 is renormalized by a local counterterm of degree 5 in the external It
was
momenta. To
generalize the
result to
an
arbitrary graph, we constructed a computing the counterterm for
renormalization procedure which involved
128
Renormalization
1PI graph only after subtracting subdivergences. We differentiated the graph S + 1 times with respect to its external momenta to prove its counterterm to be local and of degree S. This proof relied on assuming the following statement: a
If a graph has negative degree of divergence and has its subdivergences subtracted according to the rules, then it is finite. More briefly, if 5(T) < 0
then R(T) is finite. This statement sounds extremely plausible. It is nevertheless in need of proof. We have to
ensure
that the subtraction procedure actually
removing the subdivergences. (That is, there are no induced spurious divergences by the procedure.) In addition, we normally the only consider divergences as arising from regions in which some loop momenta go to infinity, all at the same rate; this generates the usual power- counting. It is necessary to eliminate more exotic possibilities. The most important problem, which is the one we will examine, is to treat accomplishes its purpose of
case that a collection of loop momenta go to infinity, but at different rates. In Section 5.2.2, we examined the special case of Fig. 5.2.1. The general case is very similar. Inductively, we assume that properties (1) to (3),
the
listed
graphs than the of the graph integration over regions loop momenta where all or some momenta go to infinity, not necessarily at the same rate. We will eliminate them as possible sources of additional divergences. If all the momenta go to infinity together, then the negative overall degree of divergence means that there is no actual divergence from at the
beginning of Section
5.8,
are true
for all smaller
G under consideration. We consider
this region. If some momenta stay finite while the others go to infinity (not necessarily at the same rate), then let y be the subgraph consisting of all those lines with the large momenta. Our inductive
hypothesis
ensures
that all the
divergences are cancelled by counterterms for subgraphs. The remaining case is that all of the loop momenta go
resulting
to infinity, but
again not at the same rate. Let k denote the components of the smallest momenta, and let / denote the rest. (Our notation is meant to copy that used
is the proof.) Let y be the subgraph consisting of all the loop momenta /. It may be a single 1PI graph or a union of 1PI subgraphs. Let these 1PI subgraphs be yl9...,yL.
for Fig. 5.2.1, and those lines
disjoint
so
carrying
Expand each subgraph in powers of its external momenta up to its degree of divergence. The remainder for each subgraph is really a graph with negative overall degree of divergence; the contribution vanishes as / goes to infinity, so we
should have set /
=
0(k). The expanded
terms contribute
just
as
they
5.8 Proof of locality
of counter terms
129
Fig. 5.2.1. After subtraction of divergences we have a factor in the integral over k corresponding to the dimension of the subgraph. We gloss over here some of the important details, notably what happens to the value of a general subgraph when some of its external momenta get large. But the main lines of the argument should be apparent. The structure of the proof is the same as in subsection 5.2.2 and the result did for
is the
same.
5.8.3 Asymptotic behavior
Weinberg (1960) not only proved the convergence theorem stated above with more complete rigor, but he also investigated what happens when several of the external momenta pt, p2,..., pn of a graph y get large in the Euclidean region. They are assumed all to be of an order Q, with the ratios
Pj/Q fixed as Q
-?
None of the
oo.
sums
of subsets
ofpf/Q vanish. Weinberg
then states how to find the asymptotic behavior:
(1) Consider any subgraph y connected to all the lines carrying the large momenta. Let all the loop momenta of y be of order Q. Compute the power of Q: Qay. (2) Look at all such subgraphs. Let ay have a maximum value a. (a) If there is a unique graph with this maximum power, then T oc Qa
Q^oc. (b) If there such
r
=
are
several
subgraphs
with ay a, then let N be the number of The asymptotic behavior is:
subgraphs.
=
Qa[A0B0 + AlBl\nQ + A2B2(\nQ)2 + 0(Qa ̄ly
Here the A?s
and the B.'s
are
are
as
+
--
A^.B^^lnQf-^ (5.8.4)
functions of those momenta that
functions of the finite quantities
are
fixed
as
Q->
oo
Pj/Q.
inductively in the guts of the convergence theorem proof is similar. It is not obvious that this part of Weinberg's theorem is of much use for physics, other than for its part in this convergence proof, since the asymptotic behavior is of Euclidean momenta. However, in the deep- inelastic scattering of a lepton on a hadron, there is a photon or a weak This theorem is needed
proved in
the last subsection. Its
interaction boson that is far off-shell. The momentum carried by the boson is effectively Euclidean, and Weinberg's theorem applies. We will see this in
Chapter 14. There are also generalizations to other intrinsically Minkowskian situations (e.g. Amati, Petronzio & Veneziano (1978), Ellis et
130
Renormalization
al (1979),
Libby & Sterman (1978), Mueller (1978,1981), Stirling (1978), and are beyond the scope of the present book.
Buras (1981)). These
5.9 Oversubtractions We showed how to renormalize a Feynman graph by making subtractions for the divergent subgraphs and for the overall divergence of the graph. It is possible, however, to make subtractions on graphs that are not divergent. Subtractions can also be made with a higher degree polynomial in the external momenta than called for by the degree of divergence. Either of these cases is called oversubtraction. Now, the general form of the renormalization, either by the recursive method or by the forest formula, did not specify the exact form of the subtraction operator T. So oversubtractions can be made without changing the general formalism.
There
wish to
important uses for oversubtractions. The first is when we 'physical values' of masses or couplings as the renormalized
are two use
parameters. We will discuss this in
a moment.
The second
operator product expansions. There, subtractions are cancel UV divergences but also to extract asymptotic
use
is to construct
made not only to behavior
as some
external momenta get large. We will discuss this later in Chapter 10.
5.9.1 Mass-shell renormalization and oversubtraction We have considered renormalization
divergences. Another point
of view
as
comes
of removing from the observation that one the
procedure
coupling parameters that appear as Lagrangian. example, consider a theory where each field has a corresponding single-particle state. Then the masses that are measured are those of the single particles, and it is often sensible to cannot
observe directly the
mass
For
coefficients in the
parametrize
and
the theory in terms of these
masses.
Similar remarks
can
be
applied to couplings. (Thus in QED one normally parametrizes the theory by the electron's mass and charge, defined by the long-range part of its electric field.) It can also be convenient to rescale the fields so that each propagator has
a
pole
of unit residue.
simple renormalizable theory like 03 in six dimensions the renormalizations to accomplish such a mass-shell parametrization are In
a
precisely those necessary to cancel the UV divergences. Thus we may define the subtraction operator applied to a self-energy graph Z(p2) to be
T(ph)o2V)
=
Z(mp\) + (p2
-
mp2h)I'(mp2h)>
t5-9-1)
5,9 Oversubtractions that the inverse propagator satisfies
so
"
as
131
p2
-?
i[P2
m2h.
"
<
-
We use the
£ph(P2)]
"
=
subscript 'ph'
i(P2
"
O + 0(p2
-
mp\)2,
to indicate renormalization
(5.9.2)
according
to the mass-shell scheme. Of course, mass-shell renormalization is
renormalization prescriptions that
we
may
only
use to
one
out
of many
cancel UV divergences. But
Consider, example, (ft3 theory again, but now in four dimensions. We may continue to use (5.9.1) and (5.9.2) for the renormalization of the propagator so we have a 'physical' parametrization. But all except the one-loop self- no have so counterterms are all the wave-function energy graph divergence, we
may also choose to renormalize in the absence of divergences.
as an
finite and all but
one
of the
mass counterterms are
of the renormalization procedure
as
finite. The combinatorics
described earlier all work unchanged.
5.9.2 Remarks One important technical problem is to check that the
over subtracted
and
the normally subtracted theories differ only by a reparametrization. This can be done by the methods which we will describe in Sections 7.1 and 7.2. In the previous subsection 5.9.1, we took the point of view that renormalization is the process of reparametrizing the theory in terms of 'physical' quantities. It should be noted that this is not always a useful point of view. In the first place, other renormalization prescriptions are more convenient for handling certain types of calculation. In the second place, there may be infra-red divergences that make the mass-shell structure of a
theory
not what one would
naively expect:
thus in
QED
the electron's
propagator does not have a simple pole. And, finally, in some theories there are many more particles and couplings than independent parameters. This
is very
common
in gauge theories.
5.9.3 Oversubtraction
on
IPR
graphs
The aim of oversubtraction, generally, is to impose some condition Green's functions. So far, we have assumed the condition to be imposed
graphs, since those are divergences. However, consider (ft3
the
1PI
2
Let
us
=
(c<p)2/2
-
the +
zero
needing
counterterms for
(ft4 theory:
m2cft2/2 -f(ft3/6
choose to renormalize at
ones
on on
-
#724 + counterterms.
(5.9.3)
external momentum. Thus the self-
132
Renormalization
IHIX (b)
(a)
(c)
{d)
Fig. 5.9.1. Subtraction of one-particle-reducible subgraphs. energy I and the
three-point
l(p2=0) ̄(p2
IPI function =
dp2
ro)(Pi
Following from function
our
=
Pi
0)
P%
=
=
=
earlier work
r(4) by requiring
the
sum
r(3) satisfy
0,
0)
=
we
lowest order
=
-
if
(5.9.4)
might renormalize the four-point graphs to be equal to their
of the IPI
lowest order value at zero external momentum. However it is also sensible
impose instead the condition on the amputated four-point function T^. graphs are IPI only in the four external lines. (We should amputate the graphs since the counterterm vertex will have attached to it external propagators.) Thus in addition to the three tree graphs of Fig. 5.9.1 (a)-(c\ we require the counterterm, Fig. 5.9.1(d): Our general method of renormalization tells us that whenever we have a basic graph containing one of the graphs (a), (b\ or (c) in Fig. 5.9.1 as a subgraph, there will be counterterm graphs in which this subgraph is replaced by the counterterm vertex (d). These counterterm graphs may be divergent even when the basic graph is finite. An example is shown in Fig. 5.9.2. In Fig. 5.9.2(a) if we impose the renormalization condition on the IPI functions only the graph (b) occurs as counterterm;(a) plus (b) is finite. If to
These
impose the condition on amputated graphs we immediately meet graph (c) where the line A is replaced by its 1/3 share of the counterterm Fig. we
5.9.1(d). But
graph (c) has a divergence, so we must renormalize it by a three-point subgraph consisting of the line A and the loop B. This is
counterterm to the
[a (b)
[a
I
(a)
(b)
"TV
^T
(c)
W)
Fig. 5.9.2. The subtractions of Fig. 5.9.1, inside
a
bigger graph.
5.10 Renormalization without regulators
133
graph (d). Note that the graph consisting of line A and loop B has a subdivergence, but no overall divergence. Even so, the overall counter- term in (d) is divergent. It is not difficult to see that the extra counterterms needed to impose the renormalization condition on the 1PR amputated graphs do not change shown in
our
results
on
renormalization. The instructions for renormalization in
Sections 5.3 and 5.5
can
be used provided only that
subgraph' by 'amputated subgraph'. The use of subtractions on 1PR graphs However it is
a
is too
we
replace the term '1PI
baroque for
device that is useful for discussing the large
and the operator-product expansion (Chapters
normal
mass
use.
expansion
8 and 10).
5.10 Renormalization without regulators: the BPHZ scheme
setting up the renormalization procedure in Sections 5.3 and 5.5 we were careful not to use a specific definition of the subtraction operation. This was In
allow for the choice of one out of the infinitely many possible renormalization prescriptions. An obvious one is the mass-shell subtraction procedure indicated in the last section. Another is the minimal subtraction procedure to be defined precisely in Section 5.11; we have already made much use of it. In this section we will explain the method of Zimmermann (1969), in which the subtractions are applied directly to the Feynman integrand, so that no regulator need be used. The starting point is the method due to Bogoliubov & Parasiuk (1957) and Hepp (1966), called the BPH scheme. They observed that the overall counterterm for a graph T is a polynomial of degree d(T), its degree of divergence. So they defined the subtraction operator T(T) to be the terms up to order S(T) in the Taylor expansion of T about zero external momentum. For example, consider the one-loop self-energy graph to
Fig.
3.1.1 in
03 theory in
six dimensions. After dimensional
regularization
its unrenormalized value is
Za(p\d)
=
2(An 412 v
The terms up to order
x
dx[m2-p2x(l-x)f/2-2.
T{2-dl2)
,
p2
V
(5.10.1)
0
in its Taylor
expansion
about p
=
0
\ldxmd-4[l-(d/2-2)p2x(l-x)/m2l
are
(5.10.2)
134
Renormalization
The renormalized value of the
g2 1287T3
dx{ [m2
-
p2x(l
graph
-
o
is
Fa
x)]In [1
-
-
T°ra.
p2x(l
-
So at d
x)/m2]
=
+
6 this is
p2x(l
-
x)}.
(5.10.3) Zimmermann's (1969) achievement was to realize that this construction can be applied directly to the integrand. Subdivergences are subtracted with the aid of his forest formula. Then the result is an integral which, according to power-counting, has no UV divergences. The integral therefore has in fact no divergences (Hahn & Zimmermann (1968), Zimmermann (1968)). This method is called BPHZ renormalization. In Section 3.4, we applied this method to the above graph, with the result (3.4.7). It can be explicitly calculated by putting all the terms over a common denominator and then using standard parametric methods. The result
agrees with (5.10.3). An example involving
a
subdivergence is given by Fig. integral be
5.10.1 for
04
in
four dimensions. Let the renormalized rBPHZ 1
->3
2(2tt)8
(5.10.4)
d4/cd4//(Pl,p2,p3,p4,/c,/).
Pz
Fig. 5.10.1. A two-loop Then
we
will construct the
The unrenormalized
u
1 =
vertex
(I2
-
integrand integrand is
1
m2) [(*
+
I)2
graph in 04 theory.
/.
I
1 -
m2] [(*
+
p3)2
-
m2] [(*
-
p4)2 m2] -
Subtraction of the sole subdivergence gives R(U)  ̄U ̄
(/2-m2)2[(/c + p3)2-m2][(/c-p4)2-m2] -2k-l-k2
"
{I2
-
m2)2 [(* + I)2
-
m2] [(/c + p3)2
-
m2] [(k
-
p4)2
-
m2]' (5.10.6)
5.11 Minimal sub traction Then the overall
I
=
_
R(U) (2k-l
k2){(k2
+
(I2
divergence
R(U)-[R(U)]\pi=P2
=
-
135
is subtracted to give =
P3=P4
=
0
m2)[p2 + p\ + 2/c-(p3 p4)] + (p2 + 2k-p,)(p2 2/rp4)} m2)2(k2 m2)2[(/c + I)2 m2] [(* + p3)2 m2] [(* p4)2 m2] -
-
-
-
-
-
-
"
-
(5.10.7) The BPHZ scheme has (1) It is
applied
number of advantages:
a
integrand
to the
and generates
a
without requiring any regularization. Thus it exhibits the fact that the properties of (2) theory do not depend
Mathematically it
(3)
convergent integral a
renormalized field
which UV regulator is used. is rather elegant. In particular there is on
no
need to
required to have a theorem that tells us that a graph that is convergent according to the naive criteria is actually convergent. (4) It allows a very simple proof of the operator-product expansion. discuss directly the
There
(1)
are a
divergences
number of
of Feynman
graphs;
it is only
disadvantages:
It is not the best scheme for theories
complicated symmetries,
(especially
gauge
theories) with
where relations between counterterms have to
be preserved; the scheme does not allow direct computation of
(2)
a
of the value
divergence.
The subtractions
in
are
made at
zero momentum
and therefore
are
massless theory. divergent (3) When the scheme is generalized to handle massless theories, it becomes much more complicated (Lowenstein, Weinstein & Zimmermann, infrared
1974a,
a
b). 5.11 Minimal subtraction 5.11.1
Definition
proved (Speer (1974) and Breitenlohner & Maison (1977a, b, c)) when dimensional regularization is used, the UV divergences of that, Feynman graphs appear as poles at isolated values of the space-time dimension d. Minimal subtraction ft Hooft (1973)) the MS scheme consists of defining the counterterms to be poles at the physical value of d, 4. We have already used this scheme, in Chapter 3. Our purpose in this d section is to make precise the definition of minimal subtraction. It can be
-
-
=
The main
depends
on
complication d,
so
that
is that bare couplings have a dimension that introduce the unit of mass fi, as follows:
we must
136
Renormalization
(1) Consider in turn the coefficient g. + dgt of each term in if. Let the dimension of g. be a{ + bf(4 d). Then we replace gx + (5#- by ^,(4_d) -
Thus the renormalized
(Gi + ^/)-
coupling
gt and the counterterm
both have dimension ai9 independently of d. (2) Let T be a 1PI graph to which it is desired to operator T. Let the dimension of T be A + B(d
apply
6gi
subtraction
a
4), and suppose the just explained. Then we define -
couplings all contain T(T) The We
=
powers of \i as
/("-4){pole
part of
(/(4"d)r)
at d
=
4}.
pole part is obtained by making a Laurent expansion about d 4. have arranged to take the pole part of a function whose dimension =
does not depend
on
d.
talking about a theory in a different number of physical dimensions than four. For example, we might be in 03 theory in six dimensions. Then the '4' in the above formulae is replaced by the correct
(3) Suppose
physical
we are
value.
For a simple graph with no subdivergences, like the one-loop self-energy in (5.10.1), this prescription amounts to subtracting the pole:
Zr)(d
=
6)
=
\im\^^r(2-d/2){1dx[m2-p2x(l-x)r12-2 Af*n) Jo
d^e(
g2 1287T3
\[yE-l-ln(4n)](m2-ip2) ^
i
m2
2^ dx[m2-p2x(l-x)]ln
+
—
p2x(l —x) it
graphs with subdivergences, the subdivergences removing the overall pole. The advantages of the scheme are:
For
(5.11.1)
must of course be
subtracted before
(1)
It
automatically preserves complicated symmetries.
The
exceptions are by
chiral symmetries and the like, which in general cannot be preserved
quantization see Chapter 13. (2) It has no problems with massless theories. In fact, dimensional continuation regulates both IR and UV divergences, thus removing the need for a separate IR cut-off. -
(3) Calculations
are
very convenient.
(4) Computation of the divergent part of
a
Feynman graph
-
needed for
5.11 Minimal sub traction
renormalization-group
calculations
137
is almost trivial at the
-
one-loop
level.
disadvantages of minimal subtraction
Some
(1) (2)
unphysical. proof of the operator-product expansion is made harder than in the
It is
The
are:
BPHZ scheme.
5.11.2 MS renormalization
QCD, where it has become standard. Another disadvantage that then appears is that minimal subtraction tends to produce large coefficients in the perturbation expansion. These are primarily due to the In {An) yE 1.95 term such as appears in (5.11.1). It has become conventional to work with a modified The MS scheme has found much
use
in work
especially
 ̄
-
scheme, called the MS scheme
(Bardeen, Buras, Duke &
Here the \i of the MS scheme is written
we
Muta
(1978)).
as
X
Then
on
(5.11.2)
;0-38/i.
have, instead of (5.11.1), the cleaner form
1287T m2
dx[m2-p2x(l-x)]ln
+
—
p2x(l
—
x)
(5.11.3)
fi2
5.11.3 Minimal subtraction with other regulators Minimal subtraction could also be applied with other UV cut-offs. For example, if a lattice of spacing a is used, then the singular a -? 0 behavior of graph of degree of divergence 3 is
a ̄s[polynomial One
can
therefore define T(T)
as
the
in In
(a)].
singular part of T, with
the
general
form T(D= /?
Z"[lnM]^0;/J+ £ l^{.\n(an)fAxJ. =
After subtraction of masses
1
a=l P
=
(5.11.4)
0"
subdivergences, the coefficients Aa^ are polynomials in
and momenta. Note again the appearance of
scheme has found little
use.
a
unit of
mass.
This
6
Composite operators
In
Chapter
2
number of
we met a
equations involving products
of field
operators at the same point. Examples are given by the equations of motion (2.1.10) and the Ward identities (2.7.6). These products we will call When computed directly they have ultra-violet 0(x)(/>(y) makes unambiguous sense if x is not
composite operators.
divergences: the product
equal to y9 but if x equals y then we have 0(x)2, which diverges. Since the equations of motion and the Ward identities express fundamental properties of the theory, it is useful to construct finite, renormalized composite operators with which to express these same properties. It could be argued that there is no need to have renormalized equations of motion. One could say that one only actually needs the equations of motion in the regulated theory, where they are finite. A situation of practical importance where we actually do need renormalized composite operators is the operator-product expansion, to be discussed in Chapter 10. This is used in a phenomenological situation such as deep-inelastic scattering
(Chapter 14)
where we wish to compute the behavior of a Green's function of its external momenta get large. Equivalently, we need to know how a product of operators, like cp(x)(j)(y), behaves as x^y.
when
some
This information is contained in the
operator-product expansion
of
Wilson (1969) which has the form
<t>{x)<t>{y)
-
Cx(x
-
y)\
+
C+2(x
-
y)[cj)(y)2]
+
?
■
?.
(6.0.1)
symbol [^(x)] denotes the renormalized operator corresponding composite operator A(x). The coefficients C(x y) are onumbers, and each has a subscript which labels the operator that it multiplies. Here the
to an unrenormalized
—
Therefore in this chapter of
composite operators,
we
show how to renormalize Green's functions
e.g.,
<O|T0(x)(/>(y)(/>2(2)|O>, <O|T0(w)(/>(x)(/>(y)2(/>(2)2|O>, <O|T0(x)2(/>W2|O>. 138
(6.0.2)
6.1
Operator-product expansion
We will first motivate the
use
139
operators by seeing how the low-order graph. Then we will low-order graphs for composite
of composite
operator-product expansion arises in
a
examine the divergences that appear in operators. We will see that we must expect multiplicative renormalization:
[4>2]
=
Z^2,
[02] is finite as the UV cut-off is removed, while Z02 is a renormalization factor. The unrenormalized operator (j)(x)2 is when the cut-off is removed. where
(6.0.3)
divergent divergent
These examples will provide motivation to define renormalized composite operators by application to Feynman graphs of the same
R-
defined in Chapter 5. After discussion of a number of technical issues, we will derive some basic properties of the renormalized
operation
that
we
operators, including the equations of motion and the Ward identities.
6.1 Operator-product expansion We will postpone a complete treatment of the operator-product expansion Chapter 10. Here we merely wish to motivate our definition of composite
to
operators with
Fig.
an
6.1.1. Take q^>
example of their
oo
use.
in these graphs to obtain the lowest-order example of the operator-product expansion.
graphs of Fig. 6.1.1 for the four-point function in 03 theory. infinity with px and p2 fixed, and with the ratios of the We expand the graphs in powers of of q fixed. Then \q2 |
Consider the
We let q* go to components
q2
-?
oo.
to find:
Fig. 6.1.1 (a)
+
(ft)-
(p2-m2)(p22-m2). (q2r
(6.1.1a)
Composite operators
140
Fig. 6.1.1(c)
10
?
2\2 (Pi-m2)(p22-m2)((Pl+p2)2-m2)_ (q2)
1
-
2g'(Pi+P2) <T
,
■
+
(2?2
-
(Pi
P2)2) + 4g'(Pi+P2
+
^
-
(9:2^2
(6.1.1b) show dependence of of an operator-product expansion like (6.0.1) (after Fourier transformation into momentum space). In (6.1.1 a) the factor in square brackets is in fact the value of the lowest- order graph for the following Green's function:
In each
px and p2 has factorized. We
term the
that this is
a
now
case
<o|r^(-Pl)^(-p2)02(O)/2|o> d^exp(-iprx-ip2^)<0|T(/>(x)(/>W(/)2(0)/2|0>.
(6.1.2)
We have
not Fourier transformed the 02(O) operator, but have set it at the we If had made the Fourier transform, then we would merely pick up origin.
a
momentum-conservation 5-function, which
we
To understand the appearance of the operator
do not have in
(6.1.1a).
02(O)/2 in (6.1.2), we may
functional-integral formula for the Green's functions that appear in equation. Since 02(O) means the product of two fields at the same space- time point, such a formula follows from our work in Section 2.2. It is
find
a
this
<O|T0(x)0(j#a(O)/2|O>
=
.>K
[dA]A(x)A(y)±A2(0)eis.
The Feynman rules for this Green's function are
the usual
ones
for the Green's function
addition that each graph contains exactly
one
can
(6.1.3)
then be derived. They with the
(0\T(ft(x)(j)(y)\0} special
vertex for the
02(O)/2
operator. The lowest-order graph is shown in Fig. 6.1.2, where the special
cj)2/2
vertex is indicated
by
a cross.
The value of the vertex is unity, for the
explicit factor 1/2 in 02/2 gets cancelled. This happens in exactly the same way as the 1/4! in the c/>4 interaction or the 1/3! in the 03 interaction gets cancelled to leave The operator
composite field).
a
value
—
\g for
an
interaction vertex.
first example of a c/>2(0)/2 we this term mean, in general, By is
our
composite operator (or product of elementary
a
Fig. 6.1.2. Lowest-order graph for two-point function of
02.
6.1
fields
Operator-product expansion
(or their derivatives)
operators that we will We can write
at the same
investigate
Fig. 6.1.1(a) + (b)
point. It is chapter.
141
the
properties of
2iq2
-
such
in this
^3 <0| 7WPl)0( P2)</>(0)2/2|0>,
(6.1.4a)
-2
Fig. 6.1.1(c)
2ig2 23
id )
1
(q2
2iq" 3
q2
dx"
/ \
/
+
(2m2+D) '
(c)
\
Fig. 6.1.3. Generation of terms in the operator-product expansion from the graphs of Fig. 6.1.1 (a) and (b).
 ̄
V<f3?a
q2
<O|T^(p1)^(p2)0(x)|O>|x
x
W + ( ) ̄
r
ig
{q2)2
L]
This is illustrated in diagrams by Fig. 6.1.3. In similar fashion, we derive an operator formula for (6.1.1b):
q4 =
(6.1.4ft)
o.
coefficients y ^
x
derivatives
Fig. 6.1.4. Generation of terms in the operator-product expansion from the graph of Fig. 6.1.1 (c)
Here the form of the first square-bracket factor
means
that
we
need
an
elementary field 0(x), rather than a composite field. The pt's and /?2's in the numerators in the second square-bracket factor have turned themselves into derivatives with respect to x;
we set x
=
0 at the end.
Equation (6.1.4b)
is illustrated in Fig. 6.1.4. The form of (6.1.4) suggests the following formula:
<0\T^q)ct>m(p^(p2)\0} ^^C^)<O|7^(O)0(Pl)0(p2)|O>.
(6.1.5)
i
The sum is over a set of local operators (9t. Each of these is either the elementary field 0, one of its derivatives, or a composite operator such as 02. The coefficients C^q) are called the Wilson coefficients. In Chapter 10
generalize this result to all orders. We will have an expansion for any Green's function with large momentum on some of its external lines. Now higher-order corrections to the Green's functions of the composite
we
will
operators such as the 02(O) that appears in (6.1.4a) have ultra-violet divergences beyond those appearing in Green's functions of elementary fields. We will see this in the next section, Section 6.2. To obtain an operator product expansion, like (6.1.5), with finite coefficients, most of this
we
will need to
composite operators. This particular problem chapter.
renormalize the
will occupy
142
Composite operators 6.2 Renormalization of
composite operators: examples
6.2.1 Renormalization of (j)2 The Feynman rules for Green's functions of unrenormalized composite operators can be derived from the functional integral, in the presence of an
ultra-violet cut-off. In coordinate space they are the usual rules, modified only by having several external fields at the same point. For example, we consider
<0|T4,(x)4>0#2(2)/2|0>
(6-2.1)
03 theory in six-dimensional space-time. The connected graphs up to order g2 are shown in Fig. 6.2.1. As before, the vertex for <f>2/2 is denoted by in
a cross.
-o- (?)
(b)
(c)
M
(d)
Fig. 6.2.1. Renormalization of the operator (j)1. To work in momentum space,
G
=
=
The
we
Fourier transform,
as
usual, and define
<O|T0(p)^)0(z)2/2|O>
|d"xd''>;exp[i(p1-x
lowest-order
+
p2-3;]<O|T0(x)0(3;)0(z)2/2|O>
graph, Fig. 6.2.1(a), is equal
to
'
/-<
a ̄(p2-m2
(6.2.2)
tii
+
ie)(p2-m2
+
ie)'
^
o\
'
Observe that the factor 1/2 in the operator <\>2j2 is cancelled, just like the 1/3! that comes with the interaction vertices.
graphs of Fig. 6.2.1. They are all divergent: Fig. 6.2.1(b) logarithmically divergent, while the remaining graphs, Fig. 6.2.1(c) to (e\ are quadratically divergent (all at d 6). The divergences in the last two graphs, Figs. 6.2.1(d) and {e\ involve self-energy corrections only, so these divergences are cancelled by the usual wave- Let
us
now
turn
to
the one-loop
is
=
6.2 Renormalization
of composite operators: examples
143
-X—?- (a)
(b)
Fig. 6.2.2. Counterterm graphs for Fig. 6.2.1(d) and (e).
counterterms, Fig. 6.2.2. In these and other graphs in this
function and
mass
chapter,
indicate counterterms by
we
a
heavy dot
and
an
composite operator by The remaining two graphs have no counterterm from and they are both divergent. For Fig. 6.2.1(b) we get
insertion of
a
a cross.
i
(p2
-
the interaction,
i
m2 + ie)
ddk
(pj-m2
+
ie) l
(k2 -m2
+
m)[(*
-
Pl)2
-
m2 +
i
-T(3-d/2)x (Pi-m2)(p2-m2)64n2  ̄m2
-
dy\
dx
ie] [(jfc + p2)2
-
m2 +
ie]
x
o
p\y(\ -x-y)- p22x(\ -x-y)-(p1+ p2fxy Anil2 (6.2.4)
and for
Fig.
6.2.1
(c)
we
get
(p?-m2)(p22-m2)[(p1+p2)2-m2] -d/2
2(2*)'
r
J
(/c2-m2)[(p1+p2 + fc)2-m2] -flfAt3--'2 (p2-m2)(p2-m2)[(p1+p2)2-m2] "
-^/2"3rn 128,3
^f1. [m2-(Pl+p2)2x(l-x)r2-2
-r(2-./2)jod^
-(4;V-3
(6-2.5)
Note that there is a symmetry factor 1/2 in this last equation. The fact that the
sum
of (6.2.4) and (6.2.5) diverges
means
that the operator
02(O) is
not
finite. in the operator-product expansion we do not need precisely the operator 02. Rather, we need some local operator similar to 02 that is finite. This indicates that we should define a renormalized operator by
For
use
subtraction of the divergences. Let us agree to use minimal subtraction. Then the counterterm graphs are obtained by replacing each divergent loop by
144
Composite operators
(a) Fig.
(b)
6.2.3. Counterterm graphs for Fig. 6.2.1(6) and (c).
pole part, as illustrated in Fig. 6.2.3. Thus the counterterm graph
minus its
for Fig. 6.2.1(h) is r
ip\
-
m2)(p22
and the counterterm graph for
(P?
"
m2)(p\
-
m2)[(Pl
+
p2)2
-
m2)647r3(d
-
(6.2.6)
6)'
Fig. 6.2.1(c) is
-
m2] \(An\d
6)^  ̄iiPl P^)' +
-
(6.2.7) The
of the factors of \i is such that the counterterm in curly brackets has exactly the same dimension as the loop to which it is a
positioning
counterterm.
We thus find the renormalized values at d
R(Gb) "
(p?-m2)(p2_m2)647r3l
' ̄x
o
R(GC)
-{p21y dylJm2 L
=
+
To
6:
dx
2
p22x)(l-x-y)-(pl+p2)2xyTl 4jt)"2
-9
(6. 2.8)
JJ' -9
{j>l-m2)(p2-m2)[{Pl+p2)2-m2-\\mn\
x?j(y-l)[m2-i(p1+p2)2]+ x
=
In
f'dx^-^
+
p^xa-x^x
4nfx2
(6.2.9)
interpret these renormalizations we observe that the counterterms are <f>2(0) in (6.2.6) and for (m2 + D/6)</> in (6.2.7). Thus
vertices for
Ga
+
R(Gb) + R(GC) + R(Gd) + R(Ge) i +
+
L
64n3(d ..
9H
-
J<o|r<ft(Pl)<?(p2)l4>2(0)|0)
6)
d/2-i
64ttV-6)
<0\T¥(Pl)4>{p2)(m2+i\3)<t>(0)\0> + O(g4 (6.2.10)
6.2 Renormalization So what
we are
computing
is
of composite operators: examples a
Green's function of the operator ,.d/2-3
?2
<t>2 + 29
2
145
^m2 + *□)</> + higher order.
64n\d-6) *a
3,j
(6.2.11) renormalized
use the square brackets on the left-hand side to denote operator. In subsequent sections we will see that this result generalizes to all orders: a renormalized operator [02] can be defined to all orders by a
We
a
formula of the form:
i[02]=^02 + /id/2-3Z,m20 + //2-3ZfDc/>,
(6.2.12)
where the Z's depend only on g and d. Such renormalized operators were defined by Zimmermann (1973a). He called them normal products and used the notation N[(/>2]. His definition differed from
ours
only in that he
used BPHZ renormalization instead of
minimal subtraction. Observe that in order to obtain
degree of divergence of at least zero, the (6.2.11) or (6.2.12) have dimension original operator 02. This is a general
a
operators that appear as counterterms in
less than
equal
or
to that of the
phenomenon. Moreover, the only operators of such dimension
are
actually appearing in (6.2.12). We may write the renormalization matrix equation in the following form:
\Za ^->Zbm2 it?-\
Here
we
Ocj>
are
1
0
0
0
1
have used the fact that
said to mix with
diagonal elements Zb are
0
and Zc
cp2
0
and
D0
are
I I
(/>
].
those as
a
(6.2.13)
finite. The operators (j) and
under renormalization, because the off-
are non-zero.
needed in the renormalizations,
so
further operators and □</> are said to form a
Moreover,
02, 0
no
closed set under renormalization.
6.2.2 Renormalization of Sometimes
operators.
we
A
(j)2(x)(j)2{y)
need Green's functions involving two
or more
composite
simple example is
<oin[?p](p)i[>2](o)|o> =
for which
fd'xe-^OlriOTMK^OJlO),
the lowest-order
graph is Fig.
(6.2.14)
6.2.4. The renormalizations of
146
Composite operators
Fig.
(6.2.11)
6.2.4. Lowest-order graph for
<O|r[02]/2[>2]/2|O>.
do not appear until the next order,
1
2{2n)d) This is ultra-violet
d
V-m2)[(p
divergent,
even
so
+
the
graph
has the value
(6'115)
k)2-m2]'
in free-field theory. Even
though
the
(j)2(x) and 02(O) on the right of (6.2.14) are well-defined, we have to
operators
integrate through the point x 0. At x 0 there is a singularity, which is not integrable if d > 4. We may nevertheless define a finite Green's function =
by adding
a
=
local counterterm:
<O|n[02]W|[^?2](O)|O>J?
=
with
C(x) Once
=
(m2
+
<O|Ti[^2]Wi[02](O)|O> (6.2.16) -C(x)<0|l|0>.
D/6)d^(x))A 3r, ;N + 0(g2). ,64tt3(^-6)
have used minimal subtraction at d
more we
=
(6.2.17)
6.
6.3 Definitions We define renormalized Green's functions of composite operators by applying the K-operation to the Feynman graphs, just as we did for Green's functions of elementary fields in Sections 5.3 and 5.5. We will need to show that the counterterms generate multiplicative renormalizations of the
operators (e.g., (6.2.12)). This is similar to what we
we
did in Section 5.6, where
showed that the counterterms in elementary Green's functions
are
starting generated by Lagrangian. with the graph-by-graph renormalization is again to allow a simple treatment of the problems of subdivergences. We do not need a new proof that the counterterms for operator insertions are local; our original proof suffices. Our motivation for
counterterms in the
As before, ones
that
scheme
-
we
have
are most
see
a
prescriptions. The subsequent developments are the BPHZ
choice of many renormalization
useful for
Zimmermann
(1973a)
-
and
minimal
subtraction
-
see
Breitenlohner & Maison (1977a, b, c) and Collins (1975b). In any case we have a subtraction operator 7(G) which is applied to a graph G (after
removal of subdivergences) in order to extract the divergent part of G. In the
6.3 BPHZ scheme, T(G)
147
Definitions
gives the first 3(G) terms in the Taylor expansion about
external momentum, where d(G) is, as usual, its degree of divergence. In the case of minimal subtraction we must state how the unit of mass \i is
zero
from the examples in Section 6.2, we must always arrange to compute the pole part of a quantity whose dimension does not vary with d. So suppose we need TMS(G) for a 1PI graph G of dimension treated. As
^B(d-d0)
can
Then
be
we
seen
define 7=
/(d-do){pole
part of
(6.3.1)
G^ ̄%
where, as usual, d0 is the physical space-time dimension. The definition of a renormalized Green's function by the R-operation is rather abstract, and counterterm
we
will
show that it amounts to
now
adding
operators. In the one-loop examples of Section 6.2 this
rather obvious. In the
from
renormalization of an
(see
general case, we start arbitrary Green's function
R(G)
=
was
the formula for
(5.3.7)—(5.3.9)):
G^^Cy(G).
(6.3.2)
7
Here, as usual, the sum is over all subgraphs y that consist 1PI
subgraphs
yl9...,yn. Each y. is as in Section 5.3.
replaced by
of a set of disjoint
its overall counterterm
vertex, generated
distinguish the various y.'s that occur, according to the composite operator insertions that they contain. Consider, for example, Fig. 6.3.1, which illustrates the renormalization of <0| T(j) (j) (j)2/2\0y in the 03 theory in six dimensions. There is a one-loop subgraph ya for the three-point function. This is renormalized by its counterterm Cya in the Lagrangian. There is also a two-loop subgraph yb which contains the composite operator vertex. (This has a subdivergence, Let
us
number of
which must be subtracted.) The counterterm C,.
where
be considered
as
/
i
.
can
i
4?. 13}
Fig. 6.3.1. Renormalization of two-loop graph with insertion of composite operator.
148
Composite operators
<2> O (?)
(b)
cO O" (c)
(e)
(d)
Fig. 6.3.2. Renormalization of three-loop graph for
generated by
0(#4)
?
<0|T[>2]/2|>2]/2|0>.
Za. graph of order g4 for the Green's function <0| 7 ̄[02]/2[</>2]/2|O>. In addition to an overall logarithmic divergence, it has three divergent subgraphs. One subgraph ya is renormalized by a an
Next consider
term in the renormalization factor
Fig. 6.3.2,
a
in the interaction Lagrangian. The other two subgraphs yb and yc each look like yb of Fig. 6.3.1, and are renormalized by the same counterterm. These two counterterms are generated from counterterm
<O|7i[02](x^ + other terms from
by expanding each Za to 0(#4) and picking out the overall counterterm is obtained. It gives a term
Zb
and
terms for yb.
of
0(g*)
Zc,
(6.3.3)
Finally, the
in the C(x) of
(6.2.16). These arguments generalize easily to arbitrary consider
a
renormalized Green's function of
one
graphs. It suffices composite operator
(0\TY\4>(x^[A{y)}\0y.
to
(6.3.4)
i=l
We define the renormalization
by the recursive formula (6.3.2), and we wish
to prove that this equals
EZ^Olrn^OBMlO). B
Here
ZAB is
a
(6.3.5)
i=l
renormalization factor whose value is given by
writing,
in
analogy to (5.6.5),
M
=
Ez^=EttT^G). iyiGl B
(6.3.6)
G
Here G is any 1PI basic graph that includes a vertex for A. It has NG external lines in addition to the vertex for A, and D(G) is the operator corresponding
6.4 Operator mixing to the overall counterterm C(G). The factor
1/JVG!
(5.6.5), to organize the symmetry factors. Consider a graph for (6.3.4). We investigate The
subgraph
149
one
is just like the l/N(G) in
of its counterterms
Cy{G).
graphs yl9...9yn.Ayj that does not contain by C{y), which corresponds to one of the
y consists of 1PI
the vertex for A is replaced
counterterms in if. A y. which does contain the vertex for A must be one of
(6.3.6), so the counterterm C is generated of the counterterm by operators on the right of (6.3.6). Now sum over all graphs G for our Green's function and expand each R(G) by (6.3.2). The sum over 1PI subgraphs y. that correspond to the G's that are summed over in one
independently
the
££ can
subgraphs giving
the counterterms for the operator vertex. The result is
then the desired result
be done
of the
counterterms in
sum
over
(6.3.5). 6.4 Operator mixing
We have seen that a renormalized
composite operator \A\ by
is expressed in
terms of unrenormalized operators
\_A]
^ZABB.
=
(6.4.1)
B
In the case of counterterms
[02]
had the
we
that the operators that
saw
same or
lower dimension. Let
were
us now
needed
as
demonstrate
this for the general case. The proof is essentially
dimensional analysis. Let G be a 1PI graph containing a vertex for A and having the same number NB of external lines as a particular operator B. Now B is a product of NB fields with a certain number DB of derivatives. A counterterm can
only be generated if the degree
of divergence 3(G) is at least DB. Now in couplings have non-negative dimension, so
d(G)
=
dim (G)
-
a
renormalizable theory all
dim (couplings) (6.4.2)
<dim(G). On the other hand, since ZABB is dim
But we
only need
(G)
=
a
DB
possible + dim
B as a counterterm if
DB
counterterm we have
(6.4.3)
(ZAB). <
3(G),
so
dim(Z^)>0 for every counterterm. This
means
that the maximum dimension of
a
operator is the dimension of A9 as we wished to prove. In Section 5.8.1 we examined the dependence of counterterms in the
counterterm
150
Composite operators
Lagrangian on mass parameters. Provided we used minimal subtraction this dependence was polynomial, with the mass behaving as a dimensional coupling in determining allowed counterterms. The same argument applies here. The result is that ZAB is a polynomial in masses (and super -
renormalizable couplings) times the inevitable power of the unit of mass p. The coefficients of the polynomial are dimensionless functions of the dimensionless couplings and of d. A typical example of this is given by our see (6.2.11) and calculation in Section 6.2 of the renormalization of [</>2] -
(6.2.12). 6.5 Tensors and minimal subtraction Suppose we tempting to
minimal subtraction to define
use
[(50)2]
[d^dv4>].
and
[(S0)2]=^v[3,05v0]. This supposition is in fact false, that the
It is
suppose that
taking
of
as we
will
(6.5.1) demonstrate. This
now
does not commute with taking general. We will explain the significance of this fact. a trace
a
means
finite part, in
The lowest-order graph (in 03 theory) for either operator is d<j>2/2 before renormalization is
Fig.
6.5.1.
The 1PI part for
G(P)
=
2(2n)d J (k2 -^yrjd'fc^2
?H! J L
128tc3 x
For
d^(j)dv(j)/2
we
-
-
-i
 ̄* m2]
- -
-
3 X
4np2
x)[m2 p2x(l x)] d/2)[m2 p2x(\ x)]2}
d/2)p2x(\
(d/2)T(\
 ̄2^ ̄ M2 + k)2 m2)[(p
|d/ 2
0
{T(2 +
- -
-
-
-
-
-
(6.5.2)
have
G,M- ',,,_*, ld
k?(k
+
V-m2)t(P
+
p)v
*)2-??2] |d/2
128tt3 x
J0 *l
-
3 X
Ann2
x)[m2 p2x(l *)] <i/2)p?pvx(l + ^,r(l d/2)[_m2 p2x(l x)]2}.
{T(2
-
-
-
-
-
-
-
k+p
Fig. 6.5.1. One-loop graph for the operators in (6.5.1).
(6.5.3)
151
6.5 Tensors and minimal subtraction
Manifestly (6.5.4)
g^G^^G for the unrenormalized Green's functions. This has We can renormalize
K[G(p)]
K[<V(P)]
=
=
by minimal subtraction.
G(p)
3m* +
-128?3(d_6)C-
G?,(p)
!mV
-12g^_6)[P,Pv(X
"
K[G(p)]-^K[G?]= The
reason
why
&>*]. (6-5-5fl)
"
iVP2)
-^(X-!?V
Thus
subtraction of the
be true since
to
+
-^-^[X-imV
^4)]- (6.5.56) +
foP4]. (6.5.6)
contraction with 0"v does not commute with the is simply that taking the trace introduces d-
pole
dependence. Thus: g^
pole part pole part
of
of
d d-6'
d-6
——^ d-6
=
pole r
d-6
d-6
We must evidently be careful to specify whether a trace is inside or outside of the renormalization. The need to do this is characteristic of dimensional regularization. Which place to put the trace depends consideration.
on
the
problem under
problem arises whenever we have to consider a tensor of rank at least a trace of Dirac y-matrices except that we choose the trace of the unit Dirac matrix to be independent The
2. (It could also arise in connection with taking
our renormalized operators do not have all that the bare properties operators do. The lack of commutativity of the trace and the finite-part operation is related to a physical effect, that there is
of d.) We have discovered that
the
anomaly in the Ward identity for scale transformations see Callan (1970), Symanzik (1970b) and Brown (1980). If we were to use, say, zero-momentum subtractions (BPH or BPHZ), then the trace and the finite-part operation would commute as can be checked from our example. So it might appear that zero-momentum subtraction provides a better all-purpose definition of renormalized operators than does minimal subtraction. However, some of the properties we will prove when using minimal subtraction now disappear or become more complicated. an
-
-
152
Composite operators
example, the equations of motion which we will prove in Section 6.6 are only true if the mass terms are oversubtracted. This turns out to prevent some Ward identities from being true when the simplest renormalized operators are used, whereas they are true in their simplest form when using minimal subtraction. We will prove this in Section 6.6 also. The moral is that one cannot completely eliminate the problems. It is possible (Collins (1975b)) to construct a definition of, say, For
[d^d^/2] which uses minimal subtraction and for [(d</>)2]- This is done by writing tensors in terms
which
components. Thus we write a second-rank tensor
M^
antisymmetric term,
M?v
=
a
symmetric traceless term, and
i(M?v" MJ + i(M?v
The subtraction
MVfi
+
-
g^ld^c^/l]
=
of Lorentz-irreducible as
an
the
sum
of
an
invariant term:
{2ld)g^MD + (1/^VM?.
procedure is applied to each term separately. This definition
loses other properties of the renormalized products. For example, conservation of energy and momentum is a consequence of the fact that the bare 0. If we define a energy-momentum tensor 0fiV has zero divergence: <3"0^v renormalized energy-momentum tensor [0 ] by our original definition of =
minimal subtraction, then this is the
same as
redefinitions and it is conserved. But if we
just suggested, then it is
our
0^v
up to allowed
[0 ] by the procedure
not conserved.
6.6
One of
the bare
construct
motivations for
Properties
working
out the
theory of renormalization of
that in Chapter 2 we had proved equations of composite operators motion and Ward identities. These results involved unrenormalized was
composite operators. So
operators,
now
we must prove
that
the
we
have defined renormalized
equations
composite
of motion and Ward identities
expressed in terms of these renormalized operators. This is particularly important for the Ward identities, for these express the symmetry properties
of the theory. In this section
we
will derive
renormalized operators. Some
a
properties
will be
others will be the actual equations of motion and will be
given
for the
case
that the operators
subtraction. A typical proof of
corresponding equation is
some
true
properties of the purely technical, while Ward identities. Our proof
number of useful
are
equation for
the
renormalized by minimal
starts
by observing that the
unrenormalized
operators.
procedure applied to both sides of the unrenormalized equation, so the main problem is to find the places where the Renormalization is almost the same
renormalization procedure is not identical for the two sides.
6.6
153
Properties
It is always possible to make the theorems false by changes in the renormalization prescription. The point of using minimal subtraction is that it is
universal prescription that preserves almost all of the desirable properties. (The reason is that it amounts, roughly, to defining each counterterm by the requirement 'remove exactly the singularity'.) These a
properties
are
relations between different operators. prescription that preserves most of
The other standard renormalization these relations is the
BPH
(or BPHZ) method of zero-momentum subtraction. In fact, the proofs were first given using the BPHZ prescription (Zimmermann (1973a, b), Lowenstein (1971) and Lam (1972)). However the use
infrared
of minimal subtraction is better for gauge theories because of their singularities. The proofs were given in this case by Collins (1975b) and
Breitenlohner & Maison (1977a, b, c). All these works However, the basic ideas are simple.
Property
rather technical.
1. Linearity.
a[A] where a and b are
equation
are
+
b[S]
=
[aA
+
(6.6.1a)
bB\
numbers, while A and B are composite operators. This interpreted as an equation for Green's functions of the
pure
is to be
operator. That is, if X is any product of renormalized operators or composite), then
a<0| T[A]X\0)
+
fe<0| T[B]X\0>
=
(elementary
<0| T[aA + bB]X\0}
(6.6.16)
Proof. This property is almost obvious. If A and B have different numbers legs (e.g. 02 and 04), then there is no simple way of defining the right-hand side of (6.6.1) except as being the left-hand side. But if A and B have the same fields, like (dcj))2 and ¥2, then the Feynman graphs for of external
<0|7\4X|0>, (0\TBX\0y differences are
corresponding
and
<0|T(aA + bB)X\0>
are
the same; the
only in the placement of powers of momentum. The equation to
(6.6.16)
is true for the basic
graphs (i.e.,
without
counterterms). To obtain the renormalized Green's functions we apply the forest formula to each graph. The terms in the forest formula are the same, since the graphs for the three Green's functions
are
the
follows from linearity of the subtraction operators
Ty.
Comments. a
same.
Then (6.6.1)
(1) It is necessary to be pedantic about this proof because: (a) it is
prototype for less trivial cases, and (b) it fails for the case ofzero-momentum
subtractions. The,reason for the failure is that the linear. For example,
[(<30)2]
and
[(<30)2
+
m2</>2]
Ty operation is then not
need two extra subtractions
154
Composite operators
compared with [02], because the degree of divergence is two higher. can only have
[(Wbph
+
?2[02]bph
=
So
we
(6.6.2)
[W)2 + m202]BPH
[02] operator is oversubtracted. The coefficients a and b in (6.6.1) must be independent of d, for otherwise (2)
if the
taking a pole part is non-linear. Furthermore, we cannot use (6.6.1) to show, [(30)2]. As we saw, this equation is in fact false. The proof fails because it can only be applied to the case that we sum over a finite number of operators. It does not automatically apply to infinite for example, ^MV[3M05V0]
summations.
=
However, when
we
defined dimensional regularization in
Chapter 4, we saw that our vectors and tensors have to have infinitely many components. Property 2.
Differentiation
is distributive. Let A be the
composite operator
A=f\ <t>j(x) where each
is
(j)j
an
elementary field
£"- Again this equation
dA_
is to be
or one
^IUw
-I
ox
interpreted
^<0|T[^(x)]X|0>
=
of its derivatives. Then
(6.63a)
jfi
for Green's functions:
<0|T[^/^]X|0>.
(6.6.36)
Proof. Let p" be the momentum leaving the Green's function (6.6.3b) at the 3/cbc^gi ves a factor ipM. The point of (6.6.3)
vertex for A. Then the derivative
is
to state that we
finite-part
get the
same
—
results whether
statement is true for the basic subtraction
Comments.
(1)
take pM inside the
operator
Ty.
We can contract \i with an index in A. Thus
9** Wd*<ft
=
Q'V^M
=
[(#)2]
Since the overall derivative merely gives a factor of
or not we
operation. To prove the equation, it is enough to observe that this
introducing
contrast to the
extra case
-
+
i/^
^-dependence by contracting
we
have
(6-6-4)
[0 □ </>]? there is no
possibility
with g^. This is in
considered in Section 6.5.
(2) Note that derivatives ordering. Thus:
are
always implicitly taken outside of the
<0lr(<^)(x>itf>2(y)|0>
=
lim—<0|T^(z)</>(x)i</.2(y)|0>.
time
(6.6.5)
6.6 Properties This
155
gives the simplest Feynman rules in momentum space, with each giving a factor of momentum on the corresponding line. lowest-order graph for the Fourier transform of (6.6.5) is
derivative of a field
The
ddxdVp'x+i9,3;<O|T(05M0)(x)^2(y)|O> =
Property
?*r]?
a counterterm
-m
<6-6'6)
_2v
%)d(k2-m2)[(k-p)2-m2y
Simple equation of motion. Decompose
3.
action and
g)J(2^/i,2
(2n)dd^(p +
the action into
a
basic
action: ^
=
^b
+
^ct,
(6.6.7)
just like the decomposition (5.1.1) of the Lagrangian, except that 9"h includes both the free and interaction terms: 9?h Jddx(if0 + ifb). Then define =
functional derivatives with respect to renormalized fields
^(X)S^W=^)-^^^)' y*\x) We
already know
=
by
ttA
=
5<p(x)
&*
with counterterms omitted.
the unrenormalized
we
(6.6.9)
equation of motion (2.5.5)
<0|T<^(x)X|0>=i^<0|T*|0>. Now
(6A8)
(6.6.10)
wish to prove the renormalized equation
<0|T[^(x)]A-|0>=i-^-<0|rX|0>,
(6.6.11)
from which follows the operator equation
[^]=0. Comments.
(6.6.12)
(1) The functional derivatives are to be treated in a purely formal
sense.
(2)
Even
though
[<9%(x)]
is
zero as an
operator, its Green's functions
define them by taking derivatives outside (6.6.11) them inside gives equal-time commutators the time-ordering. Bringing are non-zero
because
we
(Section 2.5). (3) Signs for fermions are easiest to determine by examining the derivation in Chapter 2.
156
Composite operators
Example.
In the
<f>3 theory JS?
we
=
Z(d<p)2/2
m2Z (j>2/2
-
-
g0Z3l2(j)3/6,
have
5%
[^] Then
=
-
=
-
of (6.6.11)
cases
ZQ4, □ 4>
-
ti0Z3'2<l>\
(6.6.13)
tf-'Vgtfl
(6.6.14)
m2Z4>
-
m2<t>
-
-
are
<0| T[^(x)]0(y)|O>
=
=
m2K0\ T0(x)0(y)|O> -^3"d/2^<O|T[02(x)](/>(y)|O>
(
-
nx
 ̄
(6.6.15)
idid\x ̄y\
<O|r[^(x)][03(y)]0(z)0(w)|O>R 3W?>(x y)<0| T[02(y)](/>(z)(/>(w)|O> =
-
+ +
i^>(x-z)<O|T[</>3(y)]0(w)|O> i(5(d)(x-w)<O|7[(/)3(y)]0(z)|O>.
(6.6.16)
Pra^/. general proof of (6.6.11) is rather complicated because of the arbitrary number of fields. To show the main points it is sufficient to prove one case, (6.6.15) in </>3 theory. The problem in proving the renormalized from the unrenormalized the Feynman graphs are is that equation equation A
different for the different terms. We write
m2)tf>-i0/i3-d/2</>2.
(6.6.17)
graphs are given in Figs. arising from the action. we evidently have
6.6.1 and 6.6.2. The
-(D
=
Examples
of low-order
counterterms
are
those
In momentum space
<0|r^o?^)|0>
?>|ryM <?(P)|o>=(p2
-
m2) +
+
=
(p2-m2)<0|T^(p)^)|0>.
|—+ (-O- —-) (-00+^0+0^
+
Fig. 6.6.1. Low-order graphs for (6.6.15) with free part of action.
(6.6.18)
157
6.6 Properties
<°m^£*jfo)io>
(o-O- +0—) (<i^+o-)
<^+
=
+
+
?
??
Fig. 6.6.2. Low-order graphs for (6.6.15) with interaction part of action. The
p2
—
m2 multiplying the free propagator attached
to
0(p)
cancels the
denominator of the propagator. Thus in the graph where the other end of the propagator is ¥(q) we obtain the right-hand side of (6.6.15). In all the remaining graphs the other end of the propagator is
interaction vertex, either
a
basic interaction
or a counterterm.
In fact
an we
obtain
<O|7\?lnl?0te)|O>
(6.6.19)
where
=
-(Z- l)n<t> -(m2Z-m2)(f)
-ig0Z3i2<j)2.
(6.6.20)
This is exactly what we must obtain in order that the unrenormalized equation of motion (6.6.10) is true. Notice that because (6.6.18) is finite, so is (6.6.19). We must now prove that the counterterms in
(6.6.20)
are
precisely
those
[^nt J= --£#/i3 ̄d/2[</>2]
that
are
needed
to
renormalized according to
give our
the operator standard
prescription for composite operators. Now, the renormalization prescription is precisely to add to the basic term S*nu<j> —^gfi3 ̄d/2(j)2 a series of counterterm operators whose coefficients =
poles at d 6, so as to make its Green's functions finite. But this is precisely (6.6.20). The relation between the counterterms can be seen from a comparison of Figs. 6.6.1 and 6.6.3. are pure
=
<^1*¥(P)\0>= +
(0"+
*
)
(oo+o—+*o-+*—)
+'" ,
Fig. 6.6.3. Renormalization of Fig. 6.6.2.
158
Composite operators
Property 4. Equation of motion times operator. With the before we have
<0|T[^^(x)]X|0>R
=
same
notation
i<0|T^(x)^X|0>R,
where A is any product of operators at the
same
point.
as
(6.6.21)
Hence
[A,?g=0.
(6.6.22)
Comments and examples. (1) This property is crucial
to
proving
Ward
identities. (2) All operators appearing on the right of (6.6.21) (3) In 4>3 theory, cases of (6.6.21) are
are to
be renormalized.
<0| T\_ <f> □ 4> m24>2 ^3](x)M#(z)|0> tf(x-y)<0|T4>0#(z)|0> + (y<-+z), <0|T[-^2n^-m2</>3-i^](x)[^2](y)</.(z)|0>R 2i,5(x-y)<0|T[>3]0#(z)|0> + i<5(x-zK0|T[tf>2](y)[,£2](z)|<)>R. -
-
-
=
(6.6.23)
=
(6-6.24)
Proof. The unrenormalized version of (6.6.21 )follows almost directly from the previous property in its unrenormalized version. Thisin turnfollows from the functional-integral solution of the theory, as shown in Section 2.5. We have
<0|T^(x),^(x)X|0>
-<0l^w3^)l0> i<0lT^xl°>- +
Then
we use
the fact that in dimensional
£(d)(0)=
(6A25)
regularization
|ddpl=0,
(6.6.26)
according to the results in Chapter 4. This enables us to eliminate the SA{x)/d(j)(x) term. The renormalized equation of motion (6.6.21) can be proved by generalizing the method for the previous property. It is enough to consider the example (6.6.23). Low-order graphs for the left-hand side of (6.6.23) are shown in Fig. 6.6.4. The (- □ m2) factor in ?S^O>0 cancels an attached propagator. If the other end attaches to an external field (viz., 0(y) or 0(z)), then we have a contribution to the right-hand side. If it attaches to an -
interaction, then the negative of a contribution with <9^nt is obtained, such
Fig. 6.6.5. Since these manipulations
do not change the
as
one-particle-
6.6
159
Properties
<O\T(<j>yoJ¥(k)¥(q)\0> --(q2-m2) <f
+(k2-m2) *9 p + k + t
+
{(p2-m2)
+
[(p
+ fc +
<?)2-m2] P
+
(k2 + q2 ̄2m2)-
Fig. 6.6.4. Low-order graphs for the left-hand side of (6.6.23), using free action.
<0\n<p^J¥(k)¥(q)\0}
+ cts
Fig. 6.6.5. Low-order graphs for the left-hand side of (6.6.23), using interaction part of action. (ir)reducibility structure, renormalization can be performed without
changing
the result.
Since the
SfQ^ terms
(1) Since □</>
=
need renormalization two subtleties arise:
#MV<3M</)SV¥,
the
ambiguity about
the
relevant. To preserve the derivation it must renormalization is performed.
placement of #MV is
come
before the
(2) In the BPHZ scheme m2</>2 must be oversubtracted otherwise we cannot use
linearity
to combine
—
</>□</>
and
—
m202.
The case of (6.6.24) involves two further subtleties illustrated In the first graph (a) the (-
by Fig.
6.6.6.
m2) multiplies the line coming back to a
□ </>(x) factor. This term gives zero after use of (6.6.26). The second graph (b) has two —
separated by a line. In the basic graph, the q2 m2 factor cancels the propagator to give graph (c), which has a different reducibility structure. 1PI loops
—
The first two counterterm graphs give the obvious counterterm graphs in Fig. 6.6.6(d). These correspond to the first two counterterms in Fig. 6.6.6(b). But the last counterterm in (b) has two vertices while the corresponding graph vertex. Their operator structure is different: graph (e) is another counterterm to renormalize the x -? y singularity of 04(x) </>2(y). Even so the
(e) has one
160
Composite operators
(k2-m2)
,
'Q
<■>
(b)
(92-m2)
z
/y
{xC^O-z W—.
'g^
(0
*^>—
1-
I
x—?
^ x
(e)
x, y
*
z
Fig. 6.6.6. Some graphs for (6.6.24).
two counterterm
graphs
must be
equal. They
are
both
a
single
free
propagator times a pole part coefficient times a polynomial in momentum times the same power of the unit of mass. They both make the complete Green's function finite. The general proof is rather tedious and
can
be found in Collins (1975b).
scheme, but also works for the BPH(Z) scheme. In the original BPHZ proof, by Lam (1972), of (6.6.21), there is no treatment of this complication, that the counterterms for the two sides are not in manifest correspondence i.e., that the forests are different. This proof was given for the minimal subtraction
-
Property 5. Ward identities: We will use the notation of Section 2.6 for transformations under potential symmetry operations. Let
Qj^Qj + Sifrj be
an
infinitesimal transformation of the fields under which the basic
Lagrangian transforms
as
-^basic
 ̄ ̄*
^basic
+
^b
+
^M *
b*
restricting our attention to the transformations generated by one particular generator of a group. Thus, as compared to Section 2.6, we now drop the index V, which labelled the generators. The subscript 'b' on Ab and Yb indicates that we are considering transformations on the basic Lagrangian (i.e., without counterterms). In the equations below, we will add in the We
are
counterterms by use of our standard renormalization scheme. Note also that in setting up the renormalized Green's functions, in Section 2.8, we defined a free Lagrangian if 0 and a basic interaction Lagrangian jSf b. We now work with their sum:
i?basic i?0 + jS?b. We proved earlier the unrenormalized Ward identity (2.7.6). The =
161
6.6 Properties renormalized Ward
identity
is
^r<o|TC/SW]A'|o>R =
<0| 7[Ab(x)]X|0>R
-
i<0|T5b4>(x)J^-\0}R.
(6.6.27)
We will have to prove it. From it follows
3?[/6] and
by integration 0
Of course
A
=
=
all
over
=
d4x<0| T[Ab(x)]X|0>R
0 for
symmetry. The
a
(6.6.28)
[Ab]
x:
-
id(0\TX\0)R.
(6.6.29)
current is
^^?J-Cn]- Proof. one
In
defining Yg and Ab,
we
(6-6.30)
have used the basic
with the counterterms omitted. This is because
Lagrangian, i.e., the the operation
we use
symbolized by square brackets to generate the counterterms. The proof of (6.6.27)-(6.6.30) follows the usual proof of Noether's theorem, but using the previously proved properties to write it directly in terms of renormalized operators. First -
we use
[Ab]
+
[<yg]
=
=
From this the Ward
(6.6.22). We exchanged
^L/b]
=
and
linearity
[djjb]-
I
distributivity
to obtain
d^l^^js^ J -
yg
+
[a, yg
-
<5i?basic]
-I[^^J. j
(6.6.31)
identity (6.6.27) follows by
the
the order of renormalization and
This is
permitted
-
see our
equation
tracing
of motion
over \i to
write
remarks below (6.6.4).
Comments (1) The theorem appears to give an unrestricted proof of the renormalized Ward identities. This appearance is false, since there are
symmetries that, can and often do have anomalous breaking-see Chapter 13. Such symmetries are dilatation and conformal symmetries and
162
Composite operators
chiral and supersymmetries. The potential for such anomalies can be seen by computing Ab in a regulated theory; it contains a non-zero coefficient which
vanishes
as
d->4. Such is the
chiral
symmetries (where
in the
proof
for conformal transformations and for
case
the transformations involve y5
eKXflv explicitly). Minimal subtraction is then not easily applicable and the properties we used are
or
false.
(2) Corresponding problems appear with any other regulator and with any other renormalization scheme (Piguet & Rouet (1981)). Minimal subtraction confines the problems to cases with anomalies. Property 6. Non-renormalization of current: Consider
an
internal
exact
the symmetry that
symmetry (such Compute the corresponding as
gives electric charge conservation). unrenormalized current;" from the complete
Lagrangian. Now/ contains counterterms Lagrangian. We will now prove that these /
=
Comments This theorem does not
derived from the counterterm
make / finite and that
(6.6.32)
[)£]?
apply
to
space-time symmetries
-
see
Callan, Coleman & Jackiw (1970), Freedman, Muzinich & Weinberg (1974), Collins (1976), Brown & Collins (1980) and Joglekar (1976) for the case of the energy-momentum tensor. It also cannot be extended to the
case
of a
non-
conserved current unless the
breaking term has dimension below that of S£ (Symanzik (1970a)). Furthermore, the proof does not apply directly if the transformation 5(j>}\s non-linear in </> It also needs generalization for gauge ?.
theories.
Proof.
Both/
and
Q/g]
consist of the basic current;g plus
some
minimal
subtraction counterterms. The difference
(6.6.33)
z?=r-[Jtt
series of pure pole terms, and we wish to prove it vanishes. Each term has dimension 3 or less (at d 4), since the currents have dimension 3. is
a
=
Now both
;£ and [jg] satisfy the
same
Ward identity,
so
dfl(0\Tefl(x)X\0)=0. Thus
5M£M
=
0, without
use
of equations of motion; any need to
(6.6.34) use
the
equations of motion would give a non-zero right-hand side to (6.6.34). In the absence of gauge fields, it is impossible to construct such a term. The theorem is thus proyed. In the presence of gauge fields, such terms do exist. For example, in
6.7
Differentiation with respect
quantum electrodynamics
proportional
current
we
<3V.PV,
to
to
parameters in ££
163
the electromagnetic field-strength tensor dilAv might have ^oceK^v3K(^^^),
have
a counterterm to
where ¥ ^ is the
—
With non-abelian gauge fields one but the presence of the £kA/iv indicates a chiral symmetry, which in any case needs special treatment. Moreover, in a non-abelian theory, we must also
d^A^.
take account of the constraints
subject
we
imposed by
The energy-momentum tensor also has
(djA
—
gauge invariance, which is
a
will not treat until Chapter 12.
g^O)^2
-
see
counterterms, like
possible
the references quoted above.
6.7 Differentiation with respect to parameters in S£ Consider Green's function derived from the bare classical ¥£
by using
=
the functional
GN
Z(dA)2/2
m2BA2/2
-
-
Lagrangian
gBA*/4!
integral: =
(0\T<t>(x1)...<t>(xN)\0}
J<
j[dX]A{x1)...A(xN)&* Differentiating with respect
to gB
(6.7.1)
gives
[dA]A(Xl)...A{xJ^\d*yA4(y)y^
80^
[cUJe"-2, J
|J[d^]A(x1)..M(xw)e,^}||[d^]^)JdV40')ei'jr} [dA]e"^ U)
=
1
d4KO|T0(xJ...0(xN)f^-<O|^^|O>)|O>.
(6.7.2)
Similar formulae hold for differentiation with respect to the other parameters Z
or
m\.
The renormalized
equivalents of those equations are also useful.
One
use
164
Composite operators
will be to show, within can
perturbation theory, that terms quadratic in the fields Lagrangians without affecting the
be shifted between free and interaction
Green's functions. First consider differentiation with respect to the renormalized coupling, g. We differentiate the renormalized Green's function
(6.7.3)
rL
Applied
to each basic
graph
y
T in this formula, the differentiation
1
—
TT
nocounterterms"
Renormalization of (6.7.4) produces those in
a set
of counterterms
just gives (6.7.4)
isomorphic
to
(6.7.3). So
dg°N-
ddy<O|T{[0(y)4]-<O|[(/>(y)4]|O>}(/>(x1)...(/>(xN)|O>.
4!
(6.7.5) The subtraction of the because no
vacuum
connected to Suppose
some
we
=
z(d<f>)2/2
and let the free and interaction
<e0
<eh
m\
+
m\
[04] comes about </>4 vertex in dGN/dg is
of
let the basic Lagrangian be ^basic
with
expectation value
vacuum
bubbles are used in GN. Thus each external line.
=
=
=
-
m2<t>2/2
Lagrangians
(d<j>)2/2 m2<j)2/2 (z l)(d(f))2/2 (m2
-
gtpH!
-
-
m2. Notice that
-
we
-
(6.7.6)
be
m2)(t)2/2
-
have allowed the
gif/A!,
(6.7.7a) (6.7.7b)
(d(j))2 term to have an
arbitrary coefficient. We choose to put some of the terms quadratic in <j> into so that we can derive an equation for dGN/dz or dGN/dm2 like (6.7.5). Then we will show we can move the quadratic terms to the 5£0 without changing the Green's functions. the interaction,
Differentiation with respect to
JzGn
2
z or
m2 gives
d"y<0|r{[(o</))2]-<0|[(a^)2]|0>}^(x1)...</)(xJV)|0>) (6.7.8)
_d_
dm2
GJV
=
^j'd''y<O|r{[?/)();)2]-<O|[0(y)2]|O>}^(x1)..^(xN)|O>. '
,
(6.7.9)
Differentiation with respect
6.7
However,
Lagrangian.
we
may want to put all of
to parameters in 5£
(<7</>)2z/2
m2(j)2/2
—
165
into the free
In this case the free propagator is
i/(zp2-m2). (6.7.8) and (6.7.9) remain valid. In this case d/dm2 applied to an unrenormalized graph
We wish to prove that
terms
T gives
a sum over
in which each propagator is differentiated i
d dm2
zp2
—
m2
zp*
(6.7.10)
m
This gives us the same result for the unrenormalized graphs as the right-hand side of (6.7.9). Next,
we
differentiate
a counterterm
propagator is differentiated, so that the
-
graph Cy(r).
Either
a
i in (6.7.9) gives the basic vertex for
[<t>2(y)]>or a counterterm C(yx) is differentiated. In this second case there is a counterterm graph Cy(dT/dm2) with a term cyjdm2
also
Now
C{dyjdm2)
=
(6.7.11)
dC(yi)/dm2
in the minimal subtraction scheme. (The reason is that both are defined to be pure poles times p to a power the same for both graphs.) We thus obtain all -
the counterterm graph for the right-hand side of (6.7.9). Similarly (6.7.8) is true if z(d(j>)2 is all in the free Lagrangian We thus see that, for any renormalized parameter
a
in the
if0. Lagrangian S£,
have
we
-GN CA
=
i
\ddy(0\T
*^ ̄
basic
[^
(y)
<J> basic
M O)U(x,)...0(%)IO>
da
(6.7.12) From this result we can see that the Lagrangian (6.7.6) is equivalent to the one with unit kinetic term
£'h
by
a
=
(84>')2/2
-
m'2<t>'2/2
g'4>'4/4!
-
(6.7.13)
scaling of the field, with 4> m2 g
The proof is to write (6.7.6)
=
(6.7.14a) (6.7.14b) (6.7.14c)
z-v24>\
=
zm'2,
=
z2g'.
as
^baS,c= z(8<f>)2/2
-
m'2z<t>2/2
-
g'z2cf>V4!.
(6.7.15)
166
Composite operators
Then differentiation of a renormalized Green's function GN of 0 with respect to z gives
zdGN/dz
=
zdGN/dz\fixed^g +
=
m2dGN/dm2 \fixcdZjg + 2gdGN/dg\f ixcdz.m
d4y<0| T{[z(<l<f))2/2
1
—
-
vacuum
d4y(<O|T{[0f5^^]
m2</>2/2
2#4/4!]
expectation value} 0(x1)...0(xw)|O>
vacuum
-
-
expectation value}
x
(6.7.16)
x0(x1)...0(xN)|O?. We now use the
equation of motion (6.6.21) with
A
=
0
to
give (6.7.17)
zdGN/dz=-NGN/2. From this it follows that
i.e.,
<O|T</>(x1)...0(xN)|O>
z-^2<O|T0'(x1)...0'(xN)|O>,
=
(6.7.18)
exactly as we would expect. The proof is non-trivial only because we are shifting terms between the free and interaction Lagrangians. Thus we must ensure
that counterterms do not go astray.
6.8 Relation of renormalizations of
<f>2
and m2
g2 the renormalization factor Za for <f>2 in (6.2.13) and m\jm2. This relation is as we will now prove. (We are now back in 03 theory at
Observe that at order
(6.2.11) is the inverse of the renormalization factor true to all orders, d
6.) We use the renormalized formula
=
dm2
GN
=
i
I d4y< 0| T< m2^-^;
—
vacuum
expectation value
> x
X(j)(xl)...(j)(xN)\0} -1
2 But
we
2
-mz
d4KO|r{[02]-<o|[02]|o>}0(x1)...0(xN)|o>.
also have
(6.8.1) if
=
{d<j>0)2/2
-
m2Zm02/2
-
0o^/6,
(6.8.2)
6,8 Relation of renormalizations of <f>2 and m2 so
167
that
m2d£?/dm2
=
-
m2Zm<^/2.
(6.8.3)
Hence 8 dm2
N
2
ddy<O|T{^-<O|02|O>}0(x1)...^(xN)|O>. (6.8.4)
Therefore dVM=
d>o^,
(6.8.5)
from which the desired result follows. Generalizations of this method can be found in Brown (1980) and Brown & Collins (1980).
7 Renormalization group
As
we saw
in
Chapter 3,
the renormalization
arbitrariness: the counterterm for
a
graph
procedure
has considerable
must cancel its
divergence
but
may contain any amount of finite part. A rule for choosing the value of the counterterm we called a renormalization prescription. In one-loop order it
examples that a change in renormalization prescription by a change in the finite, renormalized couplings each to corresponding divergence. Thus a change in renormalization prescription does not change the theory but only the parametrization by was
clear from the
can
be cancelled
renormalized coupling and mass. What is not so easy is to see that this property is true to all orders. This we will show in Section 7.1. The
theory under such transformations is called renormalization-group (RG) invariance. A particularly useful type of change of renormalization prescription is to change the renormalization mass \i. Infinitesimal changes are conveniently described by a differential equation, called the renormalization-group invariance
of
the
equation, which is derived in Section 7.3. This leads to the concept of the effective momentum-dependent coupling. This concept is very useful in calculations of high-energy behavior, as explained in Section 7.4. The
renormalization-group equation are called the renormalization-group coefficients and are important properties of a theory. Various developments of the formalism occupy the remaining
coefficients
in
the
sections. The renormalization group was first discussed by Stueckelberg & Petermann (1953) and by Gell-Mann & Low (1954). Very similar ideas are
applied in statistical physics (Wilson & Kogut (1974)). Many important applications arise because of the asymptotic freedom of QCD. Results of calculations of renormalization-group coefficients can be
recent
found in many places: (1) Gross (1976) lists many one-loop results for theories with scalars and fermions and up to two loops for gauge theories with only fermions.
(2) Cheng, Eichten
& Li (1974) give the ^-function for able theory to one-loop order.
168
a
general renormaliz-
Change of renormalization prescription
7.1
169
(3) Tarasov, Vladimirov& Zharkov (1980) compute renormalization- group coefficients to three-loop order in gauge theories with fermions
using minimal subtraction. (4) Vladimirov, Kazakov & Tarasov (1979) compute to four-loop order in
</>4 theory. (5) Chetyrkin, Kataev
& Tkachov (1981) and Chetyrkin & Tkachov (1981) compute the anomalous dimension in ¥4 theory at five-loop order. (6) Tkachov (1981) summarizes the methods used for the above calculations. (7) Caswell
& Zanon (1981)
theories at
three-loop 7.1
Change
techniques will
we
alternative, but
all,
we can
will describe
03 theory
prescription
valid for any theory. However, to be
are
in six space-time dimensions. There
are
three
equivalent, forms in which to write the Lagrangian. First of
write it in terms of the bare field </>0: if
(As before,
of renormalization
mainly work with the theory we have been using as a source
specific, of examples, the we
calculations in supersymmetric
Change of parametrization
7.1.1
The
perform
order.
we
=
(d<j>0)2/2
-
m20<f>20/2
-
g0<t>30/6.
(7.1.1a)
ignore the term linear in </>.) The importance of this form is 0O is invariant under change of renormalization
that the bare field
normalization is determined, because it satisfies canonical commutation relations. equal-time
prescription: its When
we
renormalize the theory,
the renormalized field </>
=
Z ̄
Lagrangian is Se
=
=
1/20o.
Z(dcf>)2/2 Z(d<t>)2/2
-
-
we
obtain finite Green's functions of
In terms of the renormalized
m20Zcf>2/2
-
field, the
g0Z^2^/6
m2B(j>2/2 -gB<j>'l6.
(7.1.16)
This is the second of the three forms. In the perturbative theory of renormalization, we wrote the Lagrangian as the sum of a free Lagrangian, a basic interaction Lagrangian, and a counterterm
Lagrangian: =
ifbasic
+
(7.1.1c)
i?ct.
This is the third form of the Lagrangian. Here,
we
have chosen to define
a
170 basic
Renormalization group
Lagrangian ^basic
=
(d<P)2/2
-
m2<t>2/2
-
(7.1.2)
fi3-d'2g<p3/6,
m and g are the renormalized mass and coupling. Since we will mostly minimal subtraction, it is sensible to define g to be dimensionless, and therefore to introduce the unit of mass \i. The counterterm Lagrangian is
where use
5£cx
=
bZ(d(f))2/2
-
bm2(j)2/2
-
(7.1.3)
bg^/6,
bm2, and bg are computed as definite functions of and with the aid of some renormalization prescription. g, \i To be concrete, let us use minimal subtraction, so that and the counterterms bZ, m
bZ=
£
(7.1 Aa)
(6-rf)-V7.(0,m,/i), 00
bm2
Sg
=
m2
=
^--/2
£
(6
-
J
d)-Jbj{g9m9ri9
(6
_
7=1
(7.1.46)
d)-jaj{g^m^y
(7.1.4c)
We saw, in Section 5.8, that in fact the coefficients at, bi9 c{ are independent m and \i\ they are functions of the dimensionless coupling g only.
of
However
we
will not
use
this fact at the moment.
The three forms of the Lagrangian listed in (7.1.1) are equivalent if we use any of them in the functional integral, then the same Green's functions -
will result. The coefficients Z, 6 with #,m, and \i fixed. The
m2,, and g0 will be singular when d approaches singularities
will be just
such
as to
give
finite
Green's functions of 0 at d 6. The parametrization of the Green's functions by g, m, and \i is rather arbitrary. Suppose that we change variables to g\ m\ and \i, which are =
given functions of g, m, and \i. These functions may even depend on the regulator, d, provided that the change of variable remains non-singular at d 6. Then we get the same theory, for the collection of Green's functions is GN unchanged. It is just the numerical values of the renormalized mass and coupling and of the unit of mass that have changed. We may even change the scale of the renormalized field by writing </>' (0, where ( is finite. The Green's functions are now different:
some
=
=
G'N
=
ZNGN.
physical observable like the S-matrix. For example, consider an S-matrix element involving N particles. It is obtained from GN by (1) dividing out an external propagator G2(Pi) for But observe that the value of (is irrelevant for
a
7.1
Change of renormalization prescription
111
each external line and (2) multiplying by z1'2 for each external line, where iz is the residue of the pole of Thus S= =
G2(p). Finally we
let the momenta pt go on-shell.
\gnzn'2/y[G2} I
lim
p^m*h I
)
i=l
\im{G'N{^2flY\G'2).
(7.1.5)
But
G2 so
the
=
(7.1.6)
C2G2,
particle pole is at the same position in G2 as in G2 and the new residue
is z'
=
t2z.
(7.1.7)
Hence
S=\im{G'N(z'V2)N/Y\G'2}. and
the S-matrix is invariant,
as
(7.1.8)
claimed.
7.1.2 Renormalization-prescription dependence
parameters. So there two-parameter collection of physical theories obtained
In the bare Lagrangian (7.1.1a), there
are
only
two
should be only a from it. As we have just seen, the freedom to vary the scale of the renormalized field
cf> in
introduce a third real
the second form of ££, viz.
(7.1.16), does
not
parameter into the physics.
Unfortunately, we appear to have introduced a large and indefinite number of parameters by having to choose one out of the infinitely many possible renormalization prescriptions. One might suppose that in different renormalization prescriptions, the singular behavior of m0 and g0 as d-^6 could be different in such a way that one picks up different phases of the theory. In fact, this is not so. We will show that a change of renormalization prescription is one of the reparametrizations discussed in the previous subsection 7.1.1. This is the property group invariance. Even within
a
we
have defined
as
single renormalization prescription, we
renormalization-
have introduced
third parameter, the unit of mass, p. Notice that the basic
Lagrangian
a
does
and g separately, but only on the combination p3  ̄dl2g\ but notice also that this property is not true for, say, the renormalized Green's
not depend
on p
functions at one-loop order. However, renormalization
a
change of p is
in effect a
prescription. Indeed, prescription the requirement that p and
the renormalization
we
could include in
our m
change
in
definition of have
a
fixed
172
Renormalization group
ratio;
we
would still have two free parameters. A change in this ratio is then
change in renormalization prescription. Our proof of renormalization- group invariance will, in fact, only explicitly cover the case of a change in p. The more general case will be essentially the same. We will prove that if we change p to \i', then the physics is unchanged, provided that we choose suitable new values, g' and m', for the renormalized coupling and mass. That is, an S-matrix element S(g9m,p) satisfies a
S(g9m9fi) The bare The
S(g'9m\fif).
and coupling m0 and g0 are renormalized field 0' with the
mass
new
parameters is not the
where (is functions
=
a
same as
similarly invariant. new values (g\m\p)
of the
with the old values but is related by
finite function of g9 m, \i and p. Hence the renormalized Green's
satisfy GN(p1,-..,pN;g,m9p)
=
=
It will be
convenient, in
our
</>',
proof,
(7.1.9)
compute Green's functions of the
to
value p! of the unit of mass. Now, in terms the Lagrangian is
original field 0, but with the of the new field
t-NGN(pl9...9pN;g'9m,9fi') CNG'N.
<£
=
new
Z'd(f)'2/2
-
m'B2<j),2/2
-
g'^/6.
(7.1.10)
Here we write Z'
=
m'2
=
9B
=
Z{g'9m'lp'9d)9
(7.1.11a)
m2B{g'9m'9p'9d)9
(7.1.11b)
9B(g'^'>d).
(7.1.11c)
are the same functions of the new renormalized parameters m\ g\ and the original bare parameters were of the old renormalized parameters \x and m, g, p. To get the Green's functions of the original field 0 but with the new value of the unit of mass, we substitute £0 for 0' to obtain
These as
S£
=
Z(0',my//,rf)C2#72
-
m2B{g\m'WM2<b2
-gB(g\m\fi\d)C3(t>3/6. 7.1.3 Low-order Let
(7.1.12)
examples
remind ourselves how the changes in g and m are obtained at one- order., We must start at tree approximation, where we ignore all
us
loop
counterterms. In order that the two formulae
(7.1.16) and (7.1.12) for
5£ be
Change of renormaliza tion p rescript ion
7.1 the same in tree
approximation when 0'
=
0n.w
m'
=
m,
C
=
=
we
change
\i to
p,\
17 3
we must
write
(/V/O3"d/20,
1. (All tree
approximation.)
(7.1.13)
6 we have g' g to lowest order. We distinguish g' and defined to be exactly the value of g' in tree approximation. In higher-order calculations, g' gets corrections, but #ncw will be defined to be Note that at d
0new: 0new
=
=
ls
(\il\i? ̄dllg always. To treat
higher-order corrections, 5£
=
write the
we
Lagrangian (7.1.12)
(8<t>)2/2 m2<t>2/2 ii*-*2[g(iilitf-'l2Wl6 + d'Z(d(f))2/2 d'm2(f)2/2 (5Vy</>76. -
as
-
-
-
(7.1.14)
Our strategy will be to express the counterterms in (7.1.14) as minimal subtraction counterterms plus some new finite pieces. The finite pieces will
accomplish the change of parametrization. First we consider the one-loop graph for the
self-energy, Fig.
3.1.1. The
*)r/2-2(4V)3-'/2-
(7.1.15)
unrenormalized value is
r? This
=
'g2?-)20 ̄3rf/2) Jf 12S7T
is
invariant
g(li/fi')3 ̄d/2).
If
dx
1>2
-
P2**1
-
0
under
we use
the
unit of
(/*, m, #)-?(/*', then the counterterm is
transformation
mass \i,
m,
C(ri)=-pole(ri)
Next
and gnew counterterm changes to we
use
\i
=
(n/v')3 ̄d/2g
instead of \i and g. The
Notice that we define CfTJ to be the negative of the pole part of Yx, with the ^-dependence of #new ignored. That is, we consider the function Ti ri(P>0new>m>AO and extract its pole at d 6 with gncw fixed. This =
=
prescription ensures that we may later replace gncw by its value at d without changing the renormalized value of the graph. Since both C^) and C'iFJ cancel the divergence, their difference
=
finite. We may therefore obtain the
same
6
is
value for the graph plus
174
Renormalization group
by letting the counterterm coefficients and b'm2, each be a sum of two terms: counterterm
in
(7.1.14), namely b'Z
(7.1.18)
3S4n3(d-6)
384tt
64w3(d-6)
64tt3
[
(7.1.19)
d-6
The first term in each equation is the minimal subtraction counterterm for the new coupling, while the second term is finite as d -? 6. Using the formula (7.1.12) for the Lagrangian,
C2
=
=
we
may regroup these terms to
/new
l +
384tt3
d-6
92
1 H +
384rc3
I
64k3
Hence
(7.1.20)
d-6
L
(7.1.21)
d-6
obtain the value of m':
we
m'2
1 +
m2
=
1 +
?m
Also at d
give
=
6
we
3|_
Sg2 384tt:
+
d-6
■InMO
+
Ofo*)
0te4) as
d->6.
(7.1.22)
have C(d
We can
Sg2
384tt
apply the
=
6)
same
value is the factor in curly
l +
=
768w:
?In
(fi/fi') + 0(g4).
(7.1.23)
procedure to the vertex graph, Fig. 3.6.1, whose brackets in (5.3.5). The counterterm b'g is written
as 3
.3
?'3"d/2
?newr
.
64n3(d
-
6)
,/3-d/2 p'3-d/2r(^)fe-d-i 64jr rtrc3 d-6
onew
|_
with the result that
C3g'n'3-d/2
=
2 ?new
g^n'3-dl2U + 64jr3 r^2 d-6
(7.1.24)
7.1
Change of renormalization prescription
175
It follows that 9'
=
9(n/fi')r\3-d/2 1 + L
Our
6-d"
■{Ml111
l1+2567r3L
3^2
0(g4)\
M(p/p') +
2567T3
+
d-6
O(04) (7.1.26)
asd->6.
j
strategy for understanding the effect of a change in renormalization is to absorb the difference into
prescription
counterterm will itself generate
a
finite counterterm. The
divergent bigger graph. Finally we reorganize the Lagrangian by putting all the finite counterterms into the basic Lagrangian. Then we see that the change in renormalization prescription is exactly compensated by a change in the counterterms when we insert it
into a
parameters of the theory. What happens when we go to
higher order? An example is given by the two-loop self-energy graph of Fig. 7.1.1(a). Graphs (ft), (c), and (d) renormal- ize its subdivergences and its overall divergence. We write the renormalized graph with the original value of p as
£i(p,flf,w,/i)
Za
=
(We thatZfc Zc.) we replace p by p and g by #new. exactly as at (7.1.24), so that: used the fact
Zb(p, g, w, p)
=
=
+
2Z6
+
(7.1.27)
Zd.
The unrenormalized
Zfc(p, #new, m, p')
graph
is unchanged if
+
But the vertex counterterm is treated
[Zfc(p, g, m, p)
S6(p, #new, m, p')\
-
(7.1.28) The first term contains the counterterm for
by compensates
minimal subtraction with unit of
one
of the
mass p.
the difference. It has the counterterm
(a)
subgraphs, computed exactly replaced by the finite
The second term
(W
-X- (d) (c) Fig. 7.1.1. Self-energy graph with counterterm graphs.
176
Renormalization group
part in (7.1.24), viz. Ag
We
now
£1
=
S?(P?
^neWJ ?>
64tt3
/O
j2ri
we wrote out
Zj with
a3 u'3-di2 y^^
(7.1.29)
]'
d-6
write
+
Here
=
unit of
2Zb(p, #new, m, //)
+
+
Zd(p, #new, m, //)
^r^j+ Zd(A^w,^)-Zd(p,^new,w,^)>. (7.1.30) the first three terms
mass \jl
2rlA#/(#new//3 ̄d/2)
.
as
the minimal renormalization of
Z1 is finite. The term self-energy graph with one of its divergence which can be cancelled by a
The remainder is finite, since
is the one-loop
couplings replaced by Ag.
It has a
minimal counterterm: C Hence the term in
2riAg g^ ̄dl2
-i/>2). i^=Hm2 6)
=
647r3(rf
(7.1.31)
-
curly brackets is + 2C
+
[Sd(p,g, m, /i)
-
Sd(p, gnew, m, /i')
-
2C]. (7.1.32)
The second term is finite, since there
are no
remaining divergences.
It is of
the form
i( and
so
gives
-
Am2
+
AZp2),
rise to another finite contribution to m'2 and to £.
7.2 Proof of RG invariance To show to all orders of perturbation theory that g\ m' and ( can be chosen so that (7.1.9) holds, we generalize from our treatment of the examples. We write
(3g'n'3-<"2=gn3-<"2 + Ag, C2 1 + AC2, C2m'2 m2 + Am2. =
=
(7.2.1)
The original version of the theory has counterterms computed with unit of mass fi: &
We
now
change
=
&bisic
to unit of mass
\i'
+
&ct(n)-
and wish to show that
(7-2-2) identically
the
Proof of RG
7.2
same
Green's functions and total
changes of the form (7.2.1). The Lagrangian is written cp
with the basic
Lagrangian
Phasic
(As before,
=
define #new of the form
Lagrangian'
^c
=
cp
**^
the
are
obtained if
make
we
basic
(7.2.3)
-L.cp-L.cpf ' ""^ ' °^ c
same as
cv
before, but written
as
(7.2.4) m2?/?2/2 -gnev,n'3-"^/6. (/i///)3 ̄d/2#.) The term ifc is a 'compensating
(#)72
we
Lagrangian
177
in the form
—
°^
invariance
=
-
A(2d(f)2/2
-
Am202/2
-
(7.2.5)
Ag<j)3/6.
Lagrangian £f'ct in (7.2.3) is computed using minimal mass \i'. We may later reorganize (7.2.3) so that the basic Lagrangian is taken as if basic + J2?c. We may drop the ^-dependence of the finite counterterms Ag, AC2, and Am2, since renormalized Green's functions are finite functions of The counterterm
subtraction
with unit of
renormalized
quantities. Finally we may rescale the fields to give (7.1.10). proof is most easily given with the form (7.2.3). Each of the finite counterterms is computed as a sum of terms, one for each IPI graph contributing to the relevant Green's function: But
a
A0=£Ar(0),etc.
(7.2.6)
r
Particular
cases are
given by
^.iC2
=
m2
=
A7.i.i(C2P2 where the label
on
m2)
examples
c/new
-(n'lnf-<-\ ̄ d-6
?20new V//x)6-"-i 64n3
=
in Section 7.1. Thus
384tt3
&3.6.l9=H -
the
d-6
'3 -J/2 ?new
-
(n'/n)6-"-i
64n2
d-6
i[I,d(p,g,m,n)
-
Zd(p, gntw, m, ft')
A indicates the number of the
-
2C],
figure depicting the
basic
graph. The
graph -
general proof
is by induction
on
the size of
G contributing to some Green's function.
gn3 ̄dl2(j>3/6 we renormalize it
with unit of
a
graph.
Consider
a
Using the basic interaction
mass ft
using
the method of
Section 5.11. The renormalized value of G is then R(G)
=
G+
£ Cy(G).
(7.2.7)
Renormalization group
178 Here the
sum
is
over
subgraphs of G that consist of one or more disjoint
1PI
graphs, and Cy(G) denotes the replacement of each of these 1PI graphs by a counterterm with unit of mass \i. Similarly we can renormalize G with a different
unit
of
mass
but
\i
with
the
basic
same
interaction
-^^-"'Wtoget R'(G) Here,
we use
the
prime
=
£ C'y(G).
G+
(7.2.8)
to denote use of the unit of mass \i
counterterms. These counterterms will be used to
compensating Lagrangian j^c in (7.2.3). The and
we
will arrange them
so
new
the original value R(G). The aim will be every 1PI graph y that is
Ay's
so
graphs
that when they
a vertex or
are
instead of \i.
graphs containing finite
We will now derive a series of new basic
generate the
will need renormalization, added to
R'(G\ we get back
to have a finite counterterm
self-energy graph.
Ay for
We will arrange the
that
R(G)
=
R'
G+X AV(G) + A(G)
(7.2.9)
>' £ G
Here the sum over y is over products of 1 PI graphs. The overall counterterm for a graph is computed using minimal subtraction, but with the d- dependence of #new and of the finite counterterms Ay ignored (see our remarks below (7.1.17)). That is, the counterterms in R'
at d
=
6 with their coefficients
vertexsubgraph,
are a
power series in #new. In the case of a there is also the usual factor jx'2 ̄ d/2. The finite subtraction
A(G) for the complete graph in (7.2.9) is only non-zero if G is a
series of poles
being
a 1 PI vertex or
self-energy graph. We will prove the
following
relation between counterterms for
a
1PI
graph C(y)
=
C'(y+ X AJLy))
The above equations (7.2.9) and (7.2.10)
+
(7.2.10)
m
trivially true for tree graphs, A(tree graph) 0. So let us assume they are true for all graphs smaller than a given graph G, with all the Ay's finite. There are two cases: (a) G is not overall divergent; then we must prove (7.2.9) with no counterterm A(G). (b) G is 1PI and overall divergent; then we use (7.2.10) with y replaced by G to define A(G). We must prove A(G) finite and prove (7.2.9). We must also prove A(G) is polynomial in the external momenta of G, with degree equal to the degree of divergence
where
no counterterms are
needed, for
are
we can set
=
7.2
of G;
we
will
assume
Proof of RG
inductively
invariance
179
that this is true with G
replaced by
any
graph. Consider the
smaller
terms in (7.2.7). For each we identify a contribution in G is the same as G in R\G). Next let y be a 1PI subgraph of The term (7.2.9). G (G itself being excluded). Decompose Cy(G) by (7.2.10):
(1) (2) (3)
The
C\y)
The term
The term
term occurs in
A(y)
occurs as
Cy(A#(y))
R\G)
as
Ay(G).
occurs
C'y{G).
as a counterterm
in the renormalization
R'(AS(G)). divergent, these exhaust all of the terms in R(G) and on of side right-hand (7.2.9). But if G is 1 PI and overall divergent then there
If G is not overall the
remains C(G) in (7.2.7) and the terms C
G+
Z A,(G)
+
A(G)
6¥G
in (7.2.9). We are therefore forced to define A(G) prove
A(G) finite. This is
now
easy, since
R(G\ R'(G\ and R'\ are
all finite, while
we
by (7.2.10), and it remains to
£ A,(G)
(7.2.11)
have K'(A(G))
=
A(G).
Moreover all the terms in (7.2.10), except possibly A(G), are ordinary minimal-subtraction counterterms. So they are polynomial in the external momenta of G, with degree equal to the degree of divergence of G. So A(G) is
polynomial, of the same degree. (Note that the replacement of a subgraph y by Ay does not change the overall degree of divergence of any graph T satisfying ygrgG. This is because of our inductive assumption on the polynomial degree of Ay.) The theorem is also true if R and R' stand not for renormalization with different units of mass, but for any two renormalization
important
that the
^-dependence
prescriptions.
It is
of Ay is taken outside of the extraction of
pole parts when computing a counterterm like Cr(Ay). This can be seen from the example of Fig. 7.1.1, at (7.1.31). The primed counterterms must be
particular function of #new, //, m', and d. The fact that g' itself is a function of other variables is ignored. Equation (7.2.10) expresses the counterterm C(y) for a graph y (with unit a
of mass pt) in and
a
terjns of renormalization counterterms with unit of mass \i
finite counterterm
A(y). We can use the finite counterterms to generate
180
Renormalization group
the compensating
Lagrangian i?c in (7.2.3), and the primed counterterms to new Lagrangian is the same as the original one (with unit of mass p\ considered as a function of 0 and 3</>. If we set (j)0 Z1/2</>, with Z Z(g9 m, p), then we can deduce that the bare parameters m0 and g0 are renormalization-group invariant:
generate
<g'cV
It should be evident that the
=
=
m0(g9m9p) Qo(9> m> I*) 7.3
=
=
m0(g'9m\p')9 9o(9'> m'> V'\
(7.2.12)
Renormalization-group equation
change in the unit of mass \i mass does not change the accompanied the while Green's functions satisfy theory, We saw in Sections 7.1 and 7.2 that
a
by suitable changes in coupling and
GN(x19...9xN'9g9m9p)
=
C ̄NGN(xl9...,xN9g'9m'9p).
(7.3.1)
m29 p9 and p\ If the ratio it not is is sufficient to lowest-order use large, p/p perturbation theory, since, for example, in (7.1.26) the coefficient of g2 may be large. An important device is to consider a large change in p as being made up of a sequence of very small changes, so that g\ m'2, and ( are obtained as We wish to compute g'9 m'2 and ( as functions of g,
solutions of differential equations. This is the subject to which we now turn. The consequence of our work in Sections 7.1 and 7.2 is that for a given
physical theory, we have for each value of p a definite value of the coupling mass m(p). These are called the effective (or running) coupling and
g(p) and
will derive differential equations for g(p) and m(/i). The easiest way to derive the results is to look at the Green's functions and the Lagrangian expressed in terms of the bare field 0O. The important
mass. We
point is that the Green's functions of 0O are invariant under our change of parametrization (fi9g9m)-+(n'9g'9m'). (This is because the mass and coupling in the bare Lagrangian are invariant.) 7.3.1
Renormalization-group coefficients
Physical quantities like the S-matrix are invariant under the change of variable (/*,#(/*), m(/^))^ (//,#(//), m(//)). This invariance is conveniently expressed by considering a small change in p9 accompanied by the corresponding changes in g and m. We write the result as
/idS/d/i The total derivative with respect to p
/jd/dfi
=
=
(7.3.2)
0.
can
pd/dp + pd/dg
be written
-
as
ymm2d/dm2.
(7.3.3)
7.3
Renormalization-group equation
181
right-hand side the partial derivatives with respect to n.gorm are fixed, and the coefficients p and ym give the variations of g{fx) and m2(/i) when /* is varied. The sign of the m2d/dm2 term is the usual convention (Weinberg (1973)). We have On the
taken with the other two
P
=
l*dg(/i)/dfr
(7.3 Aa)
ym=-m2tidm2<ji)/dii.
(7.3.46)
ft and ym are called renormalization-group coefficients. they are easy to calculate in terms of the counterterms, as
The coefficients
As
we
will
see
functions of g, m, \x, and d. If we use minimal subtraction, they have no mass dependence. This means that (7.3.4) can be readily solved as differential
equations
for
and
g(fi)
m(/i).
Indeed this is the easiest way in
compute the effective coupling and
practice
to
mass.
To compute /? and ym it is convenient to consider the Lagrangian expressed in terms of the bare field </>0 see (7.1.1a). We saw in Section 7.2 -
that
ml
and g0
are
jid0o/d/i
H&ml/dix Suppose
we
be solved to An
invariant:
renormalization-group =
O,
=
0.
have computed g0 and m0 to give p and ym.
example of such
a
calculation
(7.3.5)
some
comes
order in g. Then (7.3.5)
from
our
results
on
can
</>3 theory,
where
ml
m:
?}
^-"2i
=
3<72
1+2*^+<**} I+384^+0,*4>l-
9o=
"'3'6)
Thus 0
=
■%-*-*{?-<*-■£>
n^ n3-'?2\g(3-d/2)-^r-3
+
=
+
M
O^) 9a2
256k (a
-
6)
so
P(g)
=
(d/2-3)g--^ 3a3 y 256n
+
0(g5)
+0(g5)atd
=
6.
(7.3.7)
182
Renormalization group
Similarly 0
^ d/z
=
-m2ym[l
=
so
+
0(g2)-\
+
5g + 0(g3) 192n3(d-6)
m2p(g)
that
Observe that the (d/2 3)g term in /? is important in these derivations, even though the term disappears at d 6. Observe also that, even though the —
=
coefficients in g0 and m0 diverge at d 6, it is crucial to expand strictly in powers of g. A phenomenon true to all orders is that p and ym are =
independent
of
m
and \i,
provided
that
we
renormalize by minimal
subtraction. The general calculation of p and ym can be organized with the aid of (7.1.4) for the counterterms. Since we use minimal subtraction, the m- and \i-
dependence of
the bare parameters is 9o
ml
=
=
simple:
^ ̄dl29o(9>^ m2Zm{g,d).
(7.3.9)
Then
p(g,d)
=
ym(g) The
expressions (7.3.9)
are to
=
(d/2-3)g0 P(g,d)
Sg0 dg' (7.3.10)
dg
be expanded in powers of g with the aid of
(7.1.4). Now g0
=
=
(ti3-d/2g + ¥g)Z-3/2 axig)-lc,{g) /i3-d/2 g+- 6-d
Zm
=
(1
+
+ higher poles
}
8m2/m2)Z-
1+^4^ +higher poles' 6
where
we
h^ve picked
—
out the
counterterms (7.1.4). (These
(7.3.11)
d
single poles
are
in the series
expansion
of the
all that will be relevant.) Then (7.3.10)
7.3
183
Renormalization-group equation
becomes
r P(g,d)
=
=
y?(?)
f>
=
(rf/2
-
a 9 +
-
6--d
[i+a-\{g)- 6- 3)0
+ i
1
te)
g—
-
ifo) +
5g
+
higher poles higher poles
[fcto)
-
fll(ff)]
+
poles
(d/2-3)ff + /KffX
=
=
(7.3.12a)
"fci(g) ̄ci(g) +
[(d/2-3) + j5^)]-
6-d
higher poles
^[^1to)-*1to)].
These manipulations
are
made
(7.3.126) in powers of g. used have the fact that although
by expanding
In the last line of each of (7.3.12)
pole principle present, they must cancel in order that p and ym be 6. Notice that p and ym are independent of m and /*, and that the finite as d only ^-dependence is the (d/2 3)g term in p. Only the single-pole terms are we
terms are in -?
—
needed for the calculation. There is a series of relations between these and
higher poles that investigate these later. the
ensures
that the
poles
cancel in
(7.3.12);
we
will
7.3.2 RG equation
computed from two out of three equations (7.3.4) then be computed. To complete the calculation we
The RG coefficients p and ym
are
combinations of the counterterms. The differential
enable g and m' in (7.3.1) to
need £. This is related to the wave-function renormalization. It is easiest to obtain by observing that the bare Green's functions G(§) ZN/2GN are =
renormalization-group invariant,
so
that
N
Let
us
d
y^dn Z)GN.
(7.3.13)
d7lnZ;
(7.3.14)
define the finite coefficient ^
=
184
Renormalization group
GN satisfies the following renormalization Weinberg (1973)):
then
[^+r']G?=[^
+
=
38471-
2
""'"
dg
[(d/2-3)g + p(g)]
"2
N
Pir-ymm2 8m2
In the minimal subtraction scheme for y
group
dg
<p3
we
6-d
+
equation (e.g.
' ̄k=o.
(7.3.15)
find
higher poles
+0(g*).
(7.3.16)
7.3.3 Solution
equations to find g(n'), m(fi') and C,(n',n) in (7.3.1), The RG equation tells us that m(fi)
We wish to solve the RG
given
that
g{n)
=
g and
=
m.
"'d7ln c=^'d7ln tG?M/GN<M)l =
So
we
must
-M0&O].
solve this equation together with (7.3.4) for g and
boundary conditions
=
The
m,
(7.3.18)
C(/i,/i)=L can
m.
are
m(fj) Explicit solutions
(7-3-17)
be written:
ln(Ai7Ai)
=
"
m2([i')
=
m2([i)exp
dji
W) =
C(M'^)
=
m2(/i)exp<^
y
(a')
-
^pi-i\fljy(^))
H-Cvm[
(7'3I9)
Approximations can be made by taking a finite number perturbation series for /?, ym and y. For example: <
ln(Ai7rt=-
3
ruin')
dg
J g(n)
[1
+
1
128tt3
185
of Greens functions
7.4 Large-momentum behavior
of terms in the
O(02)] + 0
In
(7.3.20)
0(/*)
This is
accurate if g(fi') and #(/*) are small. Notice that g(ti')->0 is the property called asymptotic freedom. It is determined This \i the negative sign of the first term in ft at d 6. -?
oo.
as
by
=
The full solution to the RG
equation
is
N f dp, f  ̄—\  ̄| —y(g(fi))\GN(x\g(/i,)9nifi%fif). "
GN(x;g,m^)
=
exp\
(7.3.21)
7.4 Large-momentum behavior of Green's functions The most important application of the renormalization group is to compute large-momentum behavior. In this section we treat the simplest case, that of a Green's function
momenta
are
made
GN(pu...,pN) all of whose external large. (Notice that we have used our standard notation,
where the tilde indicates Fourier transformation into momentum space.) Let
suppose initially that all the Lorentz invariants formed from the momenta are non-zero. Then we scale all the momenta by a factor k: Pi
-+
by
us
KPn and let k get large. Thus all the Lorentz invariants pi-pj are scaled factor k2 and become large. Under these conditions, Weinberg's
a
theorem tells
us
that at least in
graphs for GN carry
asymptotic behavior
a
renormalizable theory all internal lines of and that graphs for GN have the
large momenta,
/cdim ^'(logarithms
of k).
(7.4.1)
Since all propagator denominators are large, we should be able to neglect masses and make only an error a power of k smaller than the leading
behavior (7.4.1). For example consider the propagator in </>3 theory at d 6. The at like is the while correction graph goes i/p2 large p2, one-loop =
g
128tt3
&>-!)+
dxx(l-x)ln
°bln
p2x(l
—
x)
47T/i2
m
(We used (3.6.10) for the self-energy graph
tree
(7.4.2) to derive this
equation.)
186
Renormalization group
Now there is a term proportional to ln(- p2/p2)/p2 tnat 8ets large relative to the tree graph if p2 is large enough. Thus the perturbation series has large coefficients and is not directly useful. However we may use the
renormalization group to set p2 0(p2). This makes the coefficient small again. So we use the following strategy to compute GN(icp) at large k: =
(1) Set p!
=
Kp, and use the solution of the RG
GN(Kp, g, m, p) (2) Neglect analysis
=
to
to write
C(/c/i, p)-N,2GN(Kp, g{Kp\ m(Kp\ Kp).
(if m(Kp) does give
m
equation
not get too
large). Then
use
(7.4.3)
dimensional
GN(Kpy g,myp) ̄( ̄ ",2GN(Kp, g{Kp\ 0, Kp)
^^a^py^'G^g^Apy (7.4.4) (3) Large /c-dependent coefficients, as in (7.4.2), are now removed, so if g(Kp) =
is small,
a
low-order calculation suffices.
This procedure makes it evident that the coupling that is relevant is the effective coupling at the scale of the momenta involved. we have related the large-momentum behavior of GN to the finite-momentum behavior of the zero-mass theory. It is therefore crucial that the zero-mass limit exists. However this limit does not always
It should be noted that
exist: if
we use
mass-shell
or zero-momentum
from, for example, (3.4.7), that the
subtractions, then
we see
considered in (7.4.2)
self-energy Weinberg's theorem tells us that, in the dominant momentum region for a graph without counterterms, all lines are far off- shell; hence masses can be neglected. So the problem must be that with mass-shell or zero-momentum renormalization prescriptions, the counter- terms diverge as m -? 0. This is easily checked from our explicit calculations diverges
as
same
as
m->0. Now
(see (5.10.2)). We can now
see the practical importance of the theorem whose proof was summarized at the end of Section 5.8, that the counterterms may be chosen
polynomial in masses. It ensures that the zero-mass limit may be taken directly and used to compute large momentum behavior. The minimal subtraction scheme is one way of ensuring that counterterms are polynomial in mass. If one uses, say, zero-momentum subtractions, large-momentum behavior may be computed by changing renormalization prescription to, say, minimal subtraction. Another approach is to observe that the logarithms of p2 break a,possible symmetry of the theory under scaling transformations. The consequences of this point of view were worked out by Callan (1970)
7.5
Varieties
of high- and low-energy behavior
187
and Symanzik (1970b). They derived the Ward identity for scaling transformations. It is called the Callan-Symanzik equation and looks similar to the RG equation. This equation may also be used to discuss high- energy behavior. 7.4.1
Generalizations
GN when all momenta are scaled by a large factor k is not normally experimentally relevant, for all the external momenta are then far off-shell. In coordinate space the corresponding region is the short-distance limit of GN(xl9...xN)9 where every Xj xk^> xk is made small: xj (Xj Xk)/K. The behavior of
—
—
-
This
means
that
we
should be able to
renormalization of the
use
RG methods to discuss the
theory, for renormalization is
a
purely
short-
distance phenomenon. We will work out the details in Section 7.10. On the other hand, physical experiments involve long distances. To get results for high-energy experiments we need the so-called factorization theorems. The simplest of these is the operator-product expansion which we will treat in Chapter 10. These theorems typically give a cross-section as a product of a factor which can be computed by pure short-distance methods and of
simple factors particles.
related to wave-functions of the
We could also
use
incoming and/or outgoing
RG methods to discuss the infra-red limit
k -?
0. This is
only useful if masses can be neglected. Certainly this is true in a purely massless theory, as we will see in Section 7.5.4, and then IR behavior is computable if and only if the theory is not asymptotically free. But in a massive theory, it is not useful to take \i much less than one obtains logarithms of m//i, and these prevent
perturbative
a
typical mass, for simple use of
a
methods when \x^m. 7.5 Varieties of high- and low-energy behavior 7.5.1 Asymptotic freedom
solving the RG equation to obtain high-energy behavior, we find two according to whether fi is positive or negative. In this section we discuss the asymptotically free case, when p is negative. Suppose the effective coupling g(n) is small for one value of \i, so that p is well In
cases
approximated by its first term. Then the evolution equation (7.3.4a) shows even smaller at larger values of fj, and in fact goes to zero as
that g becomes /^-?oo.
Thus perturbation theory is reliable for
computing high-energy
188
Renormalization group
behavior. From
(7.3.7)
03
that
we see
free.
in six dimensions is
asymptotically
It is instructive to compute the behavior of g from the first two terms in Let
us
/?.
define
P=-Aig3-A2g5 + 0{g7). We have the
(7.5.1)
equation for the evolution of the effective coupling:
^
=
(7.5.2)
P(g)>
Equations of the same form as (7.5.1) and (7.5.2) hold in any renormalizable theory, for example in QCD, though A { and A2 are not necessarily positive in the general case. The solution of (7.5.2) is In /i
=
constant +
=
constant +
=
constant +
dg'/f}(g') fg{n)
The constant
can
is conventional
+
0(g2).
(7.5.3)
(Buras, Floratos, Ross & Sachrajda (1977))
mass.
\A 2A, ̄
2
to write the
In (A,), where A is a parameter with the
Then
ln(|z7A') ln(M2/A2) so
^|ln [^)]
be computed from a knowledge ofg(n) at one value of \i. It
constant in the form In A +
dimensions of
^. +^+°(9': A.g'3 A\g 2 +
^
.,-,],
-1
W
A
1
=
——-J + —
Alg(nY
\n[Ald(fi)2 ̄\ + 0(g(tf), -I A]
(7.5.4)
that 9W
AMn'/A2)
A]\n2(n2/A2)
+
U\
ln3(,i/A)
f
(0^
specification specification of A.
of g(n) at one value of n is exactly equivalent to a The precise choice of the scale of A is that in (7.5.5) the
omitted terms
of the order shown rather than of order
A
are
expansion (7.5.5) is much used in QCD. A higher-order be made from the following form of the solution ln
(^/A2)
1
=
t-1732 +
At'gW + 2
I?
l/ln2(^/A). The
calculation of g can
A2
lA i^)2] -rlln A2'
,rj_
i
l0 9\_W)+^7q A.g'3
A\g\
(7.5.6)
7.5
Varieties
of high- and low-energy behavior
It is necessary to go to
two-loop order to
of (7.5.6) that
as
diverge
obtain both the terms
189 on
the
right
#-?(). The values of m(fi) and £ may be similarly calculated. For example, if y=Cl92
then
(7.5.7)
+ -,
L2Jgw
9
At
J
"""it. [l oc
7.5.2 Maximum
+
[InOi'/A*)] "Cl/4i4S
accuracy in
an
0(^2)] as
//
- oo.
(7.5.8)
asymptotically free theory
The results above enable calculations of Green's functions to be made at
high energy. By taking more and more terms in the series, we may improve predictions. However, in general, perturbation series are asymptotic,
the
not
convergent. A trivial example is the ordinary integral /*00
I(g,m) This
can
theory at J
=
=
m(2n)-112
J
dzexp(- m2z2/2-gz4/4\) —
00
a functional integral in Euclidean 04 field space-time dimension with the normalization chosen to give 0. The perturbation expansion is
be considered to be zero
1 when g
=
I ̄m(2n)-U2
£
N
=
0
=
0
oo
■-gz*Y± dzexp(-mV/2)(^-) JV! V ^
-gy__w2r(2Af r(jv
+
w?oU?!V t (zxYh-
N
-
+
"
-
r J
?
/
l/2) i) (7-5-9)
m
Now
IN
=
NN(^\ (nNyl/2[l
+
0(l/N)l
asN^oc,
(7.5.10)
divergent. The divergence is associated with the fact that the defining integral diverges if g is negative, so that J is not analytic at g 0. The corresponding property of 04 theory is that the Hamiltonian is unbounded below (i.e., the vacuum does not exist) if g is negative. so
the series is
=
190
Renormalization group
What
say is that
we can
/
by truncating
=
the series:
(7.5.11)
+
N
error
approximate
I (#T^ + 0[(#Y 1].
/= The
we can
0
is estimated by the first term omitted, i.e.,
These results
are
standard in the
theory of asymptotic expansions for simple
integrals. All experience, together with rigorous theorems for quantum mechanics and super-renormalizable field theories (Glimm & Jaffa (1981) and references therein), indicate that this behavior is in a non-trivial dimension (i.e., d > 0).
typical
for functional
integrals
Next, let
suppose we wish to compute some quantity in free theory by truncating its perturbation expansion
us
asymptotically
an
I 9(H)2NIN-
N
We assume that the set
jj,
the unit of mass
=
=
0
quantity depends
on some momentum p,
and that
we
be of order p. A case would be the propagator with that the coefficients in the expansion behave like
\i to
0(\p\). Suppose
IN
 ̄
NNbNNad[l
+
0(1/N)],
(7.5.12)
for large N. What is the best accuracy with which we can calculate the quantity ? This is given by the minimum error, i.e., the minimum of INg2N as N
varies. The result is that the minimum possible
error in a
perturbative
calculation is of order
constant|p|-2^/e*(ln|p|)fl. that beyond a certain level, power-law corrections to the behavior computed in perturbation theory are meaningless since they are smaller than the irreducible error in using perturbation theory. Power-law corrections are those that are a power of p2 smaller than
This
means
asymptotic
the leading term.
7.5.3 Fixed point theories In four dimensions, the only theories that are asymptotically free are non- abelian gauge theories with a small enough number of matter fields see Coleman & Gross (1973) and Gross (1976). Other theories, like 04 and -
QED, have an effective coupling that increases with energy. Thus, in such theories it is impossible to compute the true high-energy behavior by perturbation theory. (Note however that the coupling in QED is a/n
of high- and low-energy behavior
7.5 Varieties
191
9* Fig. 7.5.1. fi(g) in
 ̄
1/430.
a
non-asymptotically free theory with
This is very small,
not occur until very many
a
fixed point at g
=
g*.
non-perturbative region in QED does orders of magnitude beyond experimentally so
the
accessible energies.) An interesting possibility is that P(g) has the form shown in Fig. 7.5.1, with a zero at g g*. Then #(/*) approaches the fixed point' g* as \i -? oo. At =
large momentum Green's functions behave like
GN(Kply..., KpN9g9 m9fi)
 ̄
const
Kdim^+iV^)/2GiV(p1,.. .,pN,g*Afi). (7.5.13)
This behavior is
Consequently, 0.
as
if
<j>
the function
had
an
extra
term
y(g)/2 is called
y(g*)/2
in its dimension.
the anomalous dimension of the
field
7.5.4 Low-energy behavior
If m
=
0, then the renormalization group
ofmassless theory can
be used to compute infra-red
calculability is the opposite of that for the UV behavior. Consider first an asymptotically free theory. There, the effective coupling g{fi) goes to zero when \i goes to infinity, so that short-distance behavior is computable perturbatively, as we saw in Section 7.5.1. But, when \i is small, g{jx) is large, so the infra-red behavior cannot be computed reliably by perturbation theory. (This is the case for strong interactions, according to QCD.) Let us now consider a non-asymptotically free theory. For large /*, the behavior. The
effective coupling is large,
so
the short-distance behavior is not
perturbatively computable. (For example, a perturbative calculation in low order of the position of the fixed point, g*9 in Fig. 7.5.1 and of the value of /?(#*) is subject to large errors from higher-order uncalculated corrections.) But when /* goes to zero, so does the effective coupling. We are assuming the absence of a mass term for the field, so there are no large logarithms of mj\i as fx goes to zero. Hence, we can compute IR behavior in such a theory, just
192
Renormalization group
as we
computed
U V behavior in
an
asymptotically free theory.
We will
now
do this. Now, for almost any
graph
there
large logarithms of p2/fi2
are
as
p2 ->0
massless theory, just as in the ultra-violet. In the case of the propagator, these logarithms mean that the propagator's singularity is not a pole, at in
a
least
To
order-by-order. investigate this singularity y{g)
=
let
Cl929
us
again write
(7.5.14)
P(g)=-Aig\
using the same notation as before, but now with A x
<
below the first
one.
non-zero
fixed point #*, if there is
0. We
that g is The propagator is assume
rg(Kfi)
G2(Kp;g(fi\fi) =
=
i/(/cV)[l
K ̄2G2{p;g(Kii\fi)Qxp +
J gin)
dgy(g)/fi(g)
O(0M2)]expj- !dg(CJAl9)[l
+
O^2)] (7.5.15)
-i/(K2p2)-constant[ln(l//c)]Cl/2yil, as
k:->0. Hence if
nonzero
limit
as p
Cx
is non-zero, then
p2G2(p) does
-?0; the singularity of G2(p2)
p2
not have
a
finite
0 does not correspond The massless particle that gives rise to the at
=
simple single-particle pole. singularity cannot be treated as an ordinary particle, because its long-range interactions are too strong. Positivity of the metric of the state vectors
to
a
Cx to be positive, and we assumed a theory with At negative, so K2 times the right-hand side of (7.5.15) goes to zero. (The positivity
constrains
argument is the
one
given in the the pole in
textbooks (e.g.,
Bjorken
& Drell (1966))
a propagator is less than unity if the propagator is of the canonical field. Application of this argument in the theory with an ultra-violet cut-off shows that the divergence of the self-
that the residue of
energy must be such that Cx is positive.) Notice that if Cx 0, then the propagator does have =
p
=
a
finite residue at
0:
-iK2p2G2(KP,g(ii\ii)^exp\
Fig. 7.5.2. Lowest-order self-energy graph in 04 theory.
-
dgy(g)/my
(7-5.16)
self-energy Fig. 7.5.3. Lowest-order to the contributes that graph anomalous dimension in (p4 theory.
Leading logarithms,
7.6
etc.
193
Observe that only the first term in y is relevant to the finiteness of the limit, and that the limit does not exist order-by-order. A case where Cx 0 is the =
04 theory,
because the one-loop graph
two-loop graph dimension.
Fig.
7.5.3
Fig.
7.5.2 is
independent
of p. The
provides the lowest-order term in the anomalous
7.6
Leading logarithms,
etc.
Renormalization-group logarithms
7.6.1
We saw in Section 7.4 how to compute the
large momentum behavior by approximating it by a Green's function with m 0. Then we used the renormalization group to reorganize the perturbation series into a form with small coefficients. It is of interest to examine how the complete result can be obtained by a systematic resummation of the perturbation expansion. For concreteness, let us examine the propagator G2(p, g, p) in massless 03 theory in six dimensions. We write its perturbation expansion as of a Green's function =
G2
=
(i/P2)f
n = 0
where the lowest coefficient is
T0
(7.6.1)
92nTn(-p2ln\
=
1. We will prove that each
Tn is
a
polynomial in In \i (and hence in In ( p2l\i2)) of degree at most n, with n being the number of loops. To do this we will regard the RG equation (7.3.15) not as an equation to give the variation of G2 when \i is changed with g set equal to the effective coupling g(p) (thus keeping the theory fixed), but as an equation for the /^-dependence of G2 with g fixed. Picking out the order g2n term gives —
d
T din fi'
=
-y(g)-P(9)f]ni Tn,g2n] C9 Jn'
=
0
T?
—
+
coefficient of g2n.
0(g2) and ft is 0(#3), this equation determines Tn,'s:
Since y is order
J
(7.6.2)
Tn in terms of lower-
constant
I -\y{G)^mP\g2n'[^^^TA-P2/ii'2)] {ln'<n L J 09 J
JO
coefficient of 32".
(7.6.3) Iteration of this procedure another n constants
of integration.
—
gives Tn in terms of T0 and n polynomial of degree n in ln/^ as
1 times
Evidently Tn is
a
194
Renormalization group
claimed:
i
T?=
Tnt?_l[ln{-P2/H2)y-
/=0
All but the constant term
are
(7-6.4)
determined in terms of lower-order
coefficients. A convenient way of organizing the series is to define for each term L=
-
This
number of
logarithms
g2
+ number of powers of
=
-l +
is
n.
(7.6.5)
non-negative. The sum of the terms with L 0 (viz., Tn0[ln(- p2/p2)Y) gives what is called the leading logarithm approximation to G2. Application of pd/dp, pd/dg or y to G2 (with one-loop values for p and y) increases L by 1. All the non-leading logarithms give even =
higher values of L. So the leading logarithm series exactly satisfies the one- loop approximation to the RG equation. Hence we may sum the leading logarithm series by solving this approximation to the RG equation
G2(p2^p)L=0=Up\ T I
=
Here
[1
+
%y{g{p'))\{p2-g{J-p2)J-p2)} ^
LJm
used the
we
P
A{g2\n(
J
-
notation
same
Jl=o
(7.6.6)
p2/p2)]Cli2Ali/P2-
in Section 7.5.1. This equation
as
reproduces the
approximation derived at (7.5.8). Another way of treating both the leading and non-leading
to
use
the RG equation
*Z
L
=
0
(7.6.2)
to
give
[2^1(?-l)-C1]E1T?-uDn(-pV)rl"i L
+
=
0
\2A2{n-2)-C2-}\
1=0
The
=
logarithms is TnL's:
recursion relation for the
2(L-n)Tn,L[\n(-p2/p2)f-L-i =
where y
a
Cxg2 + C2g4 H
leading logarithm -
Tn_2>L[ln(-p2//i2)]n"2-L+-", (7.6.7)
and /?= -Axg2 A2g5 + part of this equation is
2nTnt0
—
,
=
[2AM
 ̄
1)
"
CJ T?_
1>0.
??■.
(7.6.8)
This equation determines the leading logarithm series in terms of T00 and of At and Cx; this series sums to (7.6.6). Equation (7.6.7) also determines the non-leading logarithms. example the next-to-leading terms are
-2{n-\)TnA= [2AM- ^-C^Tn_1A
+
=
1,
For
[2A2(n -2) -C2]Tn_2,0. (7.6.9)
7.6 n-L
=
Leading logarithms,
195
etc.
#logs
n
=
# loops
Fig. 7.6.1. Illustrating the leading logarithms and non-leading logarithms of
a
Green's function.
Again a convergent series results. Its sum is equally accurate as the result using the two-loop approximation to p and y in the solution (7.3.21). There \i is set equal to (— p2)1/2 and we take the one-loop approximation to
of
G2(p\g(y/-p2),
0, J ̄p2\ i.e., (i/p2) (1 +^2Tn). The series for larger L may be similarly determined. In Fig. 7.6.1 m
=
we
illustrate the structure of the calculations. The diagonal lines are lines of constant L, and the recursion relation (7.6.7) determines a coefficient TnL in
terms of lower-order terms
its
and
the
higher diagonals. Suppose we have computed perturbation theory to n 1 loops for G2 and wish to compute the rc-loop term. In this term the coefficients of all but ln( —p2//i2) and the constant are fixed by the lower-order calculations. on
diagonal
on
—
information is in the nth order coefficient Cn for y and in the terms with one and no logarithms, i.e., in Tnn_ x and Tnn. The (In)0 term in Thus the
new
(7.6.7) is
"
2Tn,?_x
=
-
Cn
+
V l2Am_jj
-
i= i
CB_,] Tjj.
(7.6.10)
knowing the coefficient, Tnn_li of the singly logarithmic is Tn equivalent to knowing the rc-loop coefficient Cn in y(g). Exactly the same procedure may be applied to any Green's function GN. The only difference is that there are several external momenta. It is enough to consider the connected graphs. Let us write This shows that
term in
^(connected)
=
(p2)dta'G>/25"-2
f £
G^ln""^- pV/'V,
n=0L=0
(7.6.11) where p is
one
of the external momenta and
which is the power of g
appearing
we
have factored out
in the tree
approximation.
gN ̄2, The
196
Renormalization group
coefficients
G{£1
are
now
functions of the dimensionless ratios of the
Lorentz invariants formed from the external momenta. The leading logarithm series and all the non-leading series
convergent,
so
they
appears when
we
be summed. The n\ behavior of
can
consider the
are
large orders only
single log and constant terms, and thus in the
sum over L.
7.6.2
Non-renormalization-group logarithms
In all of the above cases there was
loop. There
are more
one logarithm of the large momentum per complicated situations where not all invariants get
large. A simple standard example is the form-factor of the electron in QED with a massive photon (Fig. 7.6.2). Here q2 (px p2)2 gets large but pi =
and
p\ are fixed. It turns out
two
logarithms
per
(Sudakov (1956), Jackiw (1968)) that there are must be in the coefficients G^l in (7.6.11),
loop. These
since the power of the logarithms is too logarithms in (7.6.11).
Fig.
—
high
for them to be the
explicit
7.6.2. The electron's form factor in QED.
large-momentum behavior in such situations. sum the leading logarithms, which are often to relatively easy compute. For the on-shell form-factor this gives a which series sums to (Jackiw (1968)) convergent One would like to find the
A much-used
technique
F
is to
 ̄
exp
[
-
(e2/l6n2)\n2q2l
(7.6.12)
(See
also Mueller (1981). For the simple cases that we considered earlier, with all momenta large, the leading logarithm approximation is justified by renormalization-group methods, as we have seen. For cases like the present one
of the form-factor, it may be
a
bad approximation
(1981,1982b)). behavior in some
However methods are available to obtain
(Collins & Soper large-momentum
of these situations. See Mueller (1979, 1981) and Collins (1980) for the electron form-factor and Collins & Soper (1981) for cases in strong interactions.
7.6.3 Landau The
ghost
leading logarithmic approximation (7.6.6)
for the propagator has
a
7J Other theories
197
singularity when (7.6.13)
p2=-Ai2exp[-l/M^2)].
This singularity, if present in the true propagator, would signal a state of squared. Since the residue has the opposite sign to that for a normal propagator pole, this would be a state with unphysical properties.
this value of mass
It is called the Landau ghost (Landau & Pomeranchuk (1955)). In
a non-
asymptotically free theory like asymptotically free theory
an
case it
QED, it occurs at very large energies and in like QCD, it occurs at low energies. In either
where perturbation theory is
occurs
leading logarithmic approximation is
a
inapplicable
and
so
where the
bad approximation.
7.7 Other theories We restricted
our
attention in
theory with one field,
to a
one
deriving the renormalization group equation coupling, and one mass parameter. However
by exactly the same method, a theory with several fields </>!,...,(j)A (each may be Bose or Fermi), several couplings gl9...9gB, and several masses. A change in the unit of mass \i is compensated by a change in each of the parameters and in the scale of the fields. The main problem is a we
may treat,
proliferation of indices. It is easiest to treat couplings and masses on the footing. So we have a collection gl9...9gc of renormalized parameters, with C being the total number of couplings and masses. Then we must write same
each function
fi} being, a priori, a function of all the parameters. For the case
theory with a single coupling, and a single mass, would (g, m2), and px f}(g) and p2= ym(g)m2. Given a function / of the renormalized parameters and of
a
=
The RG coefficients gj{0)
are
have
(gl9g2)
=
-
*h"**-(%+¥4ry can
of //,
we
have
,772)
be determined by noting that the bare couplings
invariant:
These form C equations for G unknowns. The RG equations for Green's functions are complicated by the possibility that fields of the same quantum numbers may mix under
198
Renormalization group
renormalization.
Writing <t>(0)i
we
=
IJ£ij(&d)<t)p j
find, for example, that 'kinetic energy' i
terms in ¥£ are of the form
i
hi
JJ
Hence, we have a matrix counterterm for the field-strength renormalization
Zi/g,d)=EC?Cy
=
(trOy,
(7.7.4)
T
denotes transpose. (Note that ( has a different meaning here than in Sections 7.1 and 7.2.) If we define a matrix anomalous dimension by
where
then invariance of the bare fields gives °
=
^^(0).
=
(^^-Ky)^
(7.7.6a)
i.e., d
*'-"s?- or
^Z=|{Z,y}.
(7.7.66)
If Z is diagonal (as is the case in most theories we consider): Ztj (7.7.6) reduces to
an
—
btiZh then
anomalous dimension for each field
^4>i=-hi4>i, dp
(7-7.7)
d In the case of a diagonal Z, the renormalization group equation for an Appoint Green's function is '
d
=
-
*
E KteXV
dl
(7-7.8)
7.7 Other theories
199
Here
yia is the anomalous dimension function for the ath external field of If the renormalization matrix Z is not diagonal, then we have a similar,
GN. but
complicated, set of equations for the Green's functions. equations for the evolution of the couplings are coupled, so their solution is in general complicated. Considerable simplification can be achieved by using our knowledge of the dependence of the counterterms on massive couplings. Let the couplings gl9...,gAbe dimensionless and let the more
The
corresponding
bare
couplings
be
where the g(0)i depend only on the renormalized dimensionless couplings and on the UV cut-off. For the sake of definiteness, we assume that the
physical
dimension of space-time in the theory is d 4. The wave-function also on the dimensionless couplings and only depend Zf =
renormalizations
d. Let the other parameters (masses and super-renormalizable couplings) be denoted by /s, and let the dimension of fs be (4 d)zs + as. If fs is on
-
the
mass
of a fermion, then os
=
1, ts
=
0. If it is
a
boson
mass
squared, then
2 and ts 0, while for a super-renormalizable coupling ts ^ 0 and <js > 0. Then (by Section 5.8) the bare quantity corresponding to fs has the cs form =
=
/(o,s= I Hi4-d)t*XFsX(g,d). dx
Here X is any
d
—
4) equal
Requiring
product of
(7.7.9)
-as
the dimensional
to the dimension os of
couplings with dimension (at
fs.
invariance of the bare couplings gives 0
=
yj= 0
=
(4
-
d)Pigm + X Pj7r-gm, j=i G9j
I^lnZ,
(7-7-10?) (7.7.10b)
(4-dXXXFsx+ZXPj4-FsX X
t,X
X, J
"Vj
Ch
triangular structure to the evolution equation: the evolution of a coupling depends only on couplings of the same and lower dimensions.
There is then
a
In these equations, the index; runs over the values 1 to A9 i.e., over those values that correspond to the dimensionless couplings, while the indices 5
and t
run over
the labels for the dimensionful
couplings.
Renormalization group
200 If we
minimal subtraction, the calculation of the coefficients is rather
use
easy. Let
Gt(g\ Zt(g\ and PsX(g) be the coefficients of single poles in g{0)i, Zt
and
Then
FsX.
we
have
Pj(g,d)=(d-4)Pjgj + pj(g)9 fc(g, t9d) (d 4)tJs + &(g, f), =
with
yj9
and
pj
/?s satisfying A
l
?
I
Q
{(x." Z^)(^) ^I P^tf}. -
7.8 Other renormalization It was
only for
(7.7.11)
-
the sake of simplicity that
(7-7.12)
prescriptions
we
restricted
our
attention to the
minimal subtraction procedure. The proof in Section 7.2 in fact shows that any change in renormalization prescription can be compensated by a
change in renormalized parameters and a change in the scale of the us examine what happens in more general schemes. It is sufficient to restrict our attention to a theory with a single coupling and
renormalized field. Let
mass, like
03 theory
If we choose
be
a
a
in six dimensions.
renormalization scheme with
renormalization
point,
still be defined and computed
general
an extra mass \i,
which might
coefficients
then
can renormalization-group by (7.3.4), (7.3.5), and (7.3.14). What we lose in
are:
(1) the simple formulae (7.3.12), (2) the lack of dependence of /?,
ym and y on the masses.
In order to discuss UV limits, it is sensible to choose a scheme in which the limit m->0 exists. This means that
/?,
ym9 y are finite,
order-by-order,
as
m->0.
related by finite renormali- may relate the RG coefficients in different
Now, different renormalization schemes
zations of the parameters. So we schemes by looking at the theory
then computing /id/d/i in
one
are
in the physical space-time dimension and of/id/d/i in another scheme
scheme in terms
with the aid of the chain rule.
Suppose
we
have
a
second scheme in which the
new mass,
coupling
and
7.8 Other renormalization prescriptions field
are
m', g and 0': g' m'2
=
=
g'(g,m2/n2),
m2zm(g,m2/n2),\
Then the Green's functions in the
new
G'N(p;g\m'2yp) The
201
=
scheme
are
(7.8.2)
t:NGN(p;gim\p).
coefficients in the
renormalization-group
p'(g',m'2/n2)
=
(7.8.1)
new
scheme
pl ̄ ymm2 A V^> n^g' (n ̄ \ dp dp' +
=
are:
?
dm2
og
V), (7.8.3)
ym(9'^'2/p2)^-^j-m' =
Jm
 ̄
+ L-^ {.zm{g,m21 p2)\ Py ymm2^)ln eg J \ cp  ̄
cm
(7.8.4)
Our definition of the total derivative d/dp is as the derivative with respect to p when the bare coupling g0 and bare mass m0 are held fixed. Therefore, it is the same in both schemes. Notice that there are two steps in computing /?' or
y'm\ First, compute the right-hand side expressed in terms second, change variables to the new coupling and mass. The anomalous dimension of 0' is obtained as
of g9 m, and p\
y'(g\mf2/p2)=-^p^\nG'N -2p—InC dp =*
A considerable
-
+ y
2(/V% y^to?)ln ^ m2'^  ̄
simplification
schemes. Then g\ ( and
zm
occurs
are
ym(g')
/(0')
=
=
=
relating mass-independent
functions of g alone,
renormalization-group coefficients in the
p'(g')
in
(7-8-5)
new
scheme
so
that the
satisfy
l}(g)Tg'(9l ym-P—^zm(g), 7-2/?-lnC(0). ?g
(7.8.6)
202
Renormalization group
In this case, let
g\
zm and £ have
perturbation expansions
fli03+fl205 + "O zm=l+M2 + fc204 + "-A Q'
=
0 +
and let the expansions of /?,
ym(g) yte)
=
=
The expansions of /?',
primed. Then example: =
-
ym, and y be
>
Big2 + B2g* + ---y c^2 + c2^4 +
y^,
and y'
are
gs(A'2
+
3axA\)
-
(7.8.8)
J
---
written similarly with all quantities
by using (7.8.7).
express them in terms of g
we can
A[g3
-
(7-8.7)
07(^
5^2flj
+
3^;a2
+
+ 3,4
X) +
For
?
■
?.
(7.8.9) This
must agree with the
perturbation expansion of the right-hand side of
(7.8.6) pdg'/dg=-Alg3-g5(A2
+
3alAl)-g7(A3
3A2al
+
+
5A1a2) +
' ̄.
(7.8.10) From these
equations
we see
that the first two coefficients in ft do not
prescription is changed, i.e., the above 2. By generalizing equations to all orders we also j A\, A2 see that, by adjusting the terms in the expansion of g\g\ we may choose the when
change A
=
the
renormalization
A
=
terms beyond the second in /?' to be whatever we want. In similar fashion that only the 0(g2) terms in ym and y are invariant.
we
see
one-loop term in y or ym is zero then the whole of y (or ym respectively) may be made zero by a choice of renormalization prescription. This privilege does not extend to /?: if the first non-vanishing term in p is at rc-loop order (n > 1) then that term is RG invariant (but not the (n + l)-loop term). In a theory with more than one dimensionless coupling we may try to Note that if the
apply the
same
methods. This is left
as an
exercise. It will be found that only
the first term in each p is now invariant, except in the ^-function does not mix the different couplings.
The
invariance
of
these
case
applies
coefficients
only
independent renormalization prescriptions.
If one
are
zero.
Then
we
have
f}
=
y
=
one-loop
within
were to use, say,
subtractions, then the parameter \i would not appear, respect to \i
that the
ym
so
=
mass-
on-shell
all derivatives with
0.
(The asymptotic
7.9 Dimensional transmutation
203
behavior that we extract by varying \i can no longer be computed by renormalization-group methods, if we stay within this renormalization
prescription. In this instead
-
case the Callan-Symanzik equation must be used Callan (1970) and Symanzik (1970b).)
see
7.9 Dimensional transmutation
theory with one dimensionless coupling g and no masses. A physically important case is QCD with several flavors of massless quark; with two or three flavors this is an approximation to actual Consider
a
renormalizable field
strong interactions. Since the basic theory has
no masses we must use a
renormalization
an arbitrary renormalization mass /*. Although the theory apparently has two parameters, g and /*, we saw that this is not so: a change in \i can be compensated by a change in g. In fact, as Coleman & Weinberg (1973) pointed out, the theory really has no parameters at all. The point is simple but somewhat elusive, so we explain it at length. A physically measurable quantity must be renormalization-group invariant. For example, let M(#,/i) be a particle mass. By dimensional
prescription with
analysis, it is
\i times a function of g alone. So 0
d
=
n—M d/i
=
d
(7.9.1)
p— M. dg
M +
Hence M
=
/r constant exp
t-r
1
=
=
P(9l.
A2,
['A ,{.A\g" + ^g')(A,-A2g'2)-]
nCexpl-——-—— \n(g)-\ dg 2Al92 A\ J 0 J0 ,
,
"
iiC exp
{
-
^—2
-
2Al92
Here C is a constant and
A2g'3p(g')
In (g) + 0(g2)}. -f A2 we
have written
f}(g)
(7.9.2) =
—
A{g3
—
A2g5 H
,
as
usual. Note that the Green's functions invariant :
to measure a
This definition has The formula
Green's function,
an
(7.9.2)
are
not
renormalization-group
one must
define the field operators.
arbitrariness, which is the freedom has
a
to vary its scale.
number of consequences:
particle masses cannot be computed in ordinary perturbation theory (in a theory with no mass in the Lagrangian). For to avoid large logarithms one must set \i to be of order M, where M is the particle mass
(1) Non-zero
204
Renormalization group
being computed. is
(2)
a
Then (7.9.2) tells
us
that g(M) is not
free parameter; it
a
number of order
In a
unity. non-asymptotically free theory (A,
<
0), suppose
we
have
a
small
value for g(p). Then jx<M, where M is the value of the mass of any given massive particle. Perturbation theory is therefore only useful for
Green's functions when the external momenta
(3)
threshold for
massive
In
>
producing any of the an asymptotically free theory (A x
much below the
are
particles.
0), we have p, > M whenever g(p) a theory only when
is small. Perturbation theory is useful in such momenta are much bigger than particle masses.
(4) Since the ^-dependence of (7.9.2) is universal, i.e., the same for all particles, ratios of particle masses are pure numbers independent of g and p. Let us emphasize once more that these results are true when there are no explicit mass terms in the Lagrangian. The observation of Coleman & Weinberg (1973) comes from asking what
be measured in the theory. Suppose we start with p px and g gx and ask how the theory changes when we work with the theory with a different
can
=
—
value
ofg,g g2. (We suppose gx and g2 are between g first fixed point of /?.) Each version of the theory has an satisfying =
Now evolve
0 and g effective
=
g(p)
g*, the coupling =
in the second version to the value of p where
0vers2(/*2)
=
01-
theory is just the first version with all p2jpv For example let abea cross-section
Then the second version of the
factor
by depending on momenta analysis give us momenta scaled
a
p{y...ypN. Then RG invariance and dimensional
g(Pi>--->Pn'>92>Hi)
=
=
°(Pi>' ̄>Pn'>9i>I*2)
(?l\ \/*l/
J tkp \A*2
V±pN;gi^AJ 1*2
(7.9.3)
The last factor is the cross-section in version 1 of the theory, with its momenta scaled. We
see
that
changing the dimensionless coupling
does not basically change the theory, but only its dimensional transmutation. There
are many ways
might give
the proton
in
mass
a
massless theory
scale. This is called
of specifying the scale of the theory: in QCD one For perturbative purposes it is better to use
mass.
7.9 Dimensional transmutation
205
something that can
be directly used in perturbation theory, for example the value of /i at which g(ji) has some given value (e.g., 0.1) in one's chosen renormalization prescription. One standard way is to notice that for large /*,
g([i)
has its
asymptotic 2/
)=
behavior
given by
1
A2ln(lnfcVflo)) +
constant
rin^On,l(/V/*o)) L ln3(AW7*o) x
^,
Here fi0 is a reference value of /*. If /^0 is changed then the series is reorganized; only the first two terms are unchanged. As is conventional
(Buras, Floratos, Ross & Sachrajda (1977)), we define the scale A of strong as the value of jx0 for which the l/ln2(/i2//^) term is zero. This
interactions
gives (7.5.5). If we change from,
say, minimal subtraction to momentum-space
subtraction, then the theory is
unchanged provided
the
coupling
is
adjusted. This may be done in perturbation theory. For example, we might find that g in the MS scheme and in the momentum-space subtraction scheme are related by
given by (7.5.5) with A AMS, and let gmom be given by (7.5.5) with A Amom. (We already know that Ax and A2 are the same in both schemes.) Substituting these expansions into (7.5.5) and requiring consistency gives
Now let gMS be
=
=
AMs
=
Amoinexp(a1/^i).
(7-9.6)
Notice that both A{ and ax are obtained from one-loop calculations and that there are no higher-order corrections whatever (Celmaster & Gonsalves (1979)). An amusing consequence is (7.9.2). Since M is independent
by substituting (7.5.5) for g in may let /*-? oo. The higher-order
obtained
of fi
we
terms all go away and leave M
=
CA(Al)A*<2A*.
(7.9.7)
equation is not very useful for performing perturbative calculations. If the theory is a complete theory of physics, then measurements of a and the p's in (7.9.3) will be in terms of a standard of mass. This we may take to
This
particle (say, the proton). Let us now change the theory by changing the coupling from gl to g2 Just as we did earlier. Then the standard of mass is multiplied by /^2/Vi- So if we do experiments in be the
mass M
of
some
206
Renormalization group
equal to those in version 1, the actually increased by a factor /i2/Vi Therefore, (7.9.3) tells us
version 2 with numerical values of momenta momenta are
that
o
gets multiplied by
increases sense
?
by
a
factor
the same factor,
so
(/^2/Mi)dimr7-
But its unit of measurement
unchanged. In this coupling (or different
the numerical value is
massless theories with different values of the
values of A)
are
of Coleman &
indistinguishable. This is perhaps the most important result Weinberg (1973).
However, there are many experiments that claim to measure A. There are give (without qualification) a single measured value of g.
even some that
How
can
this be? The second problem is easy to dispose of. What is being
measured is the effective coupling g in some renormalization scheme with \i set to a value of the order of the energy of the experiment (typically in e+e ̄- annihilation). Strictly one should specify not only the value of g but also the scheme and the value of \x. Now the experiments are at around 10 to 30GeV, and A is at most
a
few 100 MeV. The variation of g
over
and the variations between the usual renormalization schemes
this range
are
often
no
than the size of experimental errors. So it is possible to talk loosely. However, we just asserted that massless QCD with different values of A is the same theory. The sense of a measurement of A is that we measure the numerical value of the ratio of A (defined by (7.5.5)) to a standard of mass. For the purposes of the argument, we may regard the standard as being that the nucleon mass is 939 MeV. In terms of dimensionless quantities the measurement is of the constant C in (7.9.7) when M is the nucleon mass. (In the MS scheme, we find that C is between about 5 and 20.) The non-zero masses of the quarks make a relatively small perturbation of the above more
argument. Notice that if we
standard
mass
unchanged. In QED
play God and double the size of A, then the size of the
also doubles,
so
that numerical results of experiments
the situation is different. The electron has
a
mass,
are
and its
Coulomb field is classical at large distances. A mass-shell renormalization scheme is natural. Since there is a very important mass-scale, an unqualified statement of a measurement of the
QED with
makes good unlike QCD in the absence of sense.
7.10
It is very convenient to cutoff in
a
QED coupling, viz.,
different value of
quark
is
e
a
e
=
(47r/137)1/2,
different theory,
masses.
Choice of cut-off procedure
use
dimensional continuation
perturbation theory. However,
there is
no
as an
ultra-violet
known construction of
a
7.10 Choice
complete theory in
207
of cut-offprocedure
arbitrary complex dimension, so one must beware of significance to use of dimensional continuation. This is especially true when we use minimal subtraction, which is a procedure that exploits the form of the cut-off dependence of the theory. However, the renormalized theory with the cut-off removed does not depend on the form of the cut-off. We saw this in our one-loop calculations. an
assigning too much physical
general the fact is easiest to see by using BPHZ renormalization, in which integrand is constructed that gives a manifestly convergent integral. The only freedom left is a change of renormalization prescription, otherwise known as a change of parametrization. In this section we will examine the renormalization-group properties when a different UV cut-off is used. For definiteness we cut off the theory by using a lattice, with spacing a. We consider any theory with a single dimensionless coupling g and a single mass m. It is, of course, possible to generalize to any cut-off procedure and to any theory. In general we will In
an
need
a
renormalization
mass \i,
The bare coupling g0, bare
in order that
mass
we can
take the massless limit.
m0, and the fie Id-strength renormalization m and \i, and of the
Z are written as functions of the finite parameters g,
cutoff
a.
Then the renormalized Green's functions
are
written in terms of the
bare Green's functions
and for them the limit
0 exists.
a -?
renormalization-group structure is essentially unchanged. Let us choose a mass-independent renormalization prescription, so that again #0, The
Z, and m0 have the forms :
Z
mo
=
=
The massless theory has
>
Z{g^a\
(7.10.2)
rn2Zm(g,fia) + a ̄2Y{g9fia)J m
=
0, and,
as
before, g0 and
Z are
independent
of
the cut-off parameter is dimensional, so g0 and Z have explicit dependence on fj, as shown. But the dimension of g0 is fixed at zero,
mass. But now
so
the The
longer
^-dependent
power of fj, is not used.
m2-dependence of the bare mass squared true that m0
=
0 when
m
=
0. In the
is
again linear.
case
But it is
no
of dimensional reg-
dimensional parameter is \i, and it is not possible (Collins (1974)) to generate by minimal subtraction a counterterm 2 fi2 Y(gy d). But with a lattice cut-off a term a Y is both possible and needed, ularization the
only remaining
 ̄
as we
will
now
verify by computing
a
low-order
graph.
208
Renormalization group 7.10.1 Example:
</>4 self-energy
simplest example that shows the existence of the Y-term in (7.10.2) is the self-energy graph of Fig. 7.10.1, not in the 03 theory that we have been using, but in the 04 theory in four space-time dimensions (with m 0). The The
=
Lagrangian is (2.3.1). With dimensional regularization the value of the graph is
^O(2n)-d[d'lk/k2^0,
Fig.
but with
a
7.10.1. Lowest-order
lattice cut-off
we
self-energy graph
in
<j>4 theory.
find
-ig0(32nYl
J
d3kdcoD(co,k;a).
(7.10.3)
|fc**|<7t/a
Here the Euclidean lattice propagator is
l/(a>2
+
k2) if co
and
(7.10.3)
is
|fe|
are
much
smaller than \ja. For general values of &", it is
a2\U
| sin2(fc"a/2)|,
which is positive definite, so that the integral diverges to a number of order \ja2 as a->0. A similar
divergence
occurs
in the
self-energy
of
a
non-zero
and
scalar field in any
theory.
7.10.2 RG coefficients continue our general discussion of the renormalization group lattice cut-off is used. As in the treatment using dimensional regularization we define a renormalization-group operator We
now
when
a
%=%+Kg> ?a)ig
 ̄
U9>
?a)m2ib-
(7-104)
changed our notation slightly, and used an overbar to indicate renormalization-group coefficients in the cut-off theory. These coefficients p and ym have finite limits, fl(g) and ym{g\ as a -? 0. In our later work it will be rather important to distinguish the coefficients before and after the
We have
7.10 Choice
209
of cut-offprocedure
cut-off is removed. The coefficients
can
be computed from
>Vo=0;
dp.
with (7.10.4) used for fid/dfi. We also /id In Z/d/i, just as with dimensional y =
have the anomalous dimension
regularization. This all results in
d
}d{?aY
-[ -[
d
-d'
5(H
<3#
In
t>5
5
(7.10.5)
Zm{g,pa\
\nZ(g,pa).
In addition there is the constraint
0
=
^(a-
The information
on
the
/?, ym and y. If desired,
g0
a
/?
=
be finite set
-I
pa
g(/ifl)
d#_
g3Gl x
(7.10.6)
Y{g,pa).
divergences is all contained in the finite functions
we can use
minimal subtraction with the form
g5G2l
+ -'y
function of g alone. In order that of logarithms ap must cancel in p. This implies of relations for the counterterms, the first of which is 2G22
so that
ft
Y)
g3Gll\n(ap) + g5\_G22\n2(ap)+G21\n(ap)]
g +
=
2V\- =
3G\V
—
—
—
■
?
?
is
a
a-+0 all the
as
=
An
analogous regularization, as can
set of relations occurs when we use dimensional
from (7.3.12). These we will discuss further in Section 7.11. Note that for minimal subtraction with the lattice cut-off we
havej8
=
j5,
ym
=
be
seen
ymi etc.
7.10.3 Computation
If
we
were
to
approximation,
of g0 and Z; asymptotically free
compute the exact theory, rather than we
would need to know how
case
perturbative g0(g, \id) depends on a as a -? 0 a
with g and p fixed. A low-order calculation is not sufficient, for g0 has logarithms in its perturbative expansion. Provided the theory is
large
asymptotically free, we can remedy this by using the renormalization group to improve the calculation, just as we did for the large-momentum behavior of
210
Renormalization group
Green's functions. The
starting point is
the equation
4^40o+^°=0' which
is in effect the
it
as
(7-10-7)
renormalization-group equation for g0.
We may solve
for the Green's functions.
just Ultimately, we will let a approach zero while holding g and \i fixed. But first let us keep a non-zero. Then we can define an effective coupling g(p) by ti'dg(?')/d?'
=
(7.10.M
p(g(?n
with the boundary condition #/i)
(7.10.86)
0.
=
We will also need the effective coupling at denote it by the symbol g(ji'). It satisfies
/I'd&O/d/i' 9iv)
=
Implicitly
there is
=
dependence g
=
jJtfOi'))
=
Now, when
=
us
0),
0-
of g
on \i
g(\i\\i,g\
9o(9> W)
=
0. For the moment, let
Mv'Y^'a
g
and g, and of g
=
=
on /*, g
and
a:
g(\i\\i,a,g\
Of course, #(//)-? #(/*') as a->0. We can solve the renormalization-group 0o
a
=
equation (7.10.7)
for g0 to find
9o(9(Va)> !)?
might appear that we can replace g(l/a) by g(l/a\ approximated by the first term in its perturbation expansion (since g(l/a) is small). That is, and that
a
is small, it
g0 is well
g0
=
g(l/a) + negligible
error.
These suppositions are actually false, for two reasons. First, fi(g,an) in general depends on a\i, so we cannot just replace g at \i \ja by g(l/a) computed in the a 0 theory. Secondly, we cannot simply drop the higher- =
=
order terms in #0, since the dependence of renormalized Green's functions on g0 is singular. Thus small errors in g0 may give rise to large errors in a
Green's function computed
as a
To derive the correct formula more
closely. So 9o
=
we
quantities. a-dependence of ft expansion of g0 in the form:
function of bare we must
examine the
write the perturbation
9^93lGll\n(a^)^Gi0 + Gl(a^)] +
6f5[G22ln2M + G21lnM+G20+G2M]+---
(7-10.9)
7.10 Choice
of cut-offprocedure
211
have not specified the renormalization prescription, so in addition logarithms we need finite functions Gx(a^\ etc. We have explicit constant terms Gl0, so we define Gt(afi) to be zero at afi 0. Once and from been have subtracted divergences Feynman subdivergences Here
we
to the
=
graphs, the remainders converge with power-law convergence in momentum. This is a consequence of our treatment of Weinberg's theorem, and is further treated in
Weinberg (1960). Therefore
say
we can
Gi(aii)=0((afir) as a/i->0, for some positive number logarithms of a\i, so we can safely set First we compute the ^-function:
ct
=
general, 1/2.
G-
equals
a\i times
l8go
d
r
c(. In
?
{<■■
+
d In
(afi)
Gi
2G22ln(^)
+
G21
(l+302[G11lnM + Glo+G1]
+
d\n(an)G\
+
■
+
d(fxa) -r<G2l-3Gl0Gll
+
dln(fia)
[G2-iG2l-3GllG1\n(^) -3G10e,]V +
The relation limit a->0
2G22 gives
P(g) so
-?
ft is
as
a-+0. The
P(g,0)=-g3Gll-g5(G21-3GloGli)+-,
(7.10.11)
=
must hold in order that
our
=
=
0, their logarithmic derivatives a^dG}/d(a^,) also go
The first
of g0
finite
usual notation Ax Gn and A2 G21 3GnG10. Since and G2(a/i) go to zero like a power of a\i (times logarithms) when a\i
that with
G^api)
=
3G\X
(7.10.10)
-.
step in
implies
our
—
to zero like a power.
calculation of g0 is to observe that the RG in variance
that
go(9>l*a)=g0(g(ji'),ii'a) =
The next step is to examine the size of the
g(l/a) by
the
(7.10.12)
0O(0(1?,1). error
in the
that is made in
replacing Finally, we will
effeptive coupling g{\/a) g0(g(\/a)y 1) must be computed in order to obtain
find the accuracy to which
a
=
0 theory.
212
Renormalization group
the correct renormalized Green's functions at The difference between the two effective
g(l/a)3
turn out to be of order
when
a
=
0.
is small. So let
a
and g(l/a) will define the fractional
couplings g(l/a) us
g2A by
error
g(^;^a,g)
=
/*' l/a and a->0. (7.10.13) and the definitions of p and p we find that
We will now show that A is finite when
From
/i'^7 AM
=
r3 [j3((l -
=
+
A(//,/i, a,#) is finite
AQi') We
now
as
+
-
+
3g2A)p(g)]
0(£ V*)1/2) + 0(54A).
/i'->oc,
when a->0 and /i =
=
g2A)g, pta)- (1
/i'fl^-5,
g2(fi') ̄ \/A1ln(fi'/A)
Now
(7.10.13)
g(fi';^g)[l+g(^;^g)2A(^';^aig)Ji.
<
so \i
(7.10.14)
this equation tells
<
l/a.
G^ci/i') + 0(l/ln(l/fl)).
compute g0. It is convenient
us
implies
In fact it
that
that
(7.10.15)
to write a formula for its square:
g0(g^a)2=g0(g(l/a),l)2 =
{g(l/a)(l
+
g2A) + g\\
02A)3[G1O + ^(1)]
+
0(g5)}2
^2ln[ln(l/a2A2)]
1 _ "
^fln^l/^A2"")]
[^t ln(l/fl2A2)] +
+
+ 0{ln2[ln(l/a)]/ln3(l/a)}. r72l^7T7^T27T [^2ln2(l/a2A2)]
(7.10.16)
#(/i) in terms of \i and A (7.5.5). The formula for g0 in terms of l/a and A is the same as (7.5.5) except for an additional 1/ln2 term. Observe that it was essential to keep the a-dependence in /%,a/i); the G^l) term in (7.10.15) canceled the G^l) in the two-loop Here
we
used the formula for
-
-
coefficient in g0.
Finally 9oW>W
terms of g, a and \i\
,4 2 In (In (l/a V))
1
 ̄Alln(l/a2fi2) +
where
(7.10.16) in
we express
1
,43ln2(l/aV)
2G10 ̄\-^\n(Aig2)-2AJ(g)
9 ,42ln2(l/aV)[ + 0{ln2[ln(l/a,<)]/ln3(l/a/z)},
fig)-
d?'[l//J(ff') 0
+
Ai
l/M,ff'3)-^2/M?ff')].
(7.10.17)
(7-10.18)
7.10 Choice of cut-offprocedure
213
Similar formulae hold for Z, for Zm, and for Y. Thus Z
[Alg2\n(l/a2n2)yCll2Al
=
x
y(g')
<fo'
exp
x
cx
1 + 0
 ̄ln(ln(fl/x))' J
\n(afi)
(7.10.19) where y
=
Cxg2
+
0(g4). 7.10.4 Accuracy needed for g0
Let
us now
suppose
we
compute the renormalized Green's functions:
GN(x19...9xN;g9m9fi;a) We must
now
let
approach
a
Z ̄Nf2G{0)N(xu...9x^r9g09m0;a).
=
zero, and ask how
accurately
(7.10.20)
we
need to
compute g0 and Z. In (7.10.17) and (7.10.19) we gave formulae for g0 and Z, with explicit estimates for the errors coming from uncalculated corrections.
These equations tell
us
a\i) when we let a -? 0 while keeping singular dependence on might affect the values of the renormalized
the value of g0(g,
g and [i fixed. Since the bare Green's functions have
g0, the uncalculated corrections Green's functions. In fact these terms do not affect the renormalized Green's
functions in the continuum limit, The
as we
will
now
key observation is that the renormalized
show.
Green's functions
are
finite
functions of the renormalized parameters. Thus we do not need to hold the renormalized coupling and mass fixed while taking the continuum limit a -?
0. We may in fact let them vary continuously, provided only that their a 0 are the same as before. Now examine (7.10.17). It is evident
values at
that
we
=
may absorb the whole of the correction term into
variation
of
In
g.
ln2(ln(l/fl))/ln(l/fl)
92o
as
fact
a->0. So
1 =
^lnO/aV) +
A\
the
necessary we
change
in
g
may choose the bare
just
is
such
of
coupling
a
order to be
^2ln[ln(l/aV)] A\\n2{\la2n2
IootK-?-^^-2^ (7.10.21a)
Hence in (7.10.16), So we have the
we can
following
also drop the
0{ln2[ln(l/a2)]/ln3(l/a)}
formula for g0 in terms of
1
^2ln[ln(l/a2A2)]
^1ln(l/a2A2)
A\\n2(\/a2A2)
+
a
2G10 ^2ln2(l/a2A2)
■
terms.
A alone:
(7.10.216)
214
Renormalization group
In the
case
corrections
of the wave-function renormalization Z, the uncalculated
can
be absorbed into
a
£iV multiplying
factor
the Green's
function GN. This factor must approach unity in the continuum limit. Hence we may use
Z
A^ln
=
-CulAy
1
d0'
exp
a2M2
y(g')
P(g')
,
+
ci
ai9'] (7.10.22)
where M is
arbitrary coupling we had a form (7.10.21 b) that had dependence on A, but not on \i or on g. This was because g0 is renormalization-group invariant: we may take \i arbitrarily large without affecting g09 provided that we also set g equal to the effective coupling at /*. When /* is very big, g is very small, and the higher-order corrections contained in f(g) go to zero. But Z is not invariant; it must depend on g. What we can say is that any dependence on a an
mass
that is irrelevant when
a -?
0. Notice that for
the
of the form Z
=
finite-[ln(l/fl)]-Cl/2Al
will produce finite Green's functions. Notice that if the one-loop divergence in Z vanishes, then
we
may let Z be
finite: Z
There will in
orders. What
=
exp
dg'y(g'W(g')
general be divergences
in the self-energy
have proved is that
must sum to
we
In the case of g0i any
gl
=
l/Ax ln(l/fl2)
-
they ^-dependence of the
graphs in higher something finite.
form
A2\n[\n(l/a)]/[A3l\n2(l/a2)]
+
finite/In2^
give finite renormalized Green's functions. Only knowledge
of A2 and and two-loop Ax is necessary for this. They are obtained from calculations. The coefficient of the l/ln2a determines the value of g.
will
one-
The formula (7.10.216) shows the fundamental significance of the A- parameter. In a renormalizable field theory, there are divergences, so one
single number
cannot
simply specify
Rather,
one must construct
a
the theory
as
as
the bare
coupling
the continuum limit of
lattice theory, with g0 depending on the lattice spacing, (7.10.216) gives g0 as a definite numerical function of a.
Unfortunately, there is
certain arbitrariness in
a.
precisely
some
Equation how
one
lattice approximation to a continuum theory. This is physically irrelevant (although some particular approximation may
constructs £ arbitrariness
a
constant.
7.10 Choice
be superior when
it is used for
a
215
of cut-offprocedure
numerical calculation). So
(7.10.216) is also
important because it expresses the bare coupling in terms of quantities (A, Al9 and A2), which have direct meaning in the continuum theory, and in G10, which depends on the lattice approximation, but which can actually be computed analytically (Hasenfratz & Hasenfratz (1980)). The result of such a lattice calculation is normally given as the ratio
terms of one number
of a
Ajattice
to the value of A in some standard continuum renormalization
scheme. The definition of
^
=
Alattice is
the value for which
l/[>?1ln(l/fl2A12Bttlce)] -^alnpna/^A^JM^^^l/^A,2.^.)]
gives the same continuum limit as (7.10.21). Alattice
=
It is
(7.10.23)
easily checked that this
is
(7.10.24)
Aexp(G10/^1).
Despite the fundamental significance of A, there is a convention dependence in its definition. In specifying a theory by its value of A, one must specify these conventions. This is analogous in its effect to the need for specifying a system of units in electromagnetism. The main convention is that of the renormalization prescription. The other convention is the
one
implicit in the choice of the constant in (7.5.3). It is sensible to follow the usual convention, to avoid confusion. We have
seen
that
higher-order
corrections
(beyond
two
loops) do not (7.10.21a),
enter into our formula for g0 in terms of A. This is in contrast to
which
expresses g0 in terms of g and \i. So it is sensible to treat A as a fundamental parameter of the theory-say in strong interactions. But measure A. A typical which a useful for measuring quantity e ̄ perturbation expansion exists (for example, a jet cross-section in e+ annihilation). The experiment therefore measures the effective coupling g(ji) at some value of pi which is of the order of the energy of the experiment. There are errors in this measurement caused by uncalculated higher-order
practical
considerations intervene if
measurement consists of
one
tries to
a
—
terms in the theoretical calculation of the
perturbative corrections. errors
We
due to corrections in
Since
g is more
directly
can
cross-section, not to mention non-
then deduce A from
(7.5.6), with further
p. related to the size of the cross-section, it is
perhaps correct to argue that experiments should quote their results as a value of g. But to give the value of A is equally valid. However, a small fractional error in g corresponds to a much can
be
seen
from (7.5.6). If
\dA/A\
=
we
change
larger fractional error in A. This holding \i fixed, then
g and A while
\dg/g\[2Aig2yi[l
+
0(g2)l
216 If
Renormalization group
one
could do
then it is the mass.
As
we saw
transmutation, the
mass
mass of, say, the proton in QCD, in the value of A that would determine the error in the
real calculation of the
a
error
in Section 7.9, when
is
proportional
discussed dimensional
we
to A.
7.10.5
m20
Unfortunately ml has a 1/a2 term, but the variation of m0 with m2 depends [\n(l/a)] ̄hll/Al term. So we need the coefficient of 1/a2 to very high
on a
Any slight error (say of order 1/a) will be equivalent to making the renormalized mass diverge like 1/a as a-+0. The resulting need to be very accurate in m0 leads many people to consider scalar field theories accuracy.
unnatural. In the case of fermion theories there is a symmetry under m
=
0,
the Y term is absent and
so
m0
=
mZm
7.10.6
 ̄
we
\jj
-?
y5\j/
when
have
[ln(l/a)]  ̄Bl/2Al.
m-constant
Non-asymptotically free
case
The values of g0, etc., as a -? 0 are not perturbatively computable unless the theory is asymptotically free. However, if we suppose that p in a non-
asymptotically
free theory has
a
fixed point, then
we
may write
(7.10.25)
^o(0*,l)as^O. Note that
the
same
g0(g(l/a), 1) is
which the limit is An
a
finite function of g, so the limit exists. However a 0 for any value of g{fi). So the way in
value is obtained for g0 at
example
approached
=
determines the value of g.
is easily constructed. Suppose
p(g)
=
we
have
a
sm2(g2)/(2g),
theory
in which
(7.10.26)
and
g0(sA) Then the effective coupling has 3
There is
fixed point g* a->0 must be a
9,o(9, W)
=
=
=
=
g.
(7-10.27)
the form
[arctan(l/ln(A/M))]1''2.
n1/2. We therefore find that the bare coupling
as
[arctan (1/ln (aA))]l'2
^l2-2^W)+°il/ln2{aA))-
(?-ia28)
217
Computing renormalization factors
7.11
It is necessary to know how
g0(g,an) approaches
its limit
g0(g*, 1)
in order
to determine the value of A.
7.11
Computing
renormalization factors using dimensional regularization
In the previous section, Section 7.10, we computed how the bare coupling g0 should behave as
a
function of the lattice spacing
a.
In this section
present the corresponding argument using dimensional continuation
we
as the
We do this by treating the defining equation (7.3.10) of p as a differential equation to compute g0. Our argument will be valid in any asymptotically free theory, like </>3 theory in six dimensions or QCD in four cut-off.
dimensions. If
let
we
regularize by going
d0
be the physical
space-time dimension, d0 s.
to a lower dimension d
=
then
we
—
First we compute the relations between lower and higher poles in the renormalization. Now we write
0o
=
^/2
£ dj(g)e-j\J
g+
(7.11.1)
j=i
and
we
 ̄p\
have the definition of
eg0/2 + p(g,d)dg0/dg Let us
=
0.
(7.11.2)
expand (7.11.2) in powers of e. The terms proportional to e and e° give
us:
p=- eg/2 We have
limit of
j5 ft
p{g)
=
changed notation from
the definitions that defined
+
we
-
eg/2
our
+
%gd/dg
original
definitions to correspond to
while
defined /?
as
we
the
pole e ̄j in (7.11.2) is
i(l -gd/8g)dj+l(g) + p{g)ddj/dg This is
we
the cut-off is removed.
Now, the coefficient of the
a
\)dx{g\
used in Section 7.10 for the lattice cut-off. There
to be RG coefficient in the cut-off theory, as
-
=
0.
(7.11.3)
differential equation which, when solved using the boundary
condition d.(0)
=
0, gives all the higher coefficients d.(g) in
single pole dx(g). Similar relations ft Hooft (1973)) hold
terms of the
for all renormalization counter-
logarithm expansion. They theory the only new information in the counterterm in a given order of perturbation theory is in the single pole. A convenient way of solving these relations is to work out the solution of terms. The structure is similar to the leading show that in each order of perturbation
the differential
equation (7.11.2).
ln[0o(0>/^ ̄£/2]=
This gives
fW[lg -2P(g)/e -VlJ Jo *
,
,W
g
+
faft
f7-11-4)
218
Renormalization group
i.e.,
g0/g^>n£'2 as #-?0 has been used. We now ask how g0 must behave as e d0 d 0, with g (and fx) fixed. If A xg3 + the theory is not asymptotically free (so that in P(g) *, A x is the then has a at negative), integrand pole The boundary condition
=
—
-?
=
—
■
■
g'2=-(d0-d)/2Ai+O(d-d0)2. (7.11.4) only unambiguously exists if g2 is less than this value, when d d0. To get to the d d0 theory with g non-zero we must continue g0 so that the integration avoids the pole. The result is that g0 has an imaginary part. This, among other things, suggests that the theory is unphysical (see Wilson (1973), Gross (1976)). Recently, evidence has The solution which is
zero
=
=
accumulated that the lattice
continuum limit
-
see
</>4 theory
Symanzik (1982) for
does not have a
a
non-trivial
review.
If the theory is asymptotically free then we may continue (7.11.4) to d d0, i.e., e 0. The integrand becomes singular when e 0, and we =
=
=
examine the singularity by expanding in powers of g'\
In0o
=
ln0
4-^ln/*
/%')] -|I>'{f^ A2g'5
1
l-eg'/l-A.g'^
+
\_-eg'/2-Aig'3y
"^lE-^-^^^-^-l^pv}. In the first
ln0o=^ln +
integral
we may set e
=
0 and have
errors
that
are
(7.11-6)
o(l). So
2A[
lh-^-2>(^hVw
+
o(s),
(7.11.7)
where/(g) is defined by (7.10.18). Thus
??-A-fe){i+#-in0)]+4
(7118)
7.12 Renormalization group
219
for composite operators
7.12 Renormalization group for composite operators We have
how
prescription for the interactions of a theory can be compensated by a change in the values of the seen
a
change
renormalized parameters. The
operators
we
in renormalization
same
property holds for the composite
defined in Chapter 6.
For example, consider the renormalized [02] operator in 03 theory at d 6. In Section 6.2 we calculated it in the one-loop approximation: =
<O|T0(x)0(y)[02](z)/2|O> =
tree
graph
+
=
{one-loop graph + counterterm graphs}
(7.12.1)
H—
A change in renormalization prescription amounts to a finite change in the graphs. Since the counterterms are of the form
counterterm
il>2] we
=
W
\&Za(f)2 + dZbm2(j)
+
+
3ZC □ 0
+
higher order, (7.12.2)
have
il>2]?ewRP
=
il>2]o.dRP + ??[>2]
+
b<j,
+
cU4>.
(7.12.3)
Here a, ft, and c are finite quantities of the same order in the coupling as the one-loop counterterms. The equation (7.12.3) is, so far, only derived at the one-loop order-so the finite counterterms are to be used with their
operators inserted in tree graphs. Let us examine the situation we expect to all orders. We will use minimal subtraction. Then the renormalization in the notation of
(6.2.12)
is
(7.12.4) Now the bare field
is
+
+
=
independent
of n,
so we may
write
^2-3ZbZ-l'2m2^0ti^-\n(fid'2-3ZbZ-1'2m2) ndl2-3zcz-ll2n<Pon-fln&'"2 ̄3zcz ̄112)
l[^2]^ln(Za/Z) +
/z"'2-3Z6mV^ln(//2-3Z6Z1/2Zfl-1m2) d^
+
/'2-3ZfD<^ln(//2-3ZcZ1'2Z;1), d/z
(7.12.5)
220
Renormalization group
which has the form
^i[*2]= v&W We
can
(6=|
formulate this
fi|>2]| <£
=
as a
+
yt>m2S'2 ̄3<t> + yy/2"3 □ *■
matrix
equation:
/z^-'m^Z-'-V'2-3 Z,Z_1/V/2"3| 0
D<M
<7-12-6)
0
Z"1'2
0
0
200
C0o/
Z-1'2
(7.12.7) dM
d
^—0 d/x
=
dp
The coefficients ya,y6, and yc Section 6.2, we have
where X
=
g
/64k
.
.,
1<b
finite at d
are
V 384rc3
=
=
(7.12.8)
y<p<b. 6. From
^
=
64^
?c
=
384tt3"6X/^
=
(7.12.9)
AM
.1
g
Thus
0 0
6
AbiV'2-3 y/2'3 a 3 6g 0
+
higher order.
arguments that
we
used to prove the
From the RG equation (7.12.7)
equations for Green's functions
% pT9 ̄ +
we
of the
same
m.
This follows from
property for ym and y.
prove renormalization-group composite operators. For example: we
ymW2i)<01 T<t>(x)<fi(y)K4>2W\o> =
where
(7.12.10)
i9
Observe that ya, yb, and yc are all independent of/z and same
calculations in
5
=
0
the
our
<*' 6'
ya
,_5 ?*=!
/z-— M
have used
(ya-y)<o\T<t>4>K<P2]\oy + (ybm2 + ycnK0\T<l>(x)4>(y)<P(z)\0>,
^.dtp/d/x
 ̄3 factors. eliminating the //'2
=
—
y<j>/2,
and
we
have
set
(7.12.11) d
=
6, thus
7.12 Renormalization group We must prove
generalizations
automatically
(7.12.8) both to
221
[02] operator and in its bare operators are Since operators.
all orders for the
to deal with any
RG invariant, the
for composite operators
only question
is whether the anomalous
finite. This is handled by a simple generalization of the proof given in Section 7.2 for the ordinary Green's functions. We will not spell out dimensions
the details
are
-
for that is
just
a
mathematical exercise.
8
Large-mass expansion situation in physics is that in investigating phenomena on a certain distance scale, one sees no hint of those phenomena that happen at A
common
much shorter distance scales. In a classical situation this observation seems evident. For
example, one can treat fluid dynamics without any knowledge physics that generates the actual properties of the fluids.
of the atomic
quantum field theory this decoupling of short-distance phenomena from long-distance phenomena is not self-evident at all. However, in
a
Consider an e+
—
e ̄ annihilation
experiment at a center-of-mass energy
GeV, the threshold for making hadrons containing the fe- There is, for practical (or experimental) purposes, no trace of the quark. existence of this quark. However, the quark is present in Feynman graphs well below 10
virtual particle, and can have an apparently significant effect on cross- sections. Our task in this chapter is therefore to prove what is known as the decoupling theorem. This states that a Feynman graph containing a as a
propagator for
a
momenta of the
graph is
field whose
mass
is much greater than the external by a power of the heavy mass.
in fact suppressed
physics at low energy is described by an effective low-energy theory that is obtained by deleting all heavy fields from the original theory. The decoupling of heavy particles is not absolutely universal. One important and typical exception is that of weak interactions. Let us
The
consider the interactions of hadrons at energies of a few GeV. The effective low-energy theory, in the sense just described, consists of strong and
electromagnetic interactions alone, without weak interactions. So weak interactions should be ignorable at low energies. However, it is well known that there are in fact many observed processes, particularly decays, that are due entirely to weak interactions. The point is that, in the absence of weak interactions, these processes are exactly forbidden by symmetries, such as parity, charge-conjugation, and strangeness conservation. Weak- interaction amplitudes for the processes in question would be power-law corrections suppressed by a power of energy divided by the mass of the W-boson were it not that they are corrections to zero. The consequence of this particular situation is that, at low energies, weak interactions are -
-
222
223
Large-mass expansion described by
raw-renormalizable theory, viz., the four-fermion
a
interaction. Efforts to find a
prediction of
a renormalizable theory led to gauge theories, and the W- and Z-bosons from phenomena at energies much
lower than their
clues In
low-energy phenomena have indeed provided as to what might happen at much higher energy. this chapter, we will treat the cases where decoupling occurs. The masses:
theorem that tells
expect decoupling to
us to
occur
in many theories
was
formalized by Appelquist & Carazzone (1975) and Symanzik (1973). They work with a renormalizable theory in which some fields have masses very
large compared with the others. They then consider Green's functions of the low-mass fields at momenta much less than the large masses. The theorem is that the Green's functions are the same as those in an effective low-energy
heavy fields. Corrections are smaller by a heavy mass. The sole effect of loops of heavy particles is that the couplings of the low-energy theory can have different values from those in the complete theory. Since the renormalized couplings have no particular a priori value, the theory by
a
obtained by
deleting all
of the
power of momentum divided
heavy particles
are
unobservable until close to threshold. The practical
importance of the theorem is that
without having
a
one can
understand low-energy
physics
complete Lagrangian for all phenomena.
applied in the the low-energy
We will also show how the renormalization group can be
computation of
the relation between the
couplings
of
effective theory and those of the full theory. There are many ramifications of the decoupling theorem, but
we
will not
treat these. One of these is the detailed application of the decoupling theorem to gauge theories (see, for example, Kazama & Yao (1982)). Another is the large-mass expansion of Witten (1976)-where Green's
heavy fields inelastic scattering.
functions of the
We will also not treat the
are
computed; this expansion is used is deep-
exceptions
to the
decoupling
theorem. These
techniques those used to prove the decoupling theorem itself. We have already mentioned weak interactions as one of the can
be treated by the
same
typical exceptions. Let
(1)
us
as
just
note two other main classes of
In theories with spontaneous symmetry
breaking,
a mass
exception:
is often made
dimensionless coupling (Veltman (1977) and Toussaint (1978)). The decoupling theorem assumes that a mass is made large by increasing dimensional parameters.
large by increasing
a
power-counting violate renormalizability of the low-energy theory (see Collins, Wilczek & Zee
(2) Some dimensionless couplings needed by (1978)).
224
Large-mass expansion
In any event the effective low-energy theory is non-renormalizable. It might be supposed that since General Relativity is non-renormalizable in
it contains some clues to phenomena at very high for energies (see, example, Hawking & Israel (1979)).
perturbation theory,
8.1 A model We will restrict
two fields in six if
=
simple model. space-time dimensions: attention to
our
a
very
(3^)2/2 + (dcf>h)2/2 m2ti/2 M20h2/2 /*3"d/2[0i</>i3/6 + 02</>i</>h/2] /2" V>i -
It is
a
03 theory with
-
-
-
+ counterterms.
(8.1.1)
Symmetry under 0h 0h has been imposed to cut down the number of Then possible couplings. (8.1.1) contains all couplings necessary for We assume that the renormalized mass, M, of the heavy renormalizability. field is made large while all other parameters are held finite. The factors of -?
the unit of
-
needed with dimensional
mass fj,
regularization
are
explicitly
indicated. All
techniques (realistic) theories. our
can
be
readily
As usual we have introduced
tadpole graphs. This
a
extended to treat
linear term in the
is determined
by
more
complicated
Lagrangian to cancel the
the renormalization condition that
<O|01|O>=O. The
remaining
counterterms can be put in the form
j5fc,=(Zl-l)^,2/2 + (Zh-l)a^/2 \m\Zm 1) + M2ZmM/2 \M\ZM 1) + m2ZMJ<p2J2 H3-dl2ttgiB 9i)<t>?/6 + (02B 02)Mh/2] (8-1-2) -/'2"3(/b-/)<£.? As usual,
-
 ̄
-
and the
we
#B's)
-
-
-
-
may choose the dimensionless renormalizations to be
independent
(viz., the
of the dimensional parameters
Z's
m2, M2,
and /. The
decoupling theorem
on energy scales much effective low-energy theory whose
asserts that phenomena
less than M are described
by
an
Lagrangian has the form Se^
=
zd(f)i/2
-
m*2z4>\l2
-
ii2 ̄dl2g*z*l2<¥>\/*>
 ̄
?d/2 ̄3z1/2f*<t>i
+ counterterms =
c0*2/2
-
m*20*2/2
+ counterterms.
-
^  ̄d/V0*3/6
-
\idl2 ̄ 7*</>* (8.1.3)
225
8.2 Power-counting Here
we
have defined
and the coefficient
a
z can
scaled field
be chosen
z1/2</>,. We will prove that g*, m* that Green's functions of </>, obtained
0*
so
=
Lagrangian (8.1.1) by by M. field </>* which has unit
from J5frff differ from those obtained from the full
terms which are of the order of a power of external momenta divided
It is usually convenient coefficient for its kinetic
functions in the full energy
to work with the scaled term in the basic
theory
Lagrangian.
Then Green's
related to Green's functions in the low-
theory by
GN(pl,...,PN;g{,gh,m,M^) =
=
as
are
M —?
oo
(0\Tfa(p1)... 0l(PN)|O>full theory
=
z-^2G*(Pl,...,pN;^,m*,/i)[H-0(l/Ma)] z-N!\0\Tr(Pl)...¥*(pN)\0)[l + O(l/Ma)l (8.1.4)
with p1,..., pN fixed. The fractional
errors
go to
zero as a power
of M times logarithms; the power is typically M ̄2. We can therefore use 1/Ma, with a slightly less than two, to bound the error. As is our convention,
the tilde signs
over
the fields and Green's functions indicate
a
Fourier
transform into momentum space.
8.2
Power-counting
finding tjie leading power of M a simple generalization of graph Weinberg's theorem, and will involve us in understanding which regions of momentum space are important. We will mostly be interested in graphs for In this section,
we
in the value of
a
will establish the rules for as M -> oo.
These form
the Green's functions of the light field </>,. Our aim will be to find those graphs that contain lines for the heavy field and that do not vanish as M goes to
infinity. 8.2.1 Tree
Because
we
containing
choose to
graphs
impose the symmetry <£h </>h, the only tree graphs heavy field have heavy external lines. An example is -?
-
lines for the
Fig. 8.2.1. Since all momenta
on
the lines
are
fixed, and since the free 0h-
propagator is i/(p2-M2) ̄ -i/M2,
-< Fig. 8.2.1. A
tree
graph with
a
heavy line.
226
Large-mass expansion
the behavior of any
given
tree
graph
as M -?
oo
is
M ̄2H,
(8.2.1)
where H is the number of
heavy lines. (We use the natural terminology of calling Feynman graph heavy or light according to whether its free propagator is for c/>h or c/>, respectively. Our graphical notation is that a
line of
heavy lines
are
a
thicker than light lines.)
8.2.2 Finite graphs with heavy loops
graph that has one or more loops but no ultra-violet divergences or subdivergences, and that has some heavy internal lines. The lowest-order example is Fig. 8.2.2(a), for the four-point function. Its external momenta (if small) may evidently be neglected on the lines of the Consider
now a
loop, whose value is then r.4 r, 2
(We
= -
V-M2)4 ̄3847r3M2'
(2n)6
label the symbol T by the
l
]
figure number.) 102 384ti3M2
/
(a)
\
Fig. 8.2.2. Large-mass behavior of
(b)
graph without
an
ultra-violet divergence.
oo. The precise power of M can be obtained the considering by possible regions of momentum space (after Wick as follows. rotation), Any region of k that is finite as M->oo gives a
The
graph
vanishes
as
M ->
graph is UV finite, the only other 0{M). Simple power-counting gives M ̄2, as found in
contribution of order M ̄6. Since the
possibility
is k
=
(8.2.2). This power-counting is the same as for the UV degree of divergence. The graph is negligible (by a power of M2) compared to graphs with no
heavy
wanted its leading contribution, then it the local four-point vertex symbolized in Fig. 8.2.2(b).
lines. If, nevertheless,
would be
we
effectively The non-renormalizability of this coupling (when the space-time dimension is six) is tied to the negative power of M2.
8.2.3 Divergent one-loop graphs Consider the logarithmically divergent vertex
graph
in Fig. 8.2.3(a). After
8.2 Power-counting
(?)
(*)
Fig. 8.2.3. Large-mass behavior of graph with minimal subtraction the
ultra-violet divergence.
dx
dyx
M2-(p2x + p22y)(l
■x-y)-
pixy
-}}
In
an
loop gives
+ 64n3 b
x
227
(8.2.3) The
same power-counting as for Fig. 8.2.2 confirms the power M° for the leading behavior as M -? oo. There is also a logarithm. This occurs because two regions contribute to the leading behavior: the first is where the loop momentum k is of order M. The second region is the UV region where
k ->
After subtraction of the ultra-violet divergence remains. oc.
a
finite contribution
Evidently the graph gives a contribution that increases with M. Fortunately the non-vanishing part of the loop is independent of the external momenta. So for large M, the loop is effectively a three-point vertex, as shown in Fig. 8.2.3(b). A proof which generalizes to higher order is to differentiate with respect to any external momentum. Since the differentiated
graph
is finite, it vanishes when M -> oo, like a power of M. decoupling theorem, that at low energies, we
Recall our statement of the
could calculate Green's functions from the effective low-energy Lagrangian (8.1.3). The result of our calculation of Fig. 8.2.3 is that the graph generates an extra
piece
in the
</>,3 coupling of the low-energy theory.
Let
us
therefore
write
z3/V
=
0i-
128ti3
y + ln
M2
4njn2
+
0(g5).
(8.2.4)
We may drop the graph Fig. 8.2.3 and replace it by the order g3 term on the right of (8.2.4). The loop has been replaced by a local vertex where all the
lines
come to a
single point.
line is far off-shell and
can
This corresponds to the fact that the internal only exist for a short time.
228
Large-mass expansion
Fig. 8.2.4. Large-mass behavior of graph with
The self-energy graph, Fig. 8.2.4, gives value of the loop is
a
a
quadratic ultra-violet divergence.
leading
term of order
M2. The
102
]}
f-"W1-*)]lnL A& ̄ i dx[M2-p2x(l -&{4"+"(&)]-fr+,-(£)] +
+ 0
(8.2.5)
M2
Again the loop momentum so
there will be at most
k can be either UV or of order M to
a
single logarithm of M2/fi2. Since
differentiate three times with respect to p^ before obtaining
graph, the non-vanishing terms, effective
Lagrangian (8.1.3),
ztn*2 We
=
m2
we see
that the
9l
convergent
quadratic in p. From the graph may be replaced by a
i[(z
-
+
768n3
—
have to
as M-> oo, are
contribution to the basic self-energy vertex the low-energy theory, with z=l-
a
contribute, we
g\M2 1287T3
i)p2
—
(m*2z
—
0(g%
m2)]
in
(8.2.6)
+
0(g%
(8.2.7)
+
O(05),
(8.2.8)
compute g* and m*:
can now
g* m
=
gl(g2
9i-:
-
i
*2
y +
Notice that there is
a
ln(^?III>
+
O(04)-
(8-Z9)
contribution of order M2 to the self-energy and
8.2 hence to m*2. In order to
m* finite
229
Power-counting
keep the physical mass of </>j finite and hence keep let m2 have a term proportional to g\M2 (with
as M -* oo, we must
higher-order corrections): m2 On
finite +
=
#2
expanding m*2 "*2 m*
=
M2
y
-
'128tc3L#
M2
1 + In  ̄
in powers of
'
+
"W
coupling,
we
higher order.
(8.2.10)
find
finite term in m2 '
+
Since
m
is the
unnatural
y +In
6Vl287tV mass
M2
4^
4-
higher
order.
(8.2.11)
parameter for the light field, it is generally considered
to have to fine-tune it within a fractional accuracy of
is required
m*2/M2, as
oo. In the by (8.2.10), context of grand unified theories this is called the gauge hierarchy problem (Weinberg (1974,1976), Gildener & Weinberg (1976)). It is hoped to solve it by finding a phenomenologically sensible theory with no need for fine- tuning. to obtain a finite value of m* when M-?
8.2.4 More than
one
loop
divergent one-loop graphs occurring inside a larger superficially convergent graph. A typical example is Fig. 8.2.5. When We may have
one
of the
Mg*z
\r- Fig. 8.2.5. Large-mass behavior of two-loop graph with M->co with the external momenta momenta that
gives
a
non-zero
an
ultra-violet divergence.
fixed, the only region of the loop
contribution is where the outer-loop
momentum / is finite and the
larger. So the heavy loop can
inner-loop momentum k is of order M or be replaced by its effective low-energy vertex
computed at (8.2.3). This procedure does not change the overall degree of divergence. The situation at subtle a
as we
with overall-divergent graphs is more The graph of Fig. 8.2.6 is typical. Now, it contains
higher
will now
see.
order
or
subgraph, consisting of the heavy loop, which we have already considered.
230
Large-mass expansion
?P03-O] Fig. 8.2.6. Large-mass behavior of another two-loop graph with
an
ultra-violet
divergence.
Therefore, the low-energy theory contains a graph where the heavy loop is replaced by a vertex using (8.2.4) for g*z3f2 g. This graph exactly reproduces the region where k is finite and / is large for Fig. 8.2.6. We add and subtract this graph from the original graph as indicated in the figure. The subtracted term (in square brackets) has a vanishing contribution from finite k (as M-> oo). So we replace it by an effective vertex A6. The same -
arguments
used for one-loop self-energy, Fig. 8.2.4, show that it has proportional to p2, m2, and M2, with coefficients polynomial in
as we
three terms,
ln(M2//x2).
graphs there are UV divergences for the whole graph and for subgraphs. Implicitly, the counterterm graphs are to be included. Provided we use mass-independent renormalization we are guaranteed that the counterterm graphs satisfy the same power-counting as the original graphs. In particular they are polynomial in the light masses. Thus the counterterm graphs do not change the power-counting and differentiation In this and in other
arguments that
are
crucial to
our
work.
8.3 General ideas Structurally, the arguments in the last section appear similar to those used in
Chapter
7 to show that
we
renormalization-prescription dependence
by finite counterterms. In fact, as we will see in the next a proof of the decoupling theorem can be constructed exactly by changing the renormalization prescription. We will show that a can
be compensated
section, Section 8.4,
renormalization prescription
properties,
constructed
couplings
can
important simply by deleting
and
masses
decouplings and subsection 8.3.1.
we
be chosen to have
a
number of convenient
of which is that the low-energy theory is
the most
all
heavy fields without changing the
of the light fields. This property is called manifest explain it with the aid of an example in
will
231
8.3 General ideas Our
follows the method given by
approach
Appelquist
& Carazzone
(1976). This approach generates the effective theory as a series of subtractions. The simplest non-trivial case is given in Fig. 8.2.6. There is another approach due to Weinberg (1980) in which the decoupling is considered by first integrating over the heavy fields in the (1975) and Witten
functional integral. (See also Ovrut & Schnitzer (1980).) This method is less convenient for
treating graphs like Fig. 8.2.6,
so we
do not
use
it.
8.3.1 Renormalization prescriptions with manifest decoupling Suppose we used BPH(Z) renormalization instead of minimal subtraction. Then the renormalization condition is that the terms up to pd(f) are zero in the Taylor expansion of a graph T about zero external momentum. Here
5(F) is the degree of divergence. For a graph with a single loop, consisting of a heavy line, these terms are precisely those that are non-vanishing as M-> oo. Examples are given by the graphs of Figs. 8.2.3 and 8.2.4. In fact, for a general graph, the effective low-energy theory in this renormalization presciption is obtained merely by deleting all graphs containing heavy lines, together with all their counterterm graphs. The values of the couplings and masses are not changed. Therefore the BPH(Z) prescription has the property we called 'manifest decoupling'. It might appear sensible always to use a renormalization prescription that has this property. However, for many purposes it is useful to use other renormalization prescriptions, e.g. minimal subtraction and its relatives. Particular cases
are
theories
containing massless fields, especially
non-
abelian gauge theories, and theories with spontaneous symmetry breaking. In any case, it is good to have a direct method of proof of decoupling that can
work with any
subtraction is
prescription. Furthermore, convenient if
a
prescription
like minimal
high-energy behavior (Section 7.4) with the aid of the renormalization group. In fact, the method we will use will start from a mass-independent renormalization prescription defined for both the full theory and for the effective low-energy theory. Then the renormalization of the low-energy theory is extended to a renormalization prescription of the full theory in such a way as to satisfy manifest decoupling. This method was first stated by Collins, Wilczek & more
one
also wishes to compute
Zee (1978). One renormalization prescription that gives manifest decoupling at low energies and that allows the use of renormalization-group methods at high energies is due qriginally to Gell-Mann & Low (1954). In this scheme,
makes subtractions at
some
arbitrarily
one
chosen value of momentum. This
232
Large-mass expansion
applied to the large-mass problem by Georgi & Politzer (1976). The disadvantage of this scheme, compared with the scheme that we will actually use, is that renormalization-group coefficients are explicitly functions of M/p, and of mj\x\ scheme was
Pi=Pi(9i>92'>M/iJL,m/ii). This
makes calculations
some
complicated. Furthermore,
this scheme obscures
symmetries. 8.3.2 Dominant regions
a proof of the decoupling theorem, let us give a that give unsuppressed contributions (i.e. regions precise not suppressed by a power of M2). We consider each graph in the full theory together with the set of subtraction graphs needed to cancel its divergences and subdivergences. We do not consider the subtraction graphs separately.
Before actually constructing statement of the
First of all, any
graph
with
no
heavy
lines at all contributes without
suppression. A graph with one or more heavy lines cannot give a contribution unless at least one loop momentum is of order M. The contribution to a graph when M is large can be considered
possible regions of momentum
as
the
sum
space. The
of contributions from various
regions
can
be
specified by
the
sizes of the loop momenta. For our purposes, it is enough to classify a momentum as either finite or large. 'Large' we define to mean 'of order M or
power-counting for each region in the obvious way. For the loops carrying large momenta, counting powers of M is the same as for the ultra-violet degree of divergence. This gives a factor Md, where S is the bigger'.
We
can
do
ultra-violet degree of the lines carrying large momenta. A heavy line carrying finite momentum counts as M ̄2. A light line carrying finite momentum counts as M°.
power of M for a graph is obtained as the maximum of the possible regions. Let us define SM(T) to be this highest power. In general there will be logarithmic enhancements. But Weinberg's theorem The
leading
powers for the
guarantees that the power SM(T) is correctly given by considering only the regions we have listed. The graphs treated in Section 8.2 provide examples of this procedure. (The subscript 'M' is to distinguish 5M(T) from the ultraviolet degree of divergence.) A region contributing to a leading power that is M° or bigger is symbolized by contracting to a point both the heavy lines and the lines carrying large momentum. The points represent vertices in the effective low-energy theory; we have already used this notation in Section 8.2, in the
8.4 Proof of decoupling
Fig. 8.3.1. Graph with two contracted subgraphs,
233
Fig. 8.3.2. One-particle reducible sub- graphs may have to be contracted.
figures. In general (see Fig. 8.3.1) the contractions will result in several vertices. We include in our definition the restriction that a subgraph is only contracted to a point if it contains at least one heavy line. A contracted
subgraph is 1PI in the light cutting a light line then that
lines; for if it
can
be split into two parts
line is not carrying
a
large loop
by
momentum.
However, the contracted graph may be 1PR in the heavy lines. For example, in a theory where the symmetry </>h </>h is not valid, a graph -?
—
power M°; the self-energy gives a power M2 in the propagator. A subgraph that is contracted to a single vertex gives the same power of M as its UV power-counting. So
like
Fig.
8.3.2
gives
which cancels the
a
leading
1/M2
dim (subgraph)
=
power of M + dim (couplings).
Hence in a renormalizable theory (where couplings have non-negative dimension) the only contracted graphs that have a non-vanishing value as M -? oo correspond to vertices whose couplings have non-negative
dimension. These vertices
give
the difference between
g*z3/2
and g, etc. Thus the
low-energy theory satisfy the dimensional renormalizability. In a scalar theory, this implies actual renormalizability, provided all the couplings are used that have non- negative dimension and that obey the symmetries of the full theory.
couplings
in the effective
criterion for
8.4 Proof of
decoupling
8.4.1 Renormalization prescription R* with
manifest decoupling
us work with the theory defined by (8.1.1). We choose to renormalize according to a mass-independent prescription, which we will denote by a symbol R. For definiteness we choose this to be minimal subtraction. By deleting all heavy fields from (8.1.1) and by changing the values of the couplings we obtain the form of expected low-energy theory (8.1.3). We choose to renormalize the low-energy theory by a mass-independent prescription R*, which we also take to be minimal subtraction. Our proof will consist of extending R* to a renormalization of the full
Let
it
234
Large-mass expansion
theory. The
satisfy manifest decoupling. Since different prescriptions differ only by a reparametrization, the statement (8.1.4) of the decoupling theorem will hold. The structure of/?* in the full theory will give a 'mass-independent' form for g*, m*2 and z: extension will
renormalization
)
0*=0*(01>02>MM z
=
m*2
=
>
z(gl9g29M/ri9
m2zJgvg2,M/ii)
+
(8.4.1)
M2zmM(gl9g29M/fi).)
independence independence of the light mass. As before, to complication we choose to renormalize the linear coupling 0. We then ignore both the linear f(f>} by the prescription that <0|</>,|0> coupling and the tadpole graphs. The reason we use mass-independent renormalization prescriptions for all the couplings other than the term linear in 0 is that we can thereby make very clear the decoupling of phenomena at small mass scales from large mass scales. In addition, the renormalization-group equations for (8.4.1) are much simpler to work with than they would otherwise be. Mass
save
means
notational
=
It is convenient to define two concepts: (1) A heavy graph is heavy field </>h).
(2)
A
light graph is
one
containing at least one heavy line (i.e. a line for the
one
that contains
no
heavy
lines.
theory we have a series of counterterm graphs that are used to cancel its divergences. If T is a heavy graph, then we also consider its counterterm graphs to be heavy graphs, even though they may contain no explicit heavy lines. We have chosen a renormalization prescription R* for the low-energy theory. This defines the renormalized value of any graph in the low-energy theory, and therefore of any light graph in the full theory. We now wish to For each basic
graph
T in the full
extend this prescription to heavy graphs, in such a way that it satisfies manifest decoupling. That is, the renormalized value R*(T) of a heavy graph goes to zero
as M -> oo.
The basic idea is to subtract such
momentum. That this is a sensible
procedure is easily
graphs from Section 8.2. example, we saw that Fig.
seen
graphs at zero by examining a
few of the For
M-?oc,
if
subtraction
we
=
we use
8.2.3 diverges
logarithmically
when
minimal subtraction. But with zero-momentum
have
0(p2/M2)
as M -> oo.
(8.4.2)
235
8.4 Proof of decoupling
Clearly, the difference between the difference given by (8.2.4) for g*z3/2 minimal subtraction
two -
renormalizations is
high
g. At
just the
energy we would use
be neglected compared with momenta would use the R* prescription so that we can
so M can
-
but at low energy we simplify calculations by
-
-
dropping heavy graphs.
Fig.
8.4.1.
Fig. 8.4.1 for the self-energy of the heavy field. This graph light and heavy lines. It would behave like M2 In (M) for large
Consider next
contains both
M, if we used minimal K{1"'
subtraction. Instead, let
us
define the subtraction by
64n3(4nfi2Y'2-3
dx{[M2x + m2(l-x)-p2x{l-x)]d'2-2-(M2x)dl
x
2-2
-{d/2-2)(M2x)'"2-3[m2(l-x)-p2xil-x) ̄\}
dx\\_M2x I
—^773
^6)64n3J0 :ln =
m2(l-x)-p2x(l-x)]x
m2(l-x)
p2(l-x)
M2x
M2
1+-
0(1/M2)
+
-
m2{\ -x) + p2x(l
-
x)
(8.4.3)
asM->oo.
Here, we observed that when M graph is linear in m2 and
-> oo
p2.
So
the dependence of the unrenormalized we
about p
expanded
=
m
0 and
=
quadratic in m and p. This means that the polynomial in m, i.e., 'mass-independence' holds good.
subtracted the terms up to
counterterms
are
Normally subtractions at zero mass and momentum have infra-red divergences, but the presence of a heavy line prevents this here. As a final example let us examine the two-loop graph of Fig. 8.2.6. The unrenormalized
graph
cannot be
counterterm, because there
p-dependence
I
,,,
are two
expanded about m light lines. At m, p  ̄
(value of heavy loop at p fc 771 2irv + Ln: 21 (k2 m2)[(p k)2 m2] =
-
The
0
p
=
0 to
they give
give
a
m-
and
,
2\
of the form
d k- ?>m>0
=
-
=
0)
" ̄
.
(P
,
+
2
rn2)ln(p2 + m2). 2,,
,
(8.4.4) right-hand
side of this equation is schematic and
symbolizes
the
236
Large-mass expansion
o o W
(b)
Fig. 8.4.2. Counterterms for Fig. 8.2.6. maximum powers and logarithms of obtain
the
renormalized
value
m
and of p that
of the
graph
we
occur.
However,
to
first subtract
must
subdivergences by the counterterms Fig. 8.4.2, constructed by the R*- scheme. Now, the counterterm graph (b) has infra-red behavior exactly equal and opposite to that of (8.4.4), because the counterterm is minus the value of the heavy loop at p k 0. Thus the sum of the two graphs has the —
=
extra convergence we need. The overall counterterm is then linear in m2. There are no logarithms of m as M -> oo.
8.4.2
p2 and
Definition of R*
To define the renormalization
summarize and generalize what We define the renormalization
prescription we
R* in
general,
we
simply
have just done for particular graphs.
prescription R* in the full theory to be the for the low-energy theory whenever it acts
prescription purely light graph. For a heavy graph T, we assume inductively that we have defined the quantity R*{T) in the usual way to be the unrenormalized value of r plus counterterms in the K*-scheme to cancel its subdivergences. same as our chosen
on a
If T has degree of divergence S(T)
>
0, then its overall
counterterm is
by subtraction at m p 0. The renormalized value of T is R*(T) + C*(T), as usual. To define C*(T) precisely, we first expand R*(T) in a Taylor series about the point where its external momenta and the light mass m are zero. Pick defined
R*(T)
=
—
=
with dimension up to 5(T), and let the counterterm C*(T) be the negative of these terms. Our examples tell
out the terms where momenta and m2
us to
expect that with such
(1) the leading (2) there
M->
are no IR
oo
occur
a counterterm:
behavior is canceled,
singularities
in the counterterm.
general. The proof will generalize from Fig. 8.2.6. There, the UV divergent
We must prove these statements in
the simplest non-trivial
case,
unrenormalized graph is not polynomial in
m
and p, but after subtraction of
subdivergences by the fl*-scheme, it becomes polynomial. Then the R*- prescriptioi} can legitimately generate the overall counterterm. Moreover, after subtraction of the subdivergences, the leading large-M behavior is also
8.4 Proof of decoupling
237
polynomial in m and p with degree equal to the degree of divergence, so that it is cancelled by the overall counterterm. Even with the subtractions for
subgraphs, there are in general IR in the singularities Taylor expansion of a graph. For example, consider Fig. 8.4.1 and expand its integrand see (8.4.3) in powers of m2 and p2. All the terms beyond the second give divergences at x 0; it is only the terms needed to cancel the UV divergence that are non-singular. -
-
=
8.4.3 IRfinitenessofC*(r) Suppose r is a heavy graph, 1PI in its light lines. Potential infra-red divergences in C*(T) arise when m and the external momenta are made small. They come from regions where some or all of the loop momenta are of order
m.
The
simplest case is where all the internal momenta are of order
m.
If r
were a
light graph,
we
would obtain
a
contribution of order
where d(T) is the UV degree of divergence. So let us call canonical IR degree of divergence of T. If d(T) 0, this is a =
m3{r\
d(T) the logarithmic -
divergence. If (5(T)>0, then the graph is finite as m-?0. But to get the coefficients of the polynomial counterterms we differentiate up to S(T) times with respect to m and the external momenta. The highest terms in the polynomial are therefore always logarithmically IR divergent, for a light graph. However, r is actually a heavy graph. So at least one of its propagators counts
as
1/M2 instead of 1/m2. Thus all the counterterms
contribution from this
region,
where all its
This discussion is sufficient for all
graphs have small
have
loop one-loop graphs.
divergences coming momenta. For example, Fig.
IR finite
multi-loop loops divergence from the But
from regions where only
have IR
an
momenta are small.
8.2.6 has
a
seme
region where p and k are small, i.e., order m, and / is finite or large. This 2, and is given by (8.4.4). As we saw, the IR corresponds to IR degree the is canceled divergence by graph with a counterterm for the heavy loop. The general case is that some light lines carry momenta of order m and —
(a) Fig. 8.4.1, with k and p of order (b) Fig. 8.2.6, all loop
momenta
(c) Fig. 8.2.6, k and p of order
m
m
large
>
V?^
>
?
>
(
Fig. 8.4.3. Examples of reduced graphs.
^—
238
Large-mass expansion
the remainder of the lines either
are heavy or carry large momentum. Each region is symbolized by a reduced graph in which the subgraphs consisting of the lines with large momenta and of the heavy lines are contracted to points. Examples of reduced graphs are shown in Fig. 8.4.3.
such
Note: (1) Counterterm graphs can also have infra-red divergences. The counter- terms are inside the vertices of the reduced graphs. All lines of reduced
(2)
graphs are light, so at graph corresponds to a heavy subgraph.
least
one vertex
of
reduced
a
write the infra-red degree of divergence for the region corresponding to a particular reduced graph y as We
can
S1R(T;y)= ̄S(r)+
[*W +IR degree of K*(P)].
£
(8.4.5)
reduced vertices V
The
meaning of this equation
can
be
seen
from
an
example.
Consider
Fig. 8.2.6 when k and p are of order m. If the graph were purely light, we would have IR degree equal to 2, which is the negative of the UV degree. —
This would imply that the m2 and p2 terms in the expansion about m would be
divergent. However,
the
single reduced
vertex
-
as
=
p
=
0
illustrated in
Fig. 8.4.3(c) has a counterterm. This counterterm ensures that the vertex's value is of order m2/M2 instead of m°. The IR degree for the whole graph is -
thereby decreased by 2. The second term in (8.4.5), where the sum is over this 4; single vertex, indicates this reduction. The degree for the region is then therefore expand up to order m2 and p2 without an infra-red —
we can
divergence. The terms of order are not
m4, p4, etc., are infra-red divergent,
but they
needed for ultra-violet renormalization.
general case of (8.4.5), each reduced vertex V would contribute if it were light and all its internal lines had momenta of order m. But d(V) actually contributes what we will now prove is a smaller amount.
In the -
it
Remember that counterterm counterterm vertices
R*(V) is its power cases for V are:
graphs
also contribute, and
we assume
that
included inside reduced vertices. The IR degree of its external momenta are scaled like m. The possible
are
as
(1) If V is overall convergent and contains a heavy line, then its infra-red degree is greater than its ultra-violet degree. Fig. 8.4.3(a) has a vertex 2 and IR degree zero. with UV degree If and contains a heavy line then ordinarily we V is overall divergent (2) -
would expect it to behave
as
m° when
m ->
0 with fixed M. But
we
make
239
8.4 Proof of deco upling
subtractions by the R* scheme so that its behavior is actually m3{V) 2. By induction we may assume its subtractions have no IR divergence. +
Hence,
heavy graph T always has at least degree below d(T) and none to increase it.
in every region of momenta
one mechanism to reduce its IR
a
—
Thus the overall counterterm C*(T) is IR finite. It is crucial to
proof
that
we
inductive
our
first subtract subdivergences by the R* scheme.
8.4.4 Manifest decoupling for R* A purely light graph is
a
graph in both the full theory and in the low-energy
theory. heavy graphs vanish when
It survives unaltered when we let M -?
the
M
->
oo,
given
oo.
now prove that all renormalize them by
We will
that
we
the R* scheme. To do this, decompose each heavy graph into its skeleton, i.e., a series of graphs connected by lines that are not part of any loop. Since a heavy
1PI
line
loop vanishes as M-> oo, all heavy graphs vanish as graphs vanish. limit of a 1 PI graph can be related to an IR limit by scaling
that is outside
a
M-> oo, if the 1PI
The all
M
-> oo
masses
and momenta: M-*l,
p^p/M,
m-+m/M.
Then T(p, m, M)
=
(8.4.6)
Mdir)T(p/M, m/M, 1),
where d(V) is the dimension of T. So T vanishes as M -> oo infra-red behavior is less singular than m ̄d(r). But this is what
provided we
the
showed in
the proof of IR finiteness of the counterterms. (Note that the dimension of a
graph
is greater than
or
equal
to its UV
degree of divergence.)
8.4.5 Decoupling theorem We have constructed two renormalization
prescriptions, labelled R and R*,
for the theory under consideration. The Green's functions in the schemes R and R* are equal provided we make appropriate changes in the parameters:
0i-?0*, 0h-*0h> coefficient of dcp\/2
-?
z,
oid^l/l
-?
zh,
coefficient
m2^>m*2, M2-+M*2
(8.4.7)
240
Large-mass expansion
just a particular case of a renormalization-group transformation, and is proved by Section 7.2. When M may drop all heavy graphs in the R* scheme (so also g£, zh, M* drop out of consideration). This then gives (8.1.4), which is the decoupling theorem. Mass-independence is true because we have arranged all counterterms to be polynomials in the light mass of the appropriate degree. This is
-* oo
8.5
When
one
computes
a
we
Renormalization-group analysis graph containing lines for fields with widely different
finds, in general, that its value gets large as a power of the of the mass ratio. Such large coefficients are undesirable in a logarithm
masses,
one
perturbation expansion,
for
they
mean
that the reliability of
using
a
few
low-order terms is worsened. This situation arises in both strong- and weak-interaction
physics.
We will now show how to combine the
decoupling theorem and the renormalization group to do calculations without their being made unreliable by the large logarithms. A convenient method is to use a mass-independent scheme (specifically minimal subtraction) for high-momentum calculations, where one often wishes to neglect all masses, and to use the R* scheme, as defined in Section 8.4, at low momenta, where one wishes to neglect heavy graphs. An
advantage of this method is a simplification of many of the calculations high-energy and low-energy calculations. One needs only the pole parts of graphs and the values at zero external momentum. needed to match
We will explain how to use this scheme in the toy theory (8.1.1). First let us write the RG equations for the Green's functions. For a Green's function of Nx
light
and Nh
heavy fields,
we
have
{% +±2Niyi+iiVh7h) G?>=a
where
-
(M2yM +
m2ymM)^
-
(m2ym +
M2yMm)-^.
(8-5'1)
(8.5.2)
The RG coefficients are obtained from the renormalization counterterms
usual. Their lowest-order values
are
as
8.5 Renormalization-group analysis
Pl
1 =
128ti
3(502
1
Vi
=
yh
=
+
0192—z92i92) +
241
<
+ -.
02 mn3*2 1
In the effective
(8.5.3)
384rr
low-energy theory,
the RG equation is (8.5.4)
with d*
8
M
d
,
8
(8.5.5)
464tc3 *2
7*
0
=
384tc
r +
To compare the low-energy theory and the full theory, we extended the renormahzation scheme of the low-energy theory to a renormahzation scheme R* for the full
d*
d
M
theory.
8
8
?.
-(M*2y*M +
In this scheme the RG operator has the form
m*2y*mM)^i  ̄i^*2y* M*2y*Mm)-^2, +
(8-5-6)
y* and y£. In fact /?*, y*, and y* are low-energy theory (see (8.5.4) and (8.5.5)), while f}% y*M y^m y£ y£ 0. This is easily seen by examining the Green's functions which provide the normalization conditions for the and the anomalous dimensions of the fields
are
identical to those in the =
=
=
=
=
renormalizations.
heavy propagator when
For example, consider the inverse of the
and m2
are
both
p2
much less than M2:
1/G0i2= -i[p2-M*2
+
0(p\m2p\m4)]
(8.5.7)
which satisfies
^ ̄Go,i This is only consistent if y*
=
yti
=
=
ytGZ,l
y*M
=
0-
(8-5.8)
242
Large-mass expansion 8.5.1 Sample calculation
We wish to start with the full
theory renormalized by minimal subtraction. theory, we know the evolution of the couplings. Our aim is to compute Green's functions in the low-energy theory and the values of the mass and coupling. The low-energy effective couplings are In that version of the
?2/
128*3
,
3 In (p/A*) =
fixed,
m*2
=
constant
M*2
=
fixed.
qI
(8.5.9)
[In (/x/A*)] ̄1/9,
The effective couplings for the full theory with minimal subtraction are more complicated because they solve a coupled equation for two variables. To make the transition between the schemes we compute the lowest- order divergent graphs. We equate the self-energy for 0, in the two schemes,
with
use
of the Lagrangian (8.1.3) for the low-energy theory. This gives
Fig. 8.2.4 4- pole counterterm =
Fig. 8.2.4
4- zero-momentum counterterm 4-
We thus obtain
z
and not of order
and m*2
M2,
m2
+
as
i(z
-
l)p2
given by (8.2.6) and (8.2.7). replace m2 by
To
-
i(m*2z
-
m2).
keep m*2 finite,
we must
T^pM2^2[y-l +ln(M2/4^2)].
Notice the presence of logarithms invalidate the use of perturbation
(8.5.10)
of M/p. If they are large enough, they theory to compute g*, m*2, and z.
However, the equations
we write are valid at any value of p, so we may with the calculations p of order M. After computing g*, m*, and z in perform
terms of g{, g2, m, and M,
we can
evolve them to the value of p that
for calculations in the low-energy theory. point to do the matching is where g{ g*9 i.e., at where pi M2ey/4n. Then for a general value of p we have to
we
wish
use
A convenient
=
p2
=
pi,
=
^2(/z)=l/^(/,0) A similiar
+
(3/1287r3)ln^0)+--
(8-5,lla)
equation holds for m*: m*2(Ai)
The solution for
z
=
is
m2[(1287c3/3ff?) + lnOi/Ai0) + -]"l/9.
more
(*.5.Ub)
complicated since the renormalization-group prescriptions is needed.
equation for both renormalization
8.5
243
Renormalization-group analysis 8.5.2 Accuracy
We compute
g*(fi) in
low-energy theory by matching to the full theory evolving to an arbitrary renormalization mass \x from fi0. Given the accuracy in g*(fi) that we need for a particular calculation, we will find the order to which we must perform the matching the
at some fi0 of order M and then
and to which
The RG
we must
know /?.
equation for g*(fi) gives ln(fi/tt>)=
So if there
are
small
errors
fff(M) J
(8.5.12)
do'/Pig*).
0(Mo)
Ag(fi0) and A(l//?) in ^(^0) and 1//? then the error
in g(jj) is
Suppose we perform matching up to nm-loop order; then the error in g(fi0) is + of order g(fi0)2nni 3. Suppose ft is computed to rc^-loop order; then the error in
\/fi(g)
is of order
g2nfi ̄3.
These translate to
errors
in g(fi) of order
QitfgM2"'" and
g(fi)3\n[g(fi)/g(fi0)]\infi
=
l,
or +
0[g(fiV 2n>-\ For we
example,
if
we
+
wish to
O[ff0i)3^0io)2("'-1}] if ?, perform
need Ag to be much smaller than
>2.
reliable two-loop calculations, then This means that we need to do the
g3.
one loop and that the /?-function is needed to two loops. minimum This is the accuracy needed to correspond to a fractional error on A (defined in Chapter 7) which is much less than unity.
matching correct to
9 Global
In this chapter on
we
symmetries
consider the impact of global symmetries of a field theory an example consider the theory of a charged
its renormalization. As
scalar field : <£
=
dtfd<f>
-
m2^(f)
-
(9.0.1)
g(ft(t>)2/4.
e"iw</>. The quantum theory is also invariant. For this particular theory, the quantum invariance is not a very deep statement. However, symmetries do This classical Lagrangian is invariant under the transformation 0
not
always survive quantization,
as we
will
see
in
Chapter
-?
13. Thus it is
useful to examine the consequences of the symmetry in this theory. One consequence is that only invariant counterterms are needed; for example, we
do not need to
use
non-invariant counterterms proportional to
02 + 0t2
or
i((f>2
-
<f>u).
Other consequences are the Ward identities, which characterize the action of the symmetry at the level of Green's functions. The main step in proving the statements is to impose an ultra-violet
putting the theory on a lattice or by using dimensional regularization, the symmetry is preserved. The arguments given in Section cutoff. If this is done by 2.7
are
sufficient to prove Ward identities in the bare theory. From the
invariance of Green's functions follows invariance of the counterterms. As we
will
see
in Section 9.1
we can
then write renormalized Ward identities in
the renormalized theory, which therefore exhibits the symmetry. In
cases this simple procedure fails. is that the UV cut-off breaks the symmetry. For example, the theory on a lattice breaks Poincare invariance. Luckily, other
more
One
putting
general
case
regulators, like dimensional continuation, preserve this invariance, and the renormalized theory with no cut-off is Poincare invariant. Some symmetries cannot be preserved after quantization. It must be true that no regulator can preserve them. An example, to be treated in Chapter 13, is the chiral invariance of QCD. 244
9.1 Another case, which
symmetry breaking,
by the
—
we
245
Unbroken symmetry
will treat later in this
typified by the theory given
is of spontaneous by (9.0.1) with m2 replaced
chapter,
m2. This is called the Goldstone model. In this case the ground-state
vacuum
vacuum
-
is not invariant under the symmetry, and the field acquires
expectation
-
a
value:
<O|0|O>
=
\2\m2\lg]X12 + higher
order.
If we use an invariant regulator, like dimensional continuation, we will still be able to prove Ward identities. Hence, we will be able to prove that only symmetric counterterms are needed, so that the symmetry is preserved.
From the Ward identities follows Goldstone's theorem, that there is massless boson for each generator of
a
a
broken symmetry.
9.1 Unbroken symmetry a totally unbroken internal symmetry. The fields representation of the generators. Thus:
We first consider matrix
carry
S*<t>i=-i(Qij<t>j, in the notation of Section 2.6. The proof that the symmetry elementary. We in less trivial
spell
out the
steps
can so
a
(9.1.1)
be preserved under quantization is we can see what needs to be done
that
cases:
(1) Regulate in a way that preserves the symmetry. Lattice and dimensional regularization both do this since the symmetry commutes with all space-time transformations. Include in <£ all possible invariant counterterms up to the appropriate (2) dimension. Thus 8aJ? 0. For the model (9.0.1) we replace jS? by =
<£
=
Zdftdcf)
-
m2<t>i(f>
-
0B(</>W/4.
(9.1.2)
perturbation theory, let the free Lagrangian be invariant: SaJ?0 0. Then the interaction Lagrangian is also invariant. (4) At each order, choose the counterterms to cancel the divergences in 1 PI
(3)
To do
=
Green's functions. Since the free propagators and the interactions are divergences are symmetric and
all invariant under the symmetry, the
non-invariant counterterms
are not
needed.
(5) Remove the UV cut-off. The Green's functions
are
^<O|T0ji(>;1)...0J.N(yJv)|O>=O. an arrow
(9.1.3)
(9.0.1) the propagator for the charged field the direction of flow of charge. All vertices have indicating
In the case of the model
carries
symmetric:
246
Global symmetries
equal numbers of ingoing and outgoing lines. In and d(f)f i(f)\ so this equation is literally
(9.1.3) a
=
we
have
6(f>
=
—
ic/>
of charge
statement
conservation. The
current
for
a
symmetry is
defined
by
Noether's
theorem
(Section 2.6): &
In the
case
^<wz-
=
of the simple model (9.0.1) there is
/
(914) a
single
(9.1.5)
We derived the Ward identities of the bare theory
(9.0.1) these
(2.7.6). For the theory
are
^<0| Tf(x)<t>(yiy ■■<t>(yf,)4>\ziy =
current
iZ</>t^.
=
?
-0t(zw)|O>
i£ [6(x-yj)-S(x-zJfi(0\T<t>(yiy--<l>(yN)4?(ziy--<l>\zN)\0>. (9.1.6)
We showed in Section 6.6 that the current is in fact finite;
renormalization counterterms factor Z in (9.1.5). It is of interest to
see
are
needed beyond those
how the divergences that
the factor Z. For the two-point function of/ Fig. 9.1.1, up to order Z by 1 everywhere
g2.
Since Z
except
=
1 +
0(g2)
in the tree
no extra
implied by
the
are
present get cancelled by
we
have the IPI
in this theory,
we
graphs
may
graph (a). Graph (b)
of
replace
could be
logarithmically divergent by power-counting, but is in fact zero,
so no
is needed at order g. Graph (e) is also zero. Graphs (c) and (d) are finite after their subdivergences are cancelled by a counterterm; they also cancel each other. These cancellations arise since these graphs have a
counterterm
< <X (?)
(b)
<X <^K (c)
[d)
0CX<J <K (?)
(/)
(9)
Fig. 9.1.1. Graphs up to order g2 for the two-point function of y.
Spontaneously broken symmetry
9.2
subgraph
which is
for
graph
a
247
<0|T/*(x)4>tyO>)|0>. In momentum space this is of the form
that its divergence is
q*f{q2).
The Ward identity implies
zero:
^<O|T/(x)0VW|O>=i(5(x->;)<O|<5(0V)|O>=O, that
so
q2f{q2)
=
0.
Graphs (/) and (g) each have graph of the form (b), which is cancelled by using the order
9.2
which is cancelled by a Their overall divergence must be
subdivergence
zero.
Spontaneously
broken symmetry
symmetry it will be sufficient to consider the if
=
=
graph (a).
term in Z in
the renormalization of theories with
explain
To
g2
a
Zd^dty + m20V
case
spontaneously
g(ft(f))2/4 + 5m2^(f) 5g(^(f))2/4 + + (^t)2/2 (502)2/2 m2(cf>2 + 022)/2 g(<f>2 + 02)2/16 -
-
-
(9.2.1)
+ counterterms.
Here (f>
=
broken
of the Goldstone model:
have written the complex scalar field in terms of real fields: ((j)l-\- i(f>2)2 ̄1/2. The mass term is of the 'wrong sign'. This will result in we
spontaneous breaking of the symmetry under 0-x/>e"lw. The Noether current for this symmetry is / For small
=
couplings
iZ(pFl4>
=
Z((f)ld^(t)2
-
fad^).
(9.2.2)
potential
integral is dominated by in (9.2.1). This is at
\(t>\=2m/g^2.
(9.2.3)
the Euclidean functional
fields close to the minimum of the
The perturbation expansion amounts to a saddle point expansion about the minimum. It is set up by
making
cf>1 to
=
the substitution
<t>'l
+
(9.2.4)
2m/g1'2,
give <£
=
(dcf>[)2/2 + (d(f>2)2/2 ̄m2cf>'2 -g(<f>'2 + <I>22)2/16 -mgl'2<f>'l(<f>,2 + cf>2)/2 + ^ci,
(9.2.5a)
where ^?
=
-
-
-
Wt? + 4>l)2/H> Sgmg- il2(f>\(4>\2 + <t>22)/2 <P'2(3m25g/g Sm2)/2 4>22(m2Sg/g 5m2)/2 24>\mg- ll2(m2dg/g dm2) + (Z l)^;2 + d<t>22)/2. (9.2.5b) -
-
-
-
-
-
248
Global symmetries
making this perturbation expansion is that in the functional integral we impose a boundary condition that fixes the phase of the field at oo. By the symmetry we may make this phase real, without loss of In or three more generality. space-time dimensions, fields that have a different phase over a large region have an action so much larger that The idea of
quantum fluctuations cannot destroy the boundary condition. Then forced to have a real vacuum expectation value close to 2m/gl/2. In
setting
perturbation expansion we have tadpole generate a vacuum expectation value for <f>\
up the
Fig. 9.2.1. These
<o|0;|o>
=
<j)1
is
graphs like
^
expectation value 2mg ̄l/2 tadpoles graphs appear as subgraphs. It is possible to recast the Feynman rules by writing 0, 0J + 2mg"1/2 + 8v and requiring fy\ to have zero vacuum expectation value. A better practical approach is to impose dv 0 as a renormalization that starts at order
gl/2.
It
means
+ dv. There are then
that
(pl
has
vacuum
like Fig. 9.2.2, where the
=
=
condition
dm2.
on
Fig. 9.2.1. Graphs for <O|0i|O>.
Fig. 9.2.2. Graphs containing tadpoles If
we
start
as
subgraphs.
with the theory (9.0.1) without spontaneous symmetry
breaking and vary m2 until it is negative, then we should pass through a phase transition and thereby reach the Goldstone model (9.2.1). There must be an actual phase transition because <O|0|O> is exactly zero in the phase with unbroken symmetry. Since this expectation value is non-zero in the Goldstone phase there must be non-analyticity of the theory as a function ofm2. Now,
we
normalized functions of
must
renormalize the theory: the continuation in the
m2 is sensible only if the m2 and g in the two phases. It is sensible to use
mass
re-
counterterms are the same a
mass-
independent prescription, for then the dependence on m2 of the counterterms is the simplest possible. We will prove the following: renormalization
Spontaneously broken symmetry
9.2
249
phase is accomplished by using only Lagrangian (9.2.1).
(1) Renormalization of the Goldstone
symmetric
(2)
counterterms in the
The dimensionless counterterms Z to be the same
in the
as
phase
—
1, Sg, and
Sm2/m2 can
of unbroken symmetry
be chosen
(the so-called
Wigner phase). (3) The current given by (9.2.2) is finite just
as it is in the Wigner phase. Since the bare Lagrangian is invariant under 0->0e"1<o, Ward identities are valid and from them Goldstone's theorem follows, that the
physical
mass
of (j>2 is exactly
zero.
We must also discuss the choice of a
practical renormalization prescription.
9.2.1 Proof of invariance of counterterms We will do
perturbation theory by choosing JS?0
=
the free
Lagrangian
m2(f>[2,
(9.2.6)
mg^VM? + *D/2.
(9-27)
d(f>[2/2 + d<f>22/2
-
and the basic interaction
JS?b
=
-
9(<t>? + 0D/16
-
The counterterms are given the form (9.2.5b) and Sg, Sm2/m2, and Z are given the same values as in the unbroken theory with a mass-independent renormalization scheme. We will prove that these counterterms are
sufficient to make the broken-symmetry theory finite. Some of the interaction vertices are the same as in the Wigner phase. The others are obtained by substituting 2m/gl/2 for <j)v Therefore graphs
involving the extra vertices are obtained by erasing external (f>1 lines on symmetric graphs. Examples are shown in Fig. 9.2.3. The only complication is that mass terms generated from the basic interaction go into the free rather than the interaction Lagrangian. This is the sole source of complications
in
our
proof. + other terms
+ other terms
1
1
Fig. 9.2.3. Generation of graphs in theory with spontaneously broken symmetry from graphs in the symmetric theory.
250
Global symmetries
theory let
To relate the counterterms to those in the unbroken
the free propagators
^
as
i
,
i
-
2m2
p2
-
M2
+
a
-
-
-iM2
^''p2' p2 -M2+ (p1 Here M2 is
-
-
i
j__
write
i(2m2 M2)2 i(2m2 M2) + (p2 M2)2 (p2 M2)2(p2 2m2)' -
_  ̄
>2
us
follows:
iM4
-M2)1* (p2 -M2)^1'
arbitrary parameter.
(9.2.8)
We substitute (9.2.8) for every line in
a
graph. Suppose we substitute the first term on the right of (9.2.8) for every line of a basic graph which has only four-point basic vertices. Then we obtain a graph in the symmetric theory with mass M. The difference between these symmetric graphs and the true theory is given by: (1) graphs with one or more three-point vertices, (2) graphs with the second or third term on the right of (9.2.8) substituted for one or more propagators. In either
the degree of divergence is reduced. Now the maximum divergence is two. So substitution of the third term in (9.2.8)
case
degree of
always makes a graph overall convergent. We substitution of the second term.
Let
us
counterterm
Zm
one
graphs with fewer than N loops are symmetric counterterms. We will prove graphs are renormalized. The induction starts
in ¥£ct
renormalization. We decompose the
^'2[_3m2Sg/g + (Zm 1)( h<t>l\jn25glg + (Zm 1)( -
=
mass
as
-
Here
allowed at most
suppose that all renormalized by our
now
successfully inductively that all AMoop because tree graphs need no -
are
-
(m2 + Sm2)/m2
is the
-
-
mass
M2) + (Zm 1)M2] m2 M2) + (Zm 1)M2].
m2
-
-
-
-
(9.2.9)
renormalization factor.
After substitution of (9.2.8) for each propagator in a basic 1PI graph with AMoop all subdivergences are cancelled by counterterms of lower order,
according to the inductive hypothesis.
We
are
left with the following overall
divergences: (1)
Logarithmically divergent graphs
for the
four-point
function with all
M2) and with only four-point vertices. Such propagators set to i/(p2 graphs have an overall divergence independent of M which is removed -
by counterterms in 8g for the symmetric theory. No other 1PI graph for the four-point function has an overall divergence.
Spontaneously broken symmetry
9.2
(2) Self-energy graphs with four-point vertices only
i/(p2
-
M2).
Field renormalization and the
renormalize these, again exactly as in (3) Self-energy graphs as in (2) but with
the
and with propagators
(Zm 1)M2 terms symmetric theory.
one
—
2m2
-M2
M2 =
The terms
(
m2
3m2
in
(9.2.9)
propagator replaced by
second term in (9.2.8). In the numerators of (9.2.8) -
251
we
a
write
(m2 + M2) (2m/gl/2)2{3g/4) (m2 + M2\ m2- (m2 + M2) (2m/gi/2)2{g/4) (m2 + M2). (9.2.10) =
=
-
-
=
with
M2) parts
—(m2
M2)
+
of the
-
are
renormalized
by the
(Zm—1)
They correspond to the of the effect differentiating self-energy graphs with respect to M2. The other terms in (9.2.10) we will regard as an insertion of a four-point vertex on a line when two <j>l fields are replaced by 2m/g1/2. These terms are considered under (4). (4) Graphs of classes (1) and (2) in which one or more external (f>1 fields are deleted and replaced by 2m/gl/2. Examples are Fig. 9.2.3(b) and Fig. 9.2.4. The same replacement generates the counterterm Lagrangian (9.2.5b) from the symmetric theory, so we have counter- —
—
mass counterterms.
terms for them. This
Fig.
completes
the
proof.
9.2.4. Generation of graphs with loops in theory with spontaneously broken symmetry from graphs in the symmetric theory.
9.2.2 Renormalization The same
procedure shows /
=
of the
current
that the current
Z((j),1d^2
-
4>2d^\) + 2Zmg ̄l/2dtl(t)2
2 looP
2
Fig. 9.2.5. Renormalization of
(9.2.11)
current in
spontaneously broken theory.
252
Global symmetries
has finite Green's functions. Note that the term 2Zmg ̄ graphs like Fig. 9.2.5.
l'2dfi(j)2 contains the
counterterms that renormalize The Ward identities
are
then true and involve finite quantities. A typical
is
case
^<O|T/(x)02(y)|O>=i<O|<502(y)|O> -i<O|01(y)|O> =
=
-i(2m/gl/2
+
dv).
(9.2.12)
By multiplying by the inverse propagator for 02 and going
to momentum
space, we find
P^UP) where
r£2
is the set of
is illustrated in
has
a zero at
Fig.
p2
=
=
graphs
for
9.2.6. Since F1
<O|T/02|O> that are 1PI in 02. This p" as p2 0, (9.2.13) implies that G22l
oc
->
0, in other words that 02 is massless
perturbation theory.
(9.2.13)
(2m/9l/2 + Sv)(i/G22(p2)),
to all orders of
This is the Goldstone theorem (Goldstone, Salam &
Weinberg (1962)).
Fig. 9.2.6. The Ward identity that implies Goldstone's theorem.
9.2.3 Infra-red divergences
self-energy insertion on a </>2 line have infra-red graph is illustrated in Fig. 9.2.7, and the divergence region where the momentum k on the 02 line is close to
Individual graphs with
divergences. Such comes
from the
a
a
zero:
If uncancelled, this to a
divergence indicates
value other than
energy is
zero
at
zero.
Jc
=
that the self-energy shifts the
But the Goldstone theorem tells
0. So the infra-red
divergences in other graphs of the
same
us
mass
that the self-
divergence cancels against
order.
Fig. 9.2.7. Graph with infra-red divergence.
9.3 Renormalization methods
253
9.3 Renormalization methods One of the practical problems that arises in making calculations in a theory with spontaneous symmetry breaking is to find the most convenient renormalization prescription. Fundamentally, there is no problem, for all renormalization prescriptions are related by renormalization-group transformations, and are therefore equally good. But, in practice, choice of one prescription over another can save some labor. The problems become particularly acute in gauge theories of weak interactions (Beg & Sirlin (1982)). Among the issues to be considered in choosing a renormalization prescription are:
(1) If we ignore higher-order corrections, then some parameters are equal to quantities, like particle masses, that are easily measurable. It is often convenient to impose exact equality as a renormalization condition. (2) One must treat tadpole graphs. Their effect is to provide an additional shift Sv in the vacuum expectation value of the field. Leaving these
graphs
as
they
contributing to
considerably increases the given Green's function. Shifting
are a
number of graphs the field
many extra terms in the formulae for the coefficients in
(9.2.5b). One
can
impose
Sv
=
0
by Sv gives (9.2.5a) and
renormalization condition, at the connection between the phases of
as a
expense of removing the simple broken and unbroken symmetry (as
was exploited in Section 9.2). to is relate It calculations done by different people. Direct necessary (3) can if be made the same renormalizations are used. It only comparisons to a is evidently useful standard. agree on (4) If the coupling is not very small or if there occur very large ratios of masses
and momenta, then
prescription
with the
ability
one
must
choose
to remove the
a
renormalization
large logarithms.
One approach is to use dimensional regularization with minimal Graphs can be renormalized by the forest formula. At one-loop order this amounts to subtraction of the pole part from each 1PI graph. We can do this without regard to the symmetry relations between counterterms for different Green's functions. Since the counterterms have the pure-pole form, these relations are automatically satisfied. subtraction.
approach is to compute Z, Sg, and Sm2 by three renormalization conditions imposed on some of the 1PI Green's functions in the broken- symmetry phase. Then the values of counterterms for other Green's functions are c6mputed from (9.2.5b). It is convenient to determine m2Sg/g -Sm2 by requiring Sv 0, i.e., <O|0j|O> 2mg ̄112 exactly. Then Another
=
=
254
Global symmetries 1
2
or 2
2
2
X
+
2
2
2
2
1
+ 2
2
2
2
Fig. 9.3.1. Renormalization of self-energy
the
1 or 2
/-^
at
one-loop order.
for
02 forces the IPI self-energy of 02 to be exactly one-loop graphs are shown in Fig. 9.3.1. Both of these approaches require explicit computation of the values of Z, mass counterterm
zero
when k2
=
0. The
8g, and dm2 to find the values of counterterms for the various Green's functions. It is also possible (Symanzik (1970a)) to use the Ward identities to generate renormalization conditions for all divergent IPI Green's functions from the three basic conditions. These conditions are simple if the three basic conditions
9.3.1
Generation
are
imposed
at zero external momentum.
of renormalization conditions by Ward identities
The general Ward identity is
=
-i
n ^-^xoi^^wn^^io). i
J=l
(9.3.i)
+j
the Goldstone model, the labels nt take the values 1' or 2, and we have 0X (</>[ + 2m/g1/2). For simplicity we impose (50i 02 and 8(f>2
In
=
=
the condition
-
<O|0i|O>
=
conditions valid up to /
=
-
0.
Suppose we have obtained renormalization loops. We will now find the appropriate
-
1
conditions for /-loop graphs. 1 was given in (9.2.12)and (9.2.13), andin Fig. The case JV =
9.2.6. Wesaw
self-energy of 02 is that it is zero 0. Another condition on the derivative can be chosen arbitrarily, at p2 corresponding to the freedom to multiply Z by a finite factor. Suppose we choose to make the residue of the Goldstone pole equal to unity, and we
that
one
renormalization condition
on
the
=
choose to make dv on
=
condition is that it is is
0. Then
the Green's function
equivalent
to
we
also obtain the renormalization condition
of the current with 02. The 0. This condition to its lowest-order value at p
<O|T/(x)02(^)|O>
equal making the
=
counterterm
equal
2m0-1/2(Z-l)d"02,
to
9.3 Renormalization methods as
required if/ is to be the Noether Similarly we may treat the case N
255
current. =
2:
d,<0\Tr(x)4>[(y)cf>2(z)\0y -id(x-yK0\Tct>2(y)(f)2(z)\0> + i<5(x-z)<O|T0;(>;)0i(z)|O>, (9.3.2) where we used <O|0i |0> 0. In terms of IPI graphs in momentum space =
=
this gives Fig. 9.3.2. After
use
of
Fig.
9.2.6
.2
find
we
Fig.
9.3.3.
+ i
^ Fig.
two-point
9.3.2. Ward identity for
P
Sj
n
2m =1
gm2
2
-<T^
+ i
now set p + q
0
=
=
2
2
Fig. 9.3.3. Result of multiplying Fig. 9.3.2 by We
function of/.
propagators.
0 to eliminate the left-hand side. This
^ir212(0,p,-p)-[p2-2m2-i:1(p2)]
+
y
=
q
two inverse
%ir212(0,p,-p)
+ 2m2
-
^(p2)
-
gives
[p2-Z2(p2)]
Z2(p2).
(9.3.3)
Zx and Z2 are self-energies and T212 is the IPI Green's function for 02 fields and one ¥[. We choose a mass renormalization condition for 0. Since we already know that I2(0) I x, say I x (0) 0, this tells us that the renormalization condition on T212 is Here
two
=
=
r212(0,0,0) Since graphs for T212 r212(p,4,r) completely
=
-
img-1/2
=
lowest-order value.
(9.3.4)
logarithmically divergent we know at this order. From (9.3.3) we can now determine Zj(p2). But the calculation of Zx(p2) from its graphs is already fixed except for
a
are at worst
renormalization condition that determines the value of the field-
256
Global symmetries
strength
counterterm. So
that the counterterm is
(9.3.3) gives (Z l)p2.
—
Similar arguments may be
all the remaining are
us
the renormahzation in such
a way
-
applied to give renormahzation conditions for divergences. They
1 PI Green's functions that have overall
easiest to express in terms of Ward identities for 1PI Green's functions.
(See
Lee (1976), and references
therein.)
The structure of these arguments generalizes what unbroken phase. For example, (9.3.2) integrated over
would be done in the
would give Zx Z2 in this phase. This condition would say that the counterterms for Zx and Z2 are equal. But in the Goldstone phase this is not so. Integrating over x is
equivalent
to
setting the
x
=
momentum at the vertex for the current to zero.
gives a factor of this momentum, but since there is a pole l/p2 the right-hand side of (9.3.2) is not zero. The argument that we had to use is more complicated. The derivative with
respect
to x
10
Operator-product expansion In this
chapter
will
we
(f>4 theory in d
investigate
two
closely
related
problems.
We work
time-ordered product of two fields, T0(x)0(O), together with its Fourier transform with
=
4 dimensions and consider
70(4)0(0)=
a
d4xe^*70(x)0(O).
(It is easiest
to work with time-ordered products. The methods work with of any pair operators TA(x) B(0) in any theory.) The first problem is to ask how T0(x)0(O) behaves as xM -> 0. If the theory
totally finite
were
then the result would
just
be
02(O). However, there
are
ultra-violet divergences that prevent the product from existing, so the limit does not exist. It was the idea of Wilson (1969) that 0(x)0(O) should behave like The
singular function of x times the renormalized [02] operator, as full result is that we have an expansion of the form
a
T0(x)0(O) as x -?
0. Here the sum is
C<p(x)'s [0] generalizations, is called and the
coefficients C6
are
are
smaller by a power The second problem
\q2\-+
More
X C,(x*)[0(O)]
0.
(10.0.1)
over a set of local renormalized composite fields onumber functions. This formula, or one of its an
operator product expansion (OPE), and the
often called Wilson coefficients. Corrections to (10.0.1)
are
oc.
 ̄
x -?
of x2 than the terms we
precisely
given.
wish to treat is the behavior of
we
T0(g)0(O)
as
will consider the momentum-space Green's
function
Sw+2
=
<O|T0te)#>)0(p1)...0(pw)|O>,
(10.0.2)
when q11 along a fixed direction with pl9...,pN fixed. In other words we scale the invariants q2^>K2q2, Pt'q^Kp^q. There is an operator product -?
oo
expansion
6N 2-EC^)<0l^(°)^i)---^iv)|0>. +
(10.0.3)
6
The relation between the
coordinate-space 257
and momentum-space expan-
258
Operator-product expansion
sions is elementary. Let
us
take the Fourier transform from
x to
q of a
Then the large-g behavior is dominated by the singularities in x-space. The only relevant 0. So C^q) is the large-4 part of the Fourier transform of singularity is at x if Q,(x). Conversely one Fourier transforms GN + 2 to get
momentum-space Green's function of
T(f)(x)(f)(0).
=
/?
<O|T0(x)0(O)^(pJ...^(pw)|O>
=
14
J^e-i?-*ejv
+ 2,
(10.0.4)
then the limit x -> 0 fails to exist if GN + 2 falls only as 1/q* or slower as q ->
00.
Thus knowing the large-g behavior is equivalent to knowing the singular small-x behavior, but the coordinate-space expansion also includes the leading non-singular part of the small-x region. expansions have a number of uses, particularly in an
information
These
on
asymptotically free theory. There the perturbation theory when improved by the renormalization group gives an effective method of computing the Wilson coefficients. Among the
uses are
the following:
(1) The expansion (10.0.1) in coordinate space provides a definition of renormalized composite operators that does not involve any reg- ularization (Brandt (1967)). (2) Although there is no physically important process which directly uses the limit taken in the momentum-space expansion (10.0.3), it is used
indirectly for deep-inelastic scattering of involves a matrix element of the form
a
lepton
on a
hadron. This
<p|M/(0)|p>. Here
q2
-?
-
00, but with the ratio
A dispersion relation relates this OPE is used indirectly,
as we
q2/q-p fixed instead
case to
will
see
of q2/qp2 fixed.
the limit used in
in
Chapter
(10.0.3), so
the
14.
(3) The form and the method of proof of short-distance operator-product expansion can be generalized to handle many interesting high-energy
scattering processes. (See Buras (1981) and Mueller (1981) for a review.) The results in the present chapter form a prototype for these other
results.
10.1 Examples 10.1.1 Cases with We will
no
divergences
mainly restrict our attention to Green's functions of </>(x)</>(0) in 0(x) and (f)(0) are connected to other external lines. This is the
which both
10.1
x
Ox
0
/\.P2
Pi
259
Examples
Pi
yC
>Pi/\p2
p2
(fl)
(b)
Fig. 10.1.1. Lowest-order graphs for operator-product expansion of 0(x)0(O). case
that is relevant to most &
First consider
d(f>2l2
=
-
applications.
m2(f)2/2
-
Our
#4/24
graphs Fig. 10.1.1(a)
the tree
theory
will be
04 theory (10.1.1)
+ counterterms.
for
<O|T0(x)0(O)^(Pl)0(p2)|O>.
(10.1.2)
These give p2
Expansion
in
_
a
m2
2
_
m2
CeXP(
power series about
l
 ̄
{Pl'X) + eXP(
x
=
0
-
(10.1.3)
iP2"^)]-
gives
—i[2-i(p1+p2)-x-(prx2 + p2-x2)/2+--]-
p2-m2p2-m2 This is equivalent
-
to the
T(f>(x)(f>(0)
=
-
"-
"
^
(10.1.4)
replacement
(f>2(0) + 12xfld,<t>2
+
:b^xv(K3v0 + *"'
(10-L5)
Fig. 10.1.1(b). This equation has the form of the operator- product expansion (10.0.1). Thus the operator-product expansion in this case (free-field theory) is 0. The power of x in each term is really a Taylor expansion of 0(x) about x as
illustrated in
=
just such that
Ce(x)
=
no
dimensional coefficients
constant x
are
needed:
|x|a, with a ==dim(0)-dim[0(x)0(O)].
(10.1.6)
This result also correctly gives the power-law behavior in the presence of renormalizable interactions, as we will see. But there will also be
logarithmic
corrections.
A feature which does not appear to survive inclusion of interactions is on the right of (10.1.4) is convergent and sums to give
that the series
T0(x)0(O). Consider next the graphs of Fig. 10.1.2 for the four-point function of T<f>(x)<f>(0). The important factor comes from the lines carrying momentum
fd^
{exP(-i,-x) + exp[i(g-p,-Pi,)-x]} m2)HpA-q)2-m2X(q-pA-PB)2-m2J
260
Operator-product expansion
¥Pa
Pb
Fig. 10.1.2. Higher-order graphs for operator-product expansion of </>(x)</>(0). We
now
expand the integrand
in powers of
'4
(10.1.7)'
)A (q2
-
m2)[(pA
-
x to
obtain
2-i(pA + pB)-x q)2 m2] [(a -pA- pB)2  ̄
-
2T + -".
m2]
(10.1.8) These first two terms
higher
are
just those we would
expect from (10.1.5). But the
terms have at least two extra powers of q in the numerator and are
divergent. The divergences are those of the Green's function of the composite operators. They indicate that modification of the higher terms of the expansion is needed. For example, the behavior of the coefficient of c/>dMdv0 is modified by a logarithm of x. therefore ultra-violet
Similar modifications will be needed for the coefficient of
<f>2,
when
we
consider
higher-order corrections. Therefore we will find it convenient just to restrict our attention to the leading-power behavior, corresponding to the (f)2 term in (10.1.5). 10.1.2 Divergent example
Aside from trivial propagator corrections the contribution of order g to the two-point function of 7c/>(x)0(O) is given by Fig. 10.1.3(a), which gives
I (pl-m When
.
fd^_
l)iP22-m2)g)(2n)\q2- m2)[(q-pl ̄p2)2-^2]
(10.1.9)
integral diverges logarithmically. This is a symptom of the fact that there are two important regions of q that contribute. The first is x -?
0 the
X/ v0 -uv
+ UV
(c) Fig. 10.1.3. Gr,aph for operator-product expansion of 0(x)0(O) with divergences for the operator.
10 A
where
q is finite as
T0(x)0(O) by
261
Examples
x-?0; the contribution is correctly given by replacing
02(O)
in the
large, up to 0(l/x)
as
graph.
The second region is where q becomes
x->0; in this region the interaction
vertex in
coordinate space is close to x and 0. In the second
space. From the
region the loop is confined to a small region in coordinate point of view of px and p2 the loop is a point. So we should
be able to represent the contribution of this region by Wilson coefficient of 02:
an extra term
70(x)0(O)^Q2(x)[02], C+2 Let
us now
calculate
(10.1.10)
i
l+(g/16n2)cdx2)1 ,
=
in the
c2(x2).
Now the contribution of the first region is given by replacing T</>(x)</>(0) by c/>2(0). However, this operator has an ultra-violet divergence. So let us
add and subtract the renormalized Green's function of
[02], i.e.,
<O|T[02(O)]^(p1)^(p2)|O>,
(10.1.11)
give the equation depicted by Fig. 10.1.3. The contribution of order 1 region is entirely contained in the term (b) representing (10.1.11): to
from the first
(p2-m2)(p2-m2)16^2|J0dXlnL
J T
4V
(10.1.12) Here
we
have used minimal subtraction. The remainder, term (c), is 1
-ig
{p\-m2)(p22-m2){2nf
fd4?rr- to2
J
-'
x
)l(q-Pl-p2)2-m^
UVdivergencei.
"—
*—J
(10.1.13)
When x->0 the contribution from finite q is of order |x|. But there is a contribution of order 1 from large q: this is the contribution to the original graph minus whatever is taken care of by graph (b). the x->0 behavior of the curly bracket factor of (10.1.13) (aside from a normalization factor), since to We
can
identify cx(x2)
in
(10.1.10)
as
lowest order
<0|7>2#(pt)#(p2)|0>=- (P?-m2)(p22-m2)' Now the
leading-power behavior of the curly-bracket factor is independent yn. This is easily seen by differentiating with respect to any of
°f PmPi and
262
Operator-product expansion
these variables. The result is power of x when x
-?
0. So
a
we
convergent integral which goes may define
cx(x) by setting m
to zero like a
=
c^-M^h^+M
pl
=
p2
=
0:
,iou4)
The integral is easily done by using
-j;pdzze-2<-?2)
l/(q2)2
=
form, with
to turn it into a Gaussian
*i(*)
=
the result
i[y + ln(
 ̄
*
V*2)].
(10.1.15)
10.1.3 Momentum space We
now
Fourier transform Fig. 10.1.3 to obtain the 0(g) contribution to
<o|r^(?)^(0)^(p1)^(p2)|o>. As
q2
->■ oo
we
find
Fig. 10.1.3(a) This
gives a (10.0.3) with
 ̄
—2
=--!, + 0[l/(o2)3]. (10.1.16)
j—
(p2-m2)(/>22-'?W)2
contribution to the term in the 0
=
02.
operator-product expansion
The coefficient is
C+*(<l2)
=
^
+
(10.1.17)
0(g2l
which is just the Fourier transform of the order g term in the coordinate- space expansion, ^c1(x)/(167i2). The g° term in the coordinate-space
coefficient is independent and hence
nothing
at
of x,
so
that it
gives
a
d(A\q) in momentum space,
large q2.
10.1.4 Fig.
10.1.3 inside bigger
graph
(10.0.3) indicates that the same asymptotic oo) is obtained independently of the Green's function considered. This happens because graphs like Fig. 10.1.3 can occur The
expansion (10.0.1)
behavior
as
x->0 (or
or
as q-+
167T
Fig. 10.1.4. Even higher-order graph for operator-product expansion of 0(x)0(O).
10.2 Strategy
subgraphs Fig. 10.1.4.
as
of
graphs
with
more
10.2
First
we
will make
expansion applies.
If
precise we are
263
ofproof
external lines. An
Strategy
of
is given in
example
proof
the limits in which the operator-product in Euclidean space (i.e., with imaginary time
and energy) then there is really only
one
way in which
we can
take xM to
zero
infinity. However, in Minkowski space we can let x2 0 without each component going to zero, and we can let components of q*1 go to infinity without g2->oo. These cases are interesting physically. For example, the q^^co limit with finite q2 is the case of high-energy scattering. Much is known about these limits, but they are beyond the scope of this cf
or
to
-?
book. We will prove the coordinate-space expansion (10.0.1) in the case that all components of xM go to zero with their ratios fixed. The corresponding
momentum-space expansion (10.0.3)
we will prove in the limit that all components of q* go to infinity with a fixed ratio, and with q* not light-like, so that q2 -?oo. These limits are essentially Euclidean.
Our proof will be
in
perturbation theory.
The first step is to
regions of loop-momentum space that give the
the
x -?
0
or q -?
oo
limits. Then
section, which applied
to
we
leading-power
identify
the
behavior in
generalize the arguments of the previous
specific graphs.
The region of large q which we investigate in the momentum-space expansion (10.0.3) is precisely the one to which Weinberg's (1960) theorem
applies. The theorem tells us to consider all subgraphs connected to the (f>(q) and c/>(0) in (10.0.2). For each such subgraph we let all its loop momentum be of order q, and count powers just as we did for UV divergences. The subgraph(s) with the largest power of q2 correspond to the dominant regions of momentum space. Then as ^2->oo, the complete graph is proportional to this power of q2 times possible logarithms of q2. smaller by a power of q2. Although Weinberg's result also the highest power of ln(^2) that appears, it is easier to determine this
Corrections tells
us
are
by first constructing the operator-product expansion and then applying renormalization-group methods (as in Section 10.5 below) to the coefficients. Essentially the same method can be applied to obtain the short-distance behavior, (10.0.1). For example, in Fig. 10.1.4 we have leading contributions with q finite or with q large (of order 1/x), but always with the lower loop momentum, /c, finite. The leading power is (x2)0. The logarithm of x in
264
Operator-product expansion
(10.1.15)
for the corresponding Wilson coefficient
over momenta
Suppose
we
comes
from integrating
intermediate between these two regions. have large momentum confined to a subgraph T. Then the
(for the momentum-space expansion) is exactly the dimension of our since T, theory is renormalizable, with dimensionless couplings. The leading power of q2 comes from subgraphs with the largest dimension, i.e., power of q
subgraphs with the smallest possible number of external lines. This number is two (beyond 4>(q) and (f)(0)), so that the leading power is l/(q2)2. The subgraphs have the form of the subgraph U in Fig. 10.2.1. In the ultra-violet
subgraph U, all lines carry momentum of order q. This subgraph is 1 PI in its lower two lines. All momenta in the infra-red subgraph / are finite.
Fig. 10.2.1. General
structure of lead-
Fig. 10.2.2. Factorization
of Fig. 10.2.1.
U is
independent
ing regions of momentum space for Appoint function of </>(x)</>(0). Now, to the
leading power of q2, the
ultra-violet
graph
of the external momenta k and / flowing into it. Thus we may replace U by 0 (and we may set the mass m l its value when k 0). We may also the infra-red subgraph / by an insertion of a vertex for c/>2/2 in an =
replace Appoint Green's structure as
=
=
function. This is illustrated in Fig. 10.2.2. This has the same operator-product expansion. But it should be emphasized
the
supposing that loop momenta are restricted to certain regions. These regions are not defined very precisely, and it is one of the tasks of the proof to remedy the impreciseness. Schematically we have that
we are
X Gn 2(4>Pi>' ̄>Pn) +
graphs
£!/(?,*
=
0, J
=
d*kd*ld(k +1
0)£ /
ZT,
-
£p,)/(Upi,...,Pn)- (10.2.1)
generalize the technique that we applied to Fig. graph U that could appear in Fig. 10.2.1, but not its momenta. restrict It can occur as a subgraph of some graph for we do the complete Green's function. If all momenta in U are of order q and if all To construct the
expansion
we
10.1.3. We consider each
265
10.2 Strategy of proof
large
momenta
small momenta
Fig. 10.2.3. A possible leading region for momenta outside are
a
subgraph of the form of
U in Fig. 10.2.1.
small, then we get a leading contribution to the cross-
leading contribution where the large momenta proper subgraph of t/-as in Fig. 10.2.3. Suppose we subtract off all of these contributions. Then we integrate over all loop section. We can also have a occur
inside
a
momenta of U and find that the result only gives a leading contribution when all its momenta are large. We therefore define the contribution of U to the Wilson coefficient
C(U) all evaluated
=
as
U- subtractions for
at k
=
/
=
m
=
regions of form Fig. 10.2.3 (10.2.2)
0.
resulting formula for C(U) is very similar to that of the formula for renormalizing the ultra-violet divergences of a graph. In fact, as Zimmermann (1970, 1973b) explains, a good way to prove the operator- product expansion is to treat it exactly as a problem in renormalization. His method, used in the next section, is not to compute directly the Wilson coefficients but to define first a quantity which is a Green's function minus The
the
leading
terms in its
operator-product expansion: C
This is constructed
as a sum over
graphs
T for
GN + 2-
Each graph has
subtracted from it not only counterterms to remove ultra-violet divergences but also counterterms to cancel the large-Q (or small-x) behavior. The result we
call
Now,
RW(T). the subtractions that
oversubtraction. So
is
remove the
large-Q behavior
of
are a sort
related to R(T) in the
style
of
simply RW{T) renormalization-group transformation. This transformation can then written as the Wilson expansion, as we will see. A disadvantage of Zimmermann's proof is that it uses BPHZ
a
be
re-
normalization. He takes advantage of certain short-cuts available through the use of zero-momentum subtractions. We will choose not to take these be
applied to ultra-violet divergences. Our method of proof will be essentially the same as the one used for problems with large masses (Chapter 8). The same techniques apply to the coordinate-space expansion. Here, the shortcuts
so
that minimal subtraction
can
266
Operator-product expansion
integrated over. When we do power-counting, large momenta are regarded as of order 1/x and small momenta as finite when x -> 0. The leading behavior again comes from graphs of the form of U in Fig. 10.2.1, and the power is (x2)0. There is a difference in the form of the possible graphs U that carry large momentum. All the graphs that we use in
momentum q is
the momentum-space case are also used in coordinate space. But we can also have the graph consisting of the vertices for 0(x) and (f)(0) and of
nothing else. 10.3 Proof We must
now
prove the operator
product expansion.
In
04 theory
we
consider the part of the Green's function, Gw + 2(x,p1,...,piv)
=
<O|T0(x)0(O)^(p1)--^(PiV)|O>.
in which each of 0(x) and (f)(0) is connected lines. We will now scale x
(10.3.1)
to some of the other external
by a factor k and construct a decomposition of the
form:
Gn 2{kx,Pi,.-',Pn) +
=
C(icV)<O|^[02](O)^(pJ--^(pw)|O> + rN
In every order of
perturbation theory,
the coefficient
(k2)0 times logarithms, when the scaling parameter remainder goes to zero like a power of k. Fourier transformation
on x
+
k
2(icx,p1,...,pAr). (10.3.2)
C(k2x2)
behaves like
goes to zero, while the
gives the result
GN 2(<i/K>Pi>-- >Pn) +
=
CtaVic2)<O|T¥[02^ (10.3.3)
4/jc-xx>, C(q2JK2) behaves like K*/q* times logarithms, while ^ + 2(^Piv) is smaller by a power. Our proof is given for a specific Green's function in a specific theory. However, it can easily be generalized. Features specific to a gauge theory will be pointed out in Chapter 12. The particular case of QCD with the application to deep-inelastic scattering will be treated in Chapter 14. When
10.3.1 Construction of remainder We consider the set of graphs
for GN
+ 2.
graph T, we will construct Each graph V we consider to be
For each
its contribution r(T) to the remainder rN +
2.
103 an
Proof
267
unrenormalized (but regulated) graph containing only basic interaction are derived from
vertices. These
^basic
=
o4>2/2
-
m2cf>2/2
-
/i4-d#4/24,
where g and m are the renormalized coupling and mass. As usual the renormalized value of the graph is R(Y), which is T, plus
a
series of counterterm graphs to cancel its ultra-violet divergences. For our proof we will use a renormalization prescription in which the theory is finite when the renormalized
mass m is set to zero. The renormalized value R(Y) is then the contribution of Y to the Green's function GN + 2. The remainder term r(Y) is equal to Y plus a somewhat different series of
counterterm graphs. These counterterms will be constructed so that they cancel not only the ultra-violet divergences but also the leading x->0 or 4
-?
oo
behavior of Y.
Now r(Y) is in effect a oversubtracted form of Y. The oversubtractions are of the form of an insertion of the operator [02] times a coefficient. Thus
r{Y) is the Wilson expansion, i.e., the first term on the right of (10.3.2) (10.3.3). The coefficient C(x2) (or C(q2)) depends on the coupling g and on the renormalization mass \i. It must be independent of all the momenta. In R(Y)
—
or
order to be able to neglect m in the ultra-violet limit x -> 0 or q -> oo, we must use a renormalization prescription in which the counterterms do not become infinite when use
m
->0
(with fixed regulator).
For concreteness
we
will
minimal subtraction in what follows.
In order to define r(Y\ let us recall the definition of the ordinary renormalization R(Y). This starts from the fact that the divergences of Y come from regions of loop momenta where all lines in some set of IPI subgraphs carry
a momentum
subgraph consisting
approaches infinity. We label each region by the all the lines carrying large momentum. Then
that of
R(H
=
r-j;qr).
(10.3.4)
y
is
subgraphs subgraph y replaced by its Cy(Y) to be non-zero only if y is The
sum
y!,...,7?.
over
all
y of T, and
Cy(Y)
is essentially Y with the
large-momentum divergence. We define a disjoint union of one or more IPI graphs
In that case each y. is
replaced by
a counterterm vertex
C{y^,
which is the divergent part of y.. To avoid double-counting, the sub- divergences are subtracted off first:
C(yt)
=
?
y,-
I
Ca(yA
(10.3.5)
268
Operator-product expansion
Here \^' denotes
'pole part
at d
=
4' and the sum is over all proper
subgraphs 8 of y-. As usual, if we use some other renormalization prescription than minimal subtraction the operator 0> is replaced by the operator appropriate to the renormalization prescription. The definition of the remainder r(T) is almost identical to the definition of R(T). Now, the leading short-distance (i.e., x -> 0) behavior of T comes from the following
regions:
(1)
where the momentum q is finite, (2) where q gets large and the momenta in Fig. 10.2.1 also get large.
Further leading contributions momenta get infinite in
contributions
come
some set
to
correspond
momentum-space expansion the region of finite q.
of
a
graph of
the form of U in
from regions where in addition divergent IPI graphs. These extra
the
ultra-violet
divergences.
In
the
(10.3.3) the same regions are leading except for
We define r(V) to be r with all ultra-violet divergences subtracted and then with all the leading small-x behavior subtracted: r(r)
=
R(r)-ZlLyUn d
Here the
graphs of the form of U in Fig. 10.2.1 and the subgraphs y of T that do not intersect 8. We use Lyu5 to subtraction operation defined below. It is used to extract the
sum over
sum over y
is
symbolize
a
(10.3.6)
y
over
8 is
over
all
all
comes from the region where the momenta in graph 8 are of order 1/x and the momenta in y go to infinity. The case of finite q in Fig. 10.2.1 is covered by the case that 8 consists of the vertices for 0(x) and
contribution that
0(0) only.
L^ud(T) to be zero unless y is a disjoint union of replaced by its overall counterterm C(yt) defined in (10.3.5) while the graph 8 is replaced by a quantity L(8). L(8) is to contain the leading behavior of 8 when all internal lines have large momenta. This is the same idea as that C(yt) is the overall divergence of y-. Now there are regions where a subgraph 8' of 8 carries momenta of order q and other lines in 8 carry small momenta. To avoid double-counting we We define the subtraction
IPI yl9..., yn. In that
case
subtract them first. So L(8)
each yt is
we
=
write: T
I
X
<5'<£ 5 ynS
Here T is to be
an
operator that picks
argument. Now this behavior is
=
0
out the
independent
(10.3.7)
L7U6.Q) leading
x ->
0 behavior of its
of m, and of the finite external
103
269
Proof
(We prove this by differentiating with respect to the variables m p?.) So we define T to set the values of m and the finite external
momenta.
and
momenta to We now
zero.
have
a
complete definition of r(r).
10.3.2 Absence In
Fig. 10.1.1(a)
of infra-red
the only
that consists of
and ultra-violet
possible graph
divergences
in
r(T).
of the form of U is the
alone. We call it d{. Its value is e"ipi
Q-1P2* jt kas no
0(x)0(O) subgraphs
so
L01) (e-,'?-* The remainder is therefore =
*
+
that
=
/-(Fig. 10.1.1)
one
+
Fig.
e-,"-*)Pi_P2=o 10.1.1
-
=
(10.3.8)
2.
(10.3.9)
Fig. 10.1.1(b).
Here we regarded L(<5 () as a [<£2] vertex. It is manifest that this remainder is exactly the graph minus its Wilson expansion. Now we turn to Fig. 10.1.3(a). It has Sx as a subgraph of form U and also the loop, which we call 32. Then by (10.3.7)
L(S2)
=
{S2-Ldi(d2)}m=pi=P2=0
\(2nY =
]
q(q2-m2)[(pl+p2-q)2-m2-\Sm=Pi=P2
^k<?>
=
0
(10.3,0)
Hence
Kr)=*(r)-Mr)-wn
W*-'\ f
i2
J'
(pi2_m2)(p2_m2) [2ny -\d"q
,
e^Z
|Jd V-?2)[(Pl+P2-?)2-?2]
(q2-m2)[(Pl+p2-q)2-m^
.■(e-1'"-!) ddq (10.3.11)
where The
we use T to
denote Fig. 10.1.3(a).
following properties
hold:
is infra-red convergent even though it has zero mass and zero external momentum. Although d2 has an infra-red divergence when m,
(1) Ld2(T)
pl9 and p2
approach
the divergence.
zero, the subtraction term
Ldl(82) exactly cancels
Operator-product expansion
270
(2) Ldi(T) is ultra-violet divergent, since in replacing the vertex St =iQ ̄'lq'x by 1 we remove the ultra-violet cut off. However, Ld2(T) contains all the large-4 behavior of T and of subtractions for subgraphs of 82. Thus Ld2(T) cancels the ultra-violet divergence of Ldi(T). (3) When m, pl9 p2 approach zero, we find that
r(d2)
= =
R(d2)-LSi(d2)-L(d2) 82-LSi(82)-L(82)
-?0. This is p2
The
=
the statement that
just
0, after subtractions of these
explanations
demonstrating that they
on
L(82) is the value subgraphs are made.
properties
are
of
82
at m
=
p1
=
convoluted, but with the aim of
in general. Refer now to the general definition of r(T), viz., (10.3.6), and refer to Fig. 10.2.1 instead of Fig. 10.1.3. Then the above
are true
properties get replaced by:
(1) L{8)
is infra-red convergent for any graph only regions that could give infra-red
of form
U in
Fig.
problems subtractions. (2) r(T) is ultra-violet and infra-red convergent if m, p\ and p\ The individual terms
are
10.2.1: the
cancelled
by
are non-zero.
Lyud(T) are IR finite. The subtractions remove all
ultra-violet behavior.
(3) r(8)
=
0 when
m
=
p{
=
10.3.3 R(T)
p2
—
0.
=
r(T)
is the Wilson
expansion
Although w(n = R(r)-r(r)
contains
ZL^
x
c/n)
(10.3.12)
0 behavior of R(T), it is not yet in the form of the expansion, which is
the leading
operator-product
=
x ->
W(D
=
Xs C(S)R(r/8).
(10.3.13)
(of form U) to the Wilson point, i.e., replaced by a <f>2 for the divergent subtractions pole-part
Here C(d) is the contribution of a subgraph 6 coefficient, while T/S is V with 8 contracted to a vertex.
R(T/d) will
now
include
(f>2. Summing(10.3.13) over T can be done by independently summing over 8
Green's functions of
and r/8. This gives the operator-product
expansion (first term of the right-
271
10.3 Proof hand side of (10.0.1)) with
Q2(x2)=IC(<5).
(10.3.14)
S
(We have used the as
same
for the
symbol
for the contribution from
a
complete Wilson coefficient C^2(x) particular graph.) Hence to prove the
expansion (10.0.1) or (10.0.3), we have to prove that W(V) as defined by (10.3.12) equals the right-hand side of (10.3.13). Any graph r for the Green's function GN + 2 can be written in the form of Fig. 10.2.1. In general there will be several possibilities for the upper
subgraph U. For the following argument we will choose U to be the biggest possible graph. Then, in (10.3.12) for W(T), all the subgraphs d are contained in U. So
we
have
W(T)
prove
divergent (10.3.13) when
(10.3.15)
W{U)?R(I\
=
since no
fd4/cd4/ wW{U)R(l) (2k)8
=
IPI
subgraphs include parts of both U and /. Suppose we r is replaced by U. Then from (10.3.15) it follows that py(D
=
=
^C((5)R((//(5)?^(/)
(10.3.16)
YC(8)R(m- s
The last line follows because (a) the
graphs for T/S are of the form U/d times divergent IPI subgraphs are entirely within U/d or
/, and (b) ultra-violet within /. Notice that the extra divergences for T/S as compared with T are due to the presence of the 02 vertex. These divergences are confined to
graphs
of the form of U/d.
(10.3.13) for the case T U, we use it C(U\ recursive in terms of C(d) for smaller d: Rather than prove
2C(U)=W(U)-
=
as a
definition of
X C(d)R(U/d).
(10.3.17)
multiplying C(U) is the lowest-order IPI Green's function of the operator <j>2. It is evidently much too good to be true that we have reduced what ought to be a deep proof to a mere definition. The important physics was at the previous step, (10.3.16). By implicit use of power- counting arguments, to restrict ultra-violet divergences to within U/d, we proved (10.3.13) given its truth for the case T=U. The Wilson coefficient The factor
2
C^2(x) as it occurs in the expansion (10.0.1) is then the same no matter what
Green's function.we take of 7</>(x)</>(0). This universality is feature of the Wilson expansion.
an
important
272
The
Operator-product expansion other
coefficient is
a
important feature of the expansion is that the Wilson purely ultra-violet object. This will enable renormalization-
techniques to be useful in its calculation. For this purpose we must C(U) is independent of m, k and /. The proof is done by differentiating the right-hand side of (10.3.17) with
group
now prove that
respect to the mass m or with respect to one of the external momenta k or /. Let A represent this operation. It is applied in turn to each propagator in
Inductively, we assume C(d) satisfies AC(d) 0. This is true for the lowest-order graph, 619 for which 0(5^ 1 follows from (10.3.8). In the general case AC(U) is therefore given by W(U) and in R(U/5).
=
=
2AC(U)
AW(U)-
=
£ C(d)AR(U/d),
(10.3.18)
6Z.U
Fig. 10.3.1. We have a series of terms in each of which one particular propagator of U is differentiated. The differentiation improves
as
illustrated in
the ultra-violet behavior,
so
the differentiated propagator cannot be
of a leading large-momentum region connected therefore factor out
a
maximal two-particle
to the vertices
graph <5',just
as we
a
part
0(x)</>(O).
We
factored out
U in (10.3.15).
Fig. 10.3.1. Differentiation of Wilson coefficient with respect to
a mass or an external
momentum.
Now we can use the result
d"
for AC(U). (This argument is analogous to (10.3.16).) The operator-product expansion is now proved.
to
give
zero
10.3.4 Formula for C(U) From the above results
C(U)
we
=
L
may deduce
R(U) ̄
£ C(8)R(U/8)
(10.3.19)
Heuristically this says that C(U) is that part of U arising when all its internal
10.4 General
273
case
lines have momenta of order 1/x, less contributions taken
care of by terms C(5) in the operator-product expansion with smaller graphs 8 contained in U. The term in square brackets is the unrenormalized U minus (a) all contributions involving momenta not all of order q, and (b) contributions with all momenta of order q taken care of by the subtractions under (a). This equation reproduces exactly the calculation performed in Section 10.1 of
the 0(g) Wilson coefficient, viz.,
C(S2)
=
{S2-QS1)R(S2/51)]m=t,i=P2=0. 10.4 General
case
The most fundamental form of the operator-product expansion is (10.0.1), proved by obtaining its matrix elements from the Green's functions
which is
(10.0.2). We proved the Green's function expansion restricted to connected graphs in </>4 theory, and restricted to the leading power of x2. The only that then contributes is (9 <f>2. Our proof generalizes. We may take the full Green's functions in any theory and include non-leading powers of x.
operator
=
general case the operator L extracts the appropriate number of Taylor expansion in the mass m2 and in the external momenta. The result is that each Wilson coefficient CG{x) behaves in each order of perturbation theory like a power of x2 times logarithms of (x2^2) times a polynomial in xM and m2 with dimensionless coefficients. If (9 is a tensor In the
terms in
a
operator then the coefficient is also order example (10.1.5). The leading operator for
a
tensor,
as
illustrated by the lowest-
04 theory is in fact the unit operator since it has
lowest dimension. We have
Tcf>(x)cf>(0)
=
Ct(x2)l
+
Q2(x2)[02] + 0(x?logs).
(10.4.1)
Operators linear in <f> are prohibited by the <f> <f> symmetry. To compute the coefficient, Cx(x2\ of the unit operator to lowest order, -?
we use
the
graph of Fig.
—
10.4.1. We will extract terms up to
u
Fig. 10.4.L Lowest-order
term for
O(x0), so that we
<O|7>(x)0(O)|O>.
274
Operator-product expansion
find:
C1(x2)
=
=
C1(x2)<0|l|0> {Fig.
10.4.1
-
C02(x2)<O|[>2(O)]|O>} expanded
in pow of m2 to order ,
ie'? ddq (2n)dq2-m2 + ie *
-
m2
Hd-*m2
+ k
]}.
q2
)expanded
J(2^1(C
1)U2+(^)2J
1
=
+
"4m2 8^ 871V-4)
m2
-^2^2+Y^[y + ln(-7iVx2)]
at J
=
(10.4.2)
4.
The Wilson coefficient is obtained by expanding in powers of mass and an appropriate degree, which is two here. We have no external momenta, so the expansion is in powers of m2. In the next section, when we apply the renormalization group to compute the Wilson coefficients we will find that each coefficient is given as a series in the effective coupling with the renormalization mass \i set to (— x2)1/2. The main application of these methods will be to asymptotically free theories. If we truncate the perturbation expansion, then the error will be of the order external momentum up to
of the first omitted term, and hence the fractional
error
is of order
l/[ln(-x2)]'+1, where p is the number of
loops
in the
highest-order graph. This
dominates any positive power of x. Consequently, it is difficult to use the
power-law corrections
error
to the
cannot Wilson coefficient. This would suggest leading power in properly use anything but the coefficient of the unit operator. However, in applications we will normally work with connected Green's functions. For these the leading coefficient is of the two-particle operator (j>2. a
we
10.5 Renormalization group To
make
calculations
for
a
Wilson
coefficient,
we
must
renormalization group to obtain maximum information from calculation in the way we will now explain. The coefficient has the functional
dependence
C{x2,g,fi).
a
use
the
low-order
10.5 Renormalization group
275
\x2 to be of order | l/x21 to avoid large logarithms, just as in calculating Green's functions when all their external momenta get large (Chapter 7). The RG equations are simple, since, to the leading power, there is no mass dependence. (If we use non-leading powers of x then we have polynomial dependence on masses.) The renormalization-group equation for the Wilson coefficient can be derived most easily from the To
this
use
we must set
renormalization-group equations fashion. In the
04 theory
we
for Green's functions in the
have the renormalization-group operator:
%-%+^-i-m'^ The
renormalization-group equations
^AG(jonn)
£GN([<t>2l? Here,
Gff+p
restricted
?
.)(conn)
_
=
{N/2
(ym
=
we
ml
We
+
are
(10.5.2)
l)yGl*?*)9
i2Ny)GN([>2],...)<conn>.
[</>2]
(10.5.3) 0-fields,
denotes the connected
operator with N 0-fields. To
used the fact that
m2[02] and
-
need
graphs. GiV([02],...)(conn)
Green's function of the renormalized
derive (10.5.3)
=
we
,i,m)
denotes the Green's function of N + 2 external
to connected
following
=
m\§\ 4- coefficient
times
1,
Zmm2.
apply
the operator
fid/dfi 4- (N/2 + l)y
to both sides of
(o\T<t>(x)cf>(0)4>(p1)...4>ipN)\oy =
C<f>2(xK0\T[ct>^4>(pl)...¥ipN)\0} + 0(x2\
(10.5.4)
to obtain
0=r^^*2' ^(y y?>c^"j<°l
^t*2]*^^
"■"
Since the
0(x2)
terms are order
-
-
*<p^)l°> -?- °c^2>- (10.5.5)
x2 independently of g,
iid/dp is applied. (As usual, 'order possible logarithms.) Immediately we obtain
remain of order x2 after x2 modulo
-
^<^2
=
(ym + y)C>
m, and \i,
x2'
means
they
order
(10-5.6)
276
Operator-product expansion
be solved
0 theory. The it is effectively in the m anomalous dimension of C^2 is the anomalous dimension of 0(x)0(O) minus This
equation
can
the anomalous dimension of
=
-
[</>2].
This is the
same
engineering dimensions. The signs in (10.5.6) arise from definitions of ym and y.
relation
as
for the
peculiarities
of our
11 Coordinate space
previous chapters we set up renormalization theory in momentum space. In this chapter, we will give a treatment in coordinate space. Now, the utility of a momentum-space description, such as we gave in the earlier In the
chapters, comes from the translation in variance of a problem. However, the momentum-space formulation rather obscures the fact that UV divergences arise from purely short-distance phenomena. Thus a coordinate-
space treatment is useful from a fundamental point of view. There are also number of situations, essentially external field problems, where
a a
coordinate-space practical appropriate point of view. A particular advantage is that the coordinate-space method from
treatment is the most
makes it easy to
see
that the counterterms
are
the
a more
same as
with
no
external
field. An important case, which
we
will treat in detail in this
chapter, is
that of
thermal field theory at temperature T (Fetter & Walecka (1971), Bernard (1974), and references therein). There one works with imaginary time using
periodic boundary conditions (period 1/T). It is first necessary to work out the short-distance
singularities of the free
propagator. This we will do in Section 11.1. A number of forms of the propagator will be given, whose usefulness will become apparent when we treat some
examples
in Section 11.2. The reader should
probably skip
to
this section first and refer back to Section 11.1 as the need for various
properties of the propagator arises. We will explicitly show that the particular counterterms that we compute are independent of temperature. Our arguments will be of a form that will readily generalize not only to higher orders but also to other situations. In Section 11.3 we will show to all orders of perturbation theory that counterterms at finite temperature are independent of T. We will do this by constructing counterterms in a theory in flat space-time in such a way as to make manifest the fact that only the short-distance singularities of the propagator affect the counterterms. Another case, which we will treat in Section
277
11.4, is flat space-time with,
278
Coordinate space
electromagnetic field. If the field is weak it can be treated as perturbation, i.e., as part of the interaction. But if the field is strong, one
say, an external a
put it in the free Lagrangian. Thus the free electron propagator satisfies
must
(i? + eA- M)SF(x,y;eA)
=
id{x
-
(11.0.1)
y),
which
cannot in general be solved by expanding in a series in the field (Schwinger (1951)). Another common external field problem is that of quantizing a quantum field theory in a curved space-time, where there is no remnant of a global Poincare symmetry. Momentum space is then an inappropriate tool. One
wishes to show that the UV counterterms We will not discuss this
space-time. the techniques given.
can
be kept the here. But
particular a fairly simple case
described should enable
11.1 Short-distance
singularities
same as an
in flat
extension of
treatment to be
of free propagator
11.1.1 Zero temperature
graph is ultimately determined by the short- distance singularities of the free propagators that make up the graph. So we The UV counterterm of a 1 PI
need to obtain these singularities; and
we
will do this in this section.
It is sufficient to consider the scalar propagator
Propagators for fields with spin can be obtained by respect to x. The propagator satisfies the equation
differentiating
(11.1.2)
(□ W)SF=-i<5(d>(x), together with appropriate boundary equation reduces to
conditions. When
(4zd2/dz2 + 2dd/dz where
z =
-
x2. Thus SF
=
Bessel equation of order determined and that
v
-
m2)SF
=
z1/2_c//4w(mz1/2), where =
d/2
-
1. The
with
x
is non-zero this
0, w
(11.1.3) satisfies the modified
particular solution
we
need is
by requiring that the (5-function in (11.1.2) be obtained at x2
SF-?0
as
x2->
-
oo.
=
0
279
singularities offree propagator
11.1 Short-distance
A standard method of solving (11.1.3) is
to
expand in
powers of
z:
00
n = 0 =
There a
=
independent
are two
*2)flK
("
-
singularity
of these solutions. The
is to observe that, if
SF(m
=
0)
=
0, i.e., it has l—d/2. SF is a linear
solutions. One is analytic at
0; the other solution is singular, with
combination
m
T(d/2
=
-
(11.1.4)
m2x2)nwn. a
=
quickest
z
=
way to
compute
the
0, then (11.1.1) gives
l)/[4nd/2(
-
(11.1.5)
x2f2 ̄ *].
This normalizes the coefficient of the singular solution. The normalization of the regular solution is obtained from the properties of Bessel functions, by
requiring
that SF
-?
0
as
x2
-?
-
oo.
Then
we
find that
f fm2x2\nV(d/2-l-n)
1
00
+
md-2(4n)-"12 £ (m2x2/4)T(l-n-d/2)/n\ n = 0
=
Here,
we
SFsing + SFana.
have used the series
(11.1.6)
expansion
coefficient of the regular solution computing, from (11.1.1), that SF
=
can
T(l
of the Bessel functions. The
quickly by explicitly
be obtained -
d/2)md-2/(4n)d/2
at x
=
0, if d is
less than 2. For the purposes of renormalization
we will need the singularities of SF. of two types: the singularities of SFsing as x->0, and the singularities of SFana as d approaches an even integer. The fact that most of
These
are
poles when d is an even integer d 2a>, reflects the fact that the ansatz (11.1.4) is then incorrect for the singular solution. However, the F-functions have
=
the correct result is obtained by expanding in powers of d letting d -> 2a>. The limits x -> 0 and d^>2a> are non-uniform.
graphs
like the
</>3 self-energy
need
x
—
2a> and then
Application to
± 0, but tadpole graphs (e.g.,
Fig. 11.1.1 in 04 theory) have a propagator with
Fig. 11.1,1. Graph with
a
x
—
0 for all values of d. So
tadpole.
280
Coordinate space
it is convenient to extract all the x
=
0
singular behavior of SF close to
=
2a> and
by writing
Op
>FSing
=
Jpsing + ^Fana
-
4n*,{
jyo-i
_
/
n = (o- 1
s JFana
=
\^oy ̄
W V"
\"
I
I
n!(
11/7//
y Z^ M-r^w (4?r ?^0V
as
a
(-l)n^V
77 I
I
—
(<//2
-
1
&))(?
(-1)"+V ̄2<°
+ 1
-
1
____^_—_|.
x|r(1_n_,/2)(_j finite limit
—
/2-a>
eo)! J j'
x
7 n!
4
{m2\il2 ̄a
Neither term is
I
—
(- nx2f
-
Trx2)""2""
-
T(d/2-l-n)
4
V
J
4
n=0
f
a
d
solution of the equation for SF. The analytic part 0. The no singular behavior at x
d^>2a>, with
=
(11,.7) now
has
singular
x^O. It contains the x->0 we then first take d < 2, then x -? 0, SF(x 0) 2a>. Then the only singular contribution is from the n
term also has a finite limit as d^2a> if
singularities.
But if
and finally d
-*
co-l term in
need
we
=
=
SFsing: (_l)^m2co-2^d-2co
w^oi^:/^:,^ (^/2-a))(47r)<>)-l)!
(in.?
?,.
This result will
be needed in evaluating
tadpole graphs.
11.1.2 Non-zero temperature Thermal Green's functions at inverse temperature p
Wick-rotating time conditions (Fetter SF(x y;p) satisfies
t
->
&
h and then
-
Walecka
1/7 are obtained by by imposing periodic boundary
(1971)).
Thus
=
the
free
propagator
—
(
-
d2/dr2
-
V2
+
m2)SF(T,x)
Sf(t,x)->0, Sf(t
+
P,x)
=
=
(5(t)(5(3)(x),
asx->oo,
Sf(t,x).
(11.1.9a) (11.1.96)
(11.1.9c)
integrals over space-time are restricted to the time range between 0 and The p. propagator is obtained from the zero-temperature propagator All
SF(x;oo) by writing
SF(*'J)=
t
S¥{T + n09i;cc).
(11.1.10)
of counterterms
11.2 Construction
(Note that in
apply the
n
-
=
The
SF(x
d2/dT2
-
—
V2
y;P) both
tx and zy
+ m2 to it the
are
in low-order
281
graphs
between 0 and p, so when we in (11.1.9a) arises from
only (5-function
0 term.)
only singularities
in
SF(x\p) are from the n
=
0 term when xM
=
0,
so
have
we
SF(x',P) where
SFsing
is the
same as
=
in (11.1.7), with
term is different. It differs from
A5F=
which, in the (D + m2)SF
=
(11.1.11)
S¥u]ag(x) + 3¥m(x;P), SFana(x; oo)
-
x2
=
at zero
x2. The analytic temperature by
t2
+
£ SF(T + nP,x;oo),
range 0 < t < p, is a solution of the homogeneous equation 0. Notice that it is not a function of x2 alone, i.e., it is not
Poincare invariant. Another, direct, way of deriving the form (11.1.11) is to expand SF(x; P) in
require the differential equation (11.1.9a) right-hand side ensures that SF SFsing(x) + SFana(x;p), where SFsing is the same singular solution as in (11.1.6). Then SFana is a solution of the homogeneous equation analytic at x 0, and for which SFsing + SFana satisfies periodic boundary conditions. We then add and subtract the poles 2Xd 2o^ to obtain (11.1.11). The coefficient of the pole in 5Fana is the same as at zero temperature, because it must cancel a
power series about xM
=
0, and then
to
to be satisfied. The (5-function on the
=
=
—
the pole in
SFsing,
which is
independent
of temperature.
11.2 Construction of counterterms in low-order graphs To explain how renormalization works in coordinate space we consider the graphs shown in Figs. 11.2.1 to 11.2.4 for 04 theory in four space-time dimensions. These
are
sufficient to show the various
complications
that
occur. Our treatment
The
works
as
simplest example
x
z
(a)
well at any temperature.
is the
one-loop
correction to the propagator,
* y
(b)
Fig. 11.2.1. Self-energy graph and counterterm.
282
Coordinate space
Fig. 11.2.1. Its contribution to the full propagator is: M,
=
-yA*4"" \ddzSF(x- z,d;l/T)SF(z
y,d;l/T)SF(0,d;l/T). (11.2.1)
SF(x z) and SF(y z) are singular singularities are integrable, so that they
The functions the
-
—
—
divergence. The factor
(11.1.11)
Mx
to write
Ml
=
"
T
SF(0,d;l/T)
at z
x
and at
z
y, but
is divergent,
so we use
(11.1.8) and
as
WZS^X
 ̄
Z)S^Z
 ̄
>^Fana(0, d
=
term
4; l/T)
+
0(^4)-
evidently exactly cancelled by
is
(1L2*2) the
renormalization counterterm, Fig. 11.2.1(b). The result for the lized propagator at order g is
! Jd4zSF(x
-
=
do not contribute any
||ddzSF(x- Z)SF(Z"y)ri^ The divergent
=
-
z)SF(z
-
y)SFana(0,4, l/T).
mass-
renorma-
(11.2.3)
Notice that the renormalization only involved the singular term in SF. Thus we have shown that the counterterm is the same at any temperature. The unrenormalized
M2
=
graph
U- w4"")2
of
Fig. 11.2.2 for
IdV*sF(*i
-
the
y)sF(*2
four-point function
-
y)SAy
SF(y
—
z)2
divergence at y
=
z; it is
M2
=
Since the divergence is
d approaches 4
as
logarithmic.
g2
*? (11.2.4)
xSF(z-x3)SF(z-x4). The only
-
is
Let
conies us
from the
singularity
of
write
L'yd'zgSrly -z)2f(y,z,{Xi}).
(11.2.5)
logarithmic, it is governed by the leading behavior of
(a)
m
Fig. 11.2.2. Vertex graph and counterterm.
11.2 Construction the
of (11.2.5)
integrand
of counterterms
as y-?z.
Thus
in low-order
we can
graphs
generate
a
283
counterterm,
illustrated in Fig. 11.2.2(6), M,
-J-Vy&Wf{y,yA*ii) [pole
singularity is entirely given by (11.1.7) of SFsing The
sF1 The
pole-part factor
=
d"zSF1 (y-*)2]. (11.2.6)
part of
the first term
r(d/2 -1 )/[47rd/2(
-
SF1
in the
expansion
x2y"2 -*].
in (11.2.6) is then
-polejj d^SF1(y-z)2|=-pole|r(^ ̄1)2J = -—-
d'z(-z>?-
dzz-
pole <
1 fiu 8n24-d*
In the first line
we
and the notation The factor
\id ̄*
j*
shifted z
_
z to z +
(11.2.7)
y. In the second line we Wick rotated
0dz indicates that only the region near z
in the last line is needed,
j
as
usual,
=
z°,
0 matters.
to preserve
explicit
dimensional correctness. We thus find that the renormalized M2 is
M2
\im[ddzddy[g2SF(y- z)2+8^QW)('-z)
=
(11.2.8)
xf(y,z;{Xi}).
4. Again The factor in square brackets is a well-defined distribution at d observe that we only used the singular part SFsing of SF in obtaining the =
counterterm, The
so
temperature-independent. logarithmic subdivergence as well
that the counterterm is
graph Fig.
11.2.3 has
,
(?)
a
>c<
x
(b)
(c)
Fig. 11.2.3. Two-loop vertex graph and counterterms.
as
an
284
Coordinate space
overall
M3
divergence. =
i(
-
=
g3
iddWddyddzSF(x1
igfi4-")3
SF(x4
x
Its unrenormalized value is
-
z)SF(w
-
y)SF(w
jd'wd'yd'zf <3>(w,
y, z,
-
-
w)SF(x2
z)SF(y
{xj )SF(w
w)SF(x3
-
y)
x
z)2
-
-
-
y)SF(w
-
z)SF(y
-
zf. (11.2.9)
If w is not close to y or z there is a logarithmic divergence when y approaches z. This is identical to the divergence in Fig. 11.2.2. There is also an overall divergence when all of the interaction points w, y, and z approach
each
other.
We first subtract the
R(M3)
=
subdivergence by
g3
the counterterm (11.2.7):
j}(3)(w,y,z, {xj})SF(w [sF(y- *+ Sn2(4-d) 8?\y-z) -
!
y)SF(w
-
z)
x
*
(11.2.10)
z. However, it is The distribution in square brackets is singular at y to 4, if it is it d that a when finite result is, goes integrable; produces z. function with The a at test function continuous y integrated =
—
/(3)(w, y, z)SF(w
—
y)SF(w
z) is continuous
-
at y
—
z, unless also w
=
y
=
z.
The remaining divergence in R(M3) comes from the region w y ̄ z, where z are both equal to if and not continuous. also is is a (There singularity y /(3) x3 or x4; however, this region is again integrable.) The divergence at w  ̄
=
y
=
z
is logarithmic,
singular
terms of the
C=
-
so
again it is determined entirely by the leading
propagators. The
counterterm is therefore
JdW/(V,w,w,{xJ)rx)lej fdVdzSF1(y)SF1(z)
[ s^-z)2+s?,f^^-) shown in
Fig. 11.2.3(c). singular but integrable at y as
=
SF(y)SF(z)
*
(11.2.11)
Recall that the factor in square brackets is z. So the only divergent behavior comes when
is singular, i.e., at y
—
z
=
0. It is
easily checked that the
counterterm C then reduces to 3?2<<-8
just
=
as
JdV(3)(w,w,w,{xi})('-^
)2
C
2T2-
1
(4-d)2
4 ̄d
(11.2.12)
in momentum space (Vladimirov, Kazakov & Tarasov (1979)).
11.2 Construction
of counter terms
in low-order
&* *¥■ (a)
285
graphs
(c)
(b)
Fig. 11.2.4. Two-loop self-energy graph and counterterms.
Our final
graph, Fig. 11.2.4,
is
a
little
more
difficult. Its unrenormalized
contribution to the propagator is M4
=
±(
-
xg^-d)2\&dy&dzS¥(x,
-
y)SF(y
-
zfSF(z
-
x2)
^^2|dVV(4)(y^;k-})sF(y-z)3.
(n.2.13)
The difficulty is that in momentum space there is not only the quadratic overall divergence, but also three logarithmic subdivergences. However (11.2.13) appears to have only an overall divergence, when y goes to z; this involves all three propagators. Furthermore, the counterterm, calculated in momentum space, for a subdivergence from two of the lines gives a graph of the form of Fig. 11.2.1 (a). There the third propagator is at y z, whereas the —
propagators in (11.2.13)
are
also used at y ^ z. case is to write
The correct way to handle this SF
where SF1 is the ^
Fsing
same
=
SF1+SF2
leading term
as
+
(H.2.14)
Srcm,
before, and SF2 is the second term
in
?'
SF2(*)
=
r(d/2-2)
 ̄
16^
(-nx2)"12-2
d-4
n
d/2-2_
(11.2.15)
z) is finite as d -* 4 and/or y-*z. We now substitute (11.2.14) for each factor of SF(y z) in (11.2.13) and obtain The remainder
STem(y
—
-
M4
=
g2\ \d'yddzf?\y,z)SF1(y + 3\
/<4>(y,z)SF1Srem U f/(4)(y,z)SF1srem|
-zf + 3 f?\y,z)S2F1SF finite.
2
(11.2.16)
SF1's has the quadratic divergence, while the terms with two SF1's are logarithmically divergent. The other terms are finite. The term with SF1 Srem has a divergence coming from the two SF1's, and its factor 3 reflects the fact that there are three divergent subgraphs. The graph with the The term with all
286
Coordinate space
counterterm for the
subdivergences
is Fig. 11.2.4(b):
2,,4-d
M*?
8^(4
=
-
^Wto'*' W>Sf<°>
g^fr^ldV(4)(y,>';{x1})[sF2(0)
=
+
srem(0)]. (i 1.2.17)
Here we used the value
we calculated at (11.2.7) for the counterterm vertex. The Srem(0) term here cancels the corresponding divergence in the third term in (11.2.16), while the SF2 term combines with the second term in
(11.2.16)
to
produce
302
a
logarithmic divergence from
iV4)(y,y){ [
ddzSF1(y-z)2Sh2(y-z)n
d'
8-2d
,4-<J
Let
/(4)
us return to
;SF2(0?. Sn2(4-d)*¥2K"'f-
(11'118)
(11.2.16). The divergence in the
first term is sensitive to
and its second derivative at y
g2
Jd'z f
z:
d"ySFl(y-z)3[/<*>(z,z) + (y -zf(df^ldy%=z +
=
=
finite +
[2ig2nd'2/r(d/2)]
Uy
-
zY(y
-
zy(d2f^ldy"dy%=z-\
x
y2 d2fA)
^-■?[/<<>M>- =
finite
—
i
Note that the
Tddy^dy.
ddyddzf{4)(y,z)nS{d)(y
16n2 term with
/(4)(z,z) gives
a
pole
-
at d
z)/(8 =
-
2d).
(11.2.19)
2 but not at d
=
4.
11.3 Flat-space renormalization In the last section
Our method
we
was not
computed counterterms for some low-order graphs. a good method for computing the finite parts. But it
made very explicit the fact that the counterterms
depend only on the short-
distance structure of the free propagator. In particular it made it obvious that the counterterms are independent of temperature. We must now spell out how to
generalize
the results to
an
arbitrary graph
in
an
arbitrary
theory. We define the renormalization of a graph G by the same structure that we had in momentum space (i.e., by the recursion method or by the forest
11.3
formula).
Flat-space renormalization
287
A counterterm is needed for the overall
divergence of every 1PI subgraph is obtained by first counting powers of position as all of its vertices approach each other, and by then taking the negative of the result (since it is an x -> 0 divergence). The integrations over relative positions of the vertices are included in the power-counting. The result coincides with the usual momentum-space
subgraph. The overall degree of divergence of a
definition for
1PI
1 PI
subgraph. The value of G is obtained by integrating over positions divergence and subdivergences come from regions where some of these positions approach each other. To each region corresponds a subgraph consisting of the vertices that go to the same point, together with all the lines joining them. It might appear that these graphs should be in one-to-one correspondence with the 1 PI subgraphs that in momentum space are divergent when all their internal loop momenta are large. However this is not so, for the following reasons: a
of its interaction vertices, and the
(1) There are graphs overall divergent in coordinate space that are not 1PI. (2) There are divergent subgraphs in momentum space that are not obtained directly in coordinate space; these are when some but not all lines connecting vertices of a 1PI graph are in the subgraph.
example of the first case is Fig. 11.3.1, where there is a logarithmically divergent graph consisting of the vertices at w, y, z, and of the lines joining them. In momentum space the counterterms for the self-energy subgraph cancel all the divergences. But in coordinate space SF(w y) is singular, so we have to justify the momentum-space result. Other graphs are divergent in momentum space but do not occur directly as divergences in coordinate space. A typical example is given by the one-loop subgraphs of Fig. 11.2.4. An
—
The divergence
comes
the trick
to
propagator,
from the region y ->z, and it appears to involve all
just two propagators. As we saw in Section 11.2, this handling problem is to make a decomposition of the
three propagators, as
never
in (11.2.14).
 ̄/w Fig. 11.3.1. The
y
V
1
z
subgraph is one-particle-reducible and overall divergence in coordinate space.
w-y-z
apparently
has
an
Coordinate space
288 First, however, let
us
consider the
of divergent 1PR
problem
graphs
like
of Fig. 11.3.1. After subtraction of the self-energy counterterms, Fig. 11.3.1 has the form the (w,y,z)
subgraph
ddwddyddzf(w)SF(w
x[SF(y-z)3 Here f(w) is
y)
-
x
(11.3.1)
AS(y-z) + B\jS(y-z)]SF(z-x2).
+
function
non-singular at w y. The factor in square brackets is the self-energy plus counterterms. At d 4, we have SF(w y) 1/ (w y)2 and SF(y z)3 l/(y z)6, so it might appear that there is a logarithmic divergence when w and y both approach z. A counterterm for this divergence would be a four-point vertex and therefore allowed. a
=
=
—
^
—
However, the momentum-space result gives
important
no
directly
to derive the same result
We first integrate
 ̄
—
—
such counterterm,
so
it is
in coordinate space.
over w, and see that
iddwf(w)SF(w-y) non-singular as a function of y. Then we can perform the y- integral. Although SF(y z) is singular at y z, it is integrated with a function with no singularity there, so the counterterms are sufficient to cancel the divergence. Then, finally, we integrate over z. The crucial point is is finite and
=
—
that
find an order of integration with the following property:-each kills the singularity on precisely one propagator without integral new introducing singularities. If we replaced Fig. 11.3.1 by Fig. 11.2.3, say, we would have a not-quite similar integral: we
ddwddyddzSF(w The
SF(w
integral —
over
w,
for
-
y)SF(w
-
example,
z)[SF(y
we
z)2
-
counterterm].
involves two propagators
z). The result becomes singular
So far
-
at y
=
SF(w
—
y),
z.
have chosen to define renormalization by the forest formula
or
that, after subtraction, only 1PI by graphs have divergences. Some subgraphs that need counterterms in the recursive method. We have
momentum space
(like Fig. 11.2.4)
in coordinate space, since
we
subgraphs
we
saw
7 as in
general and
do not appear
explicitly
as
divergences
defined the subgraph corresponding to
coordinate-space divergence to Even so,
seen
include all the lines
in Section 11.2 that
Fig. 11.2.4(b).
we
a
connecting its vertices.
have counterterms for such
To show how these counterterms arise in
to show that the counterterms are
independent of temperature,
11.3 Flat-space renormalization we
decompose SF
289
as
SF
=
SF1
+
-
+
(11.3.2)
SFN + Srem.
singular part SFsing of SF. We larger subgraph of the graph G that is being renormalized. Thus when SF is replaced by Srem, any (sub)graph y containing it becomes overall convergent. When we substitute (11.3.2) for every propagator in a graph, we obtain a sum of graphs, in each of which every propagator is replaced by one of We or renormalize each of these new graphs separately. A 5F1,..., SFN, Srem. subgraph needs a counterterm if its overall degree of divergence is a positive integer or zero, and we make counterterms for all 1PI subgraphs that are divergent. The rules for computing the degree of divergence are essentially obvious. Any propagator with SF replaced by SF1 contributes as in the original graph, and a replacement by SFN reduces the degree of divergence by IN 2. The only subtlety is that replacement of SF1 by Srem(x y) is considered as contributing 2N to the degree of divergence, even though it d + 2N does not behave as (x 0: Its analytic part behaves when x y)2 y as (x y)°. However, the analytic part never actually contributes to any overall divergence, and the singular part of Srem does go like (x y)2 ̄d+2N Here
SF1
choose N
the first N terms in the
to
SFN
so
that 2N is
are
than the
degree of divergence of any
—
—
—
"
—
—
->
—
—
relative to A typical
SF1. case
is the third term
on
of the subdivergence, this term is
if{4)(y,z)Srem(y SF1 (y z)2  ̄(y- zf logarithmic divergence at
Now a
"
-
2d
y
proportional -
z)SF1(y
as y -> z, =
z.
[/(4)(y,z)Srem(y z)[sF1(y -
the right of (11.2.16). Before subtraction
-
to
z)2ddyddz
while Srem
-
SFana(0) + 0. So there is
But after subtraction, -
z)2
+
we
have:
s*(4*_d)Hy *)]d dyddz. -
integrated with an analytic function like/(4)(y,z)SFana(y-z), the factor in square brackets gives a finite result. So any divergence comes from When
singularity in 5rem. But the singularity is of order (y z)6 ̄d, so no divergence occurs. We now generalize our treatment of Fig. 11.2.4. We examine structures of the form of Fig. 11.3.2(a). The subgraph A is 1PI and overall divergent. The lines /,,..., /Join vertices of A. A graph that consists of A and some or all of the /-'s is 1PI andjs overall divergent. In coordinate space, these divergences come from the region where all vertices of A approach the same point; it
the
—
290
Coordinate space
Fig. 11.3.2. General structure where subgraphs but appear to have
no
are divergent in momentum space, corresponding divergence in coordinate space.
lv..., la all participate in every one of the divergences. In order to preserve the correct counterterm structure we must include counterterms for graphs of form A in our definition of renormalization. appears that the lines
This
ensures that counterterms are given by terms in the action. For simplicity consider the case in 04 theory of one line / with two external lines pu p2. Schematically we have
ddwddxddyddzSF(x Here
SF(x
—
-
w)A(w, x, y, z)f(y, z).
(11.3.3)
w) is the propagator of line /, while f(y,z) is analytic at y z graph. A(w, x, y, z) is the value of graph A. We =
and represents the rest of the
subtract off all subdivergences of the graph Au{l}. Among these is the overall counterterm for A, which is of the form
CA(w,x,y9z;d9g,ri since
A
gives
a
=
C(d,g,^d\w
logarithmic divergence.
-
z)d^(x
-
z)d?(y
The result of
-
z),
replacing
(11.3.4) A
by its
counterterm is then
C(d9g9ri(ddzf(z,z)S¥(Q).
(11.3.5)
by the Feynman rules, since when four-point Green's function without additional loops it needs
This counterterm is necessarily generated A
occurs
an
in
a
overall counterterm. We wish to show how this counterterm is needed to
cancel certain parts of the divergences of (11.3.3) as y-*z. Let us now substitute each propagator by (11.3.2) with JV
=
2. The
/may be replaced by SF1, SF2, or Srem. The terms with SFx and SF2 both contribute to a divergence at y z, whether in the original or in The from (11.3.5) with SF SF1 or (11.3.5). divergence graph (11.3.3) SF2 is local and temperature-independent, so that it can be cancelled by overall wave-function and mass counterterms. The terms with Srem also give divergences,,but cancel in the sum of (11.3.3) and (11.3.5), just as in the simplest case of Fig. 11.2.4. propagator for line
=
=
291
11.4 External fields
The argument
presented above
is
considerably
more
cumbersome than
needed for the particular case considered. However it was presented so emphasize its form as a special case of a general argument. This
happens
only
to be the
as to case
present in Green's functions. However in the
one
bubbles (used to compute ground-state energies) and in the presence of composite operators the degree of divergence can be higher, with the consequence of needing a more general proof. vacuum
In any event, the moral is that if
we
divergent subgraphs including those
choose counterterms to cancel
of form A in Fig. 11.3.2, then the
counterterms need only depend on the SF1,SF2, In particular, the value of SFana is irrelevant. It is only SFana that knows the boundary conditions, so only it knows the temperature. Note that the SF/s are monomials in mass. Hence a
our counterterms are
totally different
polynomials in mass parameters, as we saw by
method in momentum space.
11.4 Externa] fields Consider
as
an
generated
by
a
2
-
=
-
example QED
classical
source
in the external electromagnetic field
J*. We have
(in covariant gauge)
iFl* + #(#+ **- M)\l* J?A^ i(Z3 l)Fjv + (Z2 l)0(tf+ eAty -
-
-
-
W(Z2M0
-
M)
-im)d-A2. (11.4.1) To separate the classical and quantum parts of the electromagnetic field, we let stf ^ be a c-number potential that satisfies the classical Maxwell equations with source J". Then we replace A^ in (11.4.1) by A + st ^ with the result &
=
"
-
+
iFl* i(Z3 (Z2
-
 ̄
-
0(tf+ est- Mty + e¥Aty 1)F*V + (Z2 l)0(i*+ eA)iP (Z2M0 M)^
(V2OS'A2
+
-
\)\\iei#\\i
-
+
st^
(11.4.2)
+ total derivative.
We assume the gauge condition d -srf
A
-
=
with the classical c-number part
3-^ Q^
=
0
=
J".
electromagnetic field is satisfying
0. The total
(11.4.3)
¥t^ is large then we are not allowed to expand in powers of st^. Indeed, as,Schwinger (1951) pointed out, the electron propagator If the field
SF(x,y;ejtf) in the external field is not necessarily
analytic
at jtf
=
0.
292
Coordinate space
Moreover, he showed that it is the
production in an electric field. without assuming that esi\
non-analytic part that is relevant for pair perturbation theory in e
We will therefore do
is small. The lowest-order propagator (in
powers of e) therefore satisfies the equation: (iy?d/dx* +
Since SF is
no
efd^
longer simple
renormalizations
are
the
-
M)SF(x, y; est)
=
i(5(4)(x
-
y)\.
(11.4.4)
in momentum space, it is not clear that the with si\ 0. Let us work in coordinate
same as
=
(11.4.4). The solution is y, plus series analytic there. We only need explicitly terms, and to prove renormalizability we decompose SF as in (11.3.2). It is important not to use an infinite series for SF at that stage, because the series will not converge for general values of x, y and si. The singular series is obtained by treating both e and M in (11.4.4) as space. We construct
power series in
a
series singular at x the first few singular a
x
-
y to solve
a
=
perturbations and expanding in powers. This is similar to the way in which the singular part of the scalar propagator is an expansion in powers of m2. We obtain the leading power of x y by solving —
vfdldx?S¥1 =i(5(x
-
(11.4.5)
y)\.
Each non-leading term SFn is obtained in terms of earlier terms by solving at x±y the equation n-l
iy?d/dx?SFn
=
(M
-
est ^f)
(11.4.6)
£ SF,
SFn uniquely, we require it to be a singular power of x y times a function of y. The operation iy^d/dx* makes SFn more singular while multiplication by M or by ey^st^ leaves the degree of singularity unchanged. After this work we see that the renormalizations are correctly given by treating as a perturbation the term JM^ in the Lagrangian (11.4.1) before To obtain
—
the shift of the field to
A^ + si ^ Since A^ is the renormalized field, this term
cannot affect the divergences
with each of
one or more
-
it
simply tells us to integrate J^(x) separately an ordinary Green's
external photon fields of
function. We
see
that, after the shift
to
the divergences are correctly si and then taking the first few terms.
A^ + si^
treated by expanding in powers of The non-analyticity is entirely confined to the remainder term in SF; this contributes to important physics, but not to the divergences.
12 Renormalization of gauge theories important to show that renormalization of a gauge theory can be accomplished without violating its gauge invariance. Gauge invariance is physically important; among other things it is used (via the Ward identities) to show that the unphysical states decouple ('t Hooft (1971a)). In Chapter 9 we considered the case that the basic Lagrangian of a theory is invariant under a global symmetry, as opposed to a gauge symmetry, such as we will be investigating in this chapter. We showed that the counterterm Lagrangian is also invariant under the symmetry. Suppose now that the basic Lagrangian is invariant under a gauge symmetry. One might suppose that the counterterms are also invariant under the symmetry, just as for a global symmetry. This is not true, however, since the introduction of gauge fixing (as explained in Sections 2.12 and 2.13) destroys manifest gauge invariance of the Lagrangian. One might instead point out that the theory with gauge fixing is BRS invariant and deduce that the counterterms are BRS invariant. This deduction is false. To see this, we recall that an ordinary internal symmetry relates Green's functions with certain external fields to other Green's functions differing only by change of symmetry labels. However, BRS symmetry relates a field to a composite It is
field (2.13.1). This wrecks the proof of BRS invariance of counterterms except in an abelian theory, where the Faddeev-Popov ghost is a free field.
Before treating the non-abelian case, let QED. The
Lagrangian
-
-
us
examine
an
abelian theory,
is
i(Z3 \)Fl + (Z2 l)0(i0 + eRA- MW Z2{p(M0 M)¥ (l/2£)d-A2. -
-
-
-
(12.0.1)
Here eR and M are the renormalized coupling and mass, while A^\p and \j/ are the renormalized fields. We have chosen to include only counterterms
invariant under the gauge transformation
^->e-^, 293
>
(12.0.2)
Renormalization
294
of gauge
theories
special case of our later results will show that coefficients of the possible gauge non-invariant counterterms actually vanish. (We will write eR fi2 ̄d/2e if dimensional regularization is used, with e being the dimension-
A
=
less renormalized charge.) In terms of unrenormalized quantities Z\/2*?, <Ao Zj'V, e0 Z" *'2eR9 and £0 ^ {Z3, we have ^
=
i(dHA0v Mo/ + <Mtf + eo4o -
"
The counterterm for the
photon-electron (Z2
which has
a
A^
=
 ̄
=
-
 ̄
Mo¥o
vertex
"
i^i0)d'A2. (12.0.3)
is, from (12.0.1),
(12.0.4)
Ije^A^
coefficient
It is easy to
proportional to the wave-function renormalization. verify that to one-loop order this result is correct. Our later
results prove it to all orders. If the non-abelian
theory of (2.11.7)
and (2.11.8)
gauge-invariant counterterms, then the
same
were
renormalized by
relation of the
iJ/Aip
counter-
quark field strength renormalization Z2 would hold. It is easily verified by explicit calculation that this is false. As we pointed out above, the
term to the
reason
that the counterterms need not be BRS invariant is that the BRS
invariance is not
a
symmetry that relates elementary fields
to
elementary
such that the Lagrangian is BRS invariant after renormalization, but under a renormalized BRS symmetry,
fields. Even so, the counterterms
as we
will show in this
are
chapter.
One result will be that whereas the gauge transformation of a fermion field in QED is given by di// ieRa>il/, with eR being the renormalized charge, =
the
where X is
is finite. There
a
fermion field in QCD is dxjj
ig^fXxjjdX, renormalization factor. The divergent composite operator dip
BRS transformation of
a
=
number of strategies for proving renormalizability. Before let us remark that the aim is to show that a finite theory exists which has gauge-invariance properties. The gauge invariance is are a
explaining them,
exhibited by Ward identities. It is possible to choose counterterms in such a way that gauge invariance does not hold. For example, we could add a term to the QED Lagrangian. Since the coefficient of this term is dimensionless, we obtain a finite theory by adding appropriate extra infinite
(A^A^2
is not gauge invariant. This implies, for that negative metric states do not decouple from physical
counterterms. But this
example,
theory
processes, and the theory is unphysical.
The proofs state that it is possible to
choose counterterms so that gauge invariance holds. Several
approaches
can
be distinguished:
295
Renormalization of gauge theories
1. Invariant regulator. Use an ultra-violet regulator that does not break gauge symmetry, for example dimensional regularization. Then Ward identities are true when the Lagrangian is given by (2.11.7), (2.12.5), and (2.12.9). Allow all parameters and fields to get renormalized. The theorems
proved are that this is sufficient to make the theory finite when the regulator is removed. This is the traditional approach. The advantages center around the manifest preservation of gauge invariance. The to be
that chiral symmetries cannot be regulated gauge invariantly; this symptomizes the fact that not all chiral symmetries can be preserved after
disadvantages
are
quantization
-
see
Chapter
13.
2. Gauge invariant regulator + MS. Let
us
again
use
dimensional
regularization (or another gauge-invariant cut-off). But now let renormalize each separate graph by the forest formula or by
us
choose to
the recursive method (as given in Chapter 5). We do not explicitly constrain the counterterms to satisfy gauge invariance; so in general we have violated gauge invariance. But if we use minimal subtraction then the counterterms are gauge invariant. The reason is simple: since we will prove that we can
renormalize the theory gauge invariantly, the lowest-order counterterm that is not gauge invariant must be finite, after summing over all graphs of this order. But in minimal subtraction the only finite counterterm is zero. This method is of great use when renormalizing the complicated non-local operators that appear in generalized operator product expansions (Collins & Soper (1981)). We can renormalize the graphs without explicitly
investigating
the Ward identities. The disadvantage is that the method is
closely tied to
a
specific
renormalization
3. Non-invariant regulator any
regulator Piguet
There
non-invariant counterterms. One can use
adjust overall counterterms, if possible, to satisfy all the (Piguet & Rouet (1981), Symanzik (1970a), 't Hooft (1971a),
and
Ward identities and
plus
prescription.
& Sibold (1982a, b, c)).
are two
be used. There
different forms of the Ward identities, either of which may the Ward identities derived in Section 2.13 for Green's
are
functions, and there
are the ones for the 1PI graphs, as derived by Lee Our will use BRS the identities for Green's functions (1976). approach a with combination of and 2. It is based on the 1 together approaches
treatment of Brandt (1976). Most other treatments have used the identities for the 1PI graphs. We will restrict general
cases
-
our
simplest theories, like QCD. More supersymmetries are not treated here. See
attention to the
with chiral
Chapter 13 for references.
or
-
296
Renormalization of gauge theories 12.1 Statement of results
simplicity we will mainly treat one case: a theory of a gauge field coupled to a Dirac field, with the gauge-fixing term being the usual one, £(3-Aa)2/(2£). We assume that the gauge group is simple (in the For
—
a
physically, this implies that there is only one independent gauge coupling. With a 1/(1) gauge group and one or more Dirac fields, this theory is quantum electrodynamics. If the gauge group is SU(3) and the matter fields are in the triplet representation, then we have quantum chromodynamics. mathematical sense);
The result to be proved is that the Green's functions are made finite by renormalizing the values of all the parameters in the basic Lagrangian
(2.11.7). These parameters
are
the gauge
coupling,
the fermion
masses
the field strength renormalizations, and the gauge-fixing parameter. The resulting Lagrangian expressed in terms of renormalized fields se
-
=
-
\zzg% + £ zftjip (l/2{)5- Aal
+
-
M, is
Mgu (12.1.1)
Z3/flD"Cfl.
We have allowed the fermion fields to be in several irreducible representations of the gauge group labelled by i. There are separate field-strength renormalizations Z(2° and bare masses M{£ for each representation. The
covariant derivative is
=
where g0 and
(dfl + igRXZ-ltaAl)il/,
gR are the bare and renormalized
dimensional
regularization Following Lee (1976)
we
couplings. (With
fi2 ̄d/2g, with gR write the bare coupling as write
we
(12.1.2)
=
dimensionless.)
g
X
0o so
that the
coupling
Ga
=
Zdfd'c. + gRXCabe(df)<*A?.
—7-l/2rTa
V°
"
MS
-
GkXZ-'c^AI.
Observe that in accordance with theTesults
gauge-fixing
(12.1.4)
tensor is
=
the
(12-L3>
2^T720R>
of the gauge field to the ghost is XgR:
ZdfD>ca The field strength
=
term
(d-A)2
to be
proved,
(12.1.5) the coefficient of
is finite, when the renormalized field is used.
12.1 Statement This
gives
a
gauge-fixing term
-
of results
297
(d'A0)2/(2£0\ when expressed in terms of
the bare field, with
?o
=
(121-6)
Z3£
The main theorem to be proved is: Theorem 1. The renormalizations
Afg>, can
be chosen
X, Z,
Zf,
and
that Green's functions of A, ij/,
so
(12.1.7)
Z3
{/},
c, and c are finite.
To prove this result we will use the Ward identities for BRS invariance. These involve a number of
composite fields, which we also composite operators are
finite. The counterterms for these
need to prove related to the
basic counterterms (12.1.7). We will prove: Theorem 2.
(5BRS(renormalized field)/(5/lR is finite. That is, its Green's functions with any number of renormalized fields are finite. We define the renormalized BRS transformation SBRS/dlR as
follows: (1) Let the BRS transformation 8BRS be defined by (2.13.1), (2.13.2) with g and { replaced by g0 and f0 and with the fields replaced by
unrenormalized fields (i.e., A -+A0, etc). Then the BRS invariant.
(2) Define 6AR
=
^
MS
V K^
Lagrangian (12.1.1) is
6XZ ̄ 1/2Z"1/2. Then
=
=
=
<Ws<A/<^r
<W"/<^r Smsca/S?.R
-
=
=
igRxtail/ca,
^caZ + gRcabccbAlX, hgRXcahcchcc \gRXc
-
=
=
-
a c,
)
(12.1.8)
1.
=
sBRSca/SAR=-d'Aa.
T
Renormalized Ward identities follow from the unrenormalized The operators
multiplication by 8X/8ar. because of Theorem 2. Certain
auxiliary operators are useful proving Theorems 1 and 2.
appearing
for
reasons
in them
by finite,
ones
are
which only become
apparent in
?
=
Z □ c* +
=
(Z/X
a
4?
-
d^A^Xc^,
l)d^/gR + cabccbA^
c a c
The operator (9a is
zero
by
the
=
cabccbcc.
ghost equations
(12.1.9)
(12.1.10) (12.1.11)
of motion. We will prove:
298
Renormalization
Theorem 3. Green's functions of (Da,
fields
are
finite.
Theorem 4. Green's functions of
than zero) of basic fields
are
(bigger
¥afl, and c
theories
a c
with any number of basic
dR<f> and any number (greater finite, where 0 is any basic field.
31^
Theorem 5. Green's functions of
number
of gauge
with
c a c
with
than zero) of basic fields
are
one
or two
<5R</>'s
and any
finite.
The last few results have
no intuitive appeal. They will be needed as part inductive proof of the important Theorem 1. We will also find it convenient to use CPT invariance of the theory (after dimensional
of
an
regularization). Now reversal of one time and one space component is equivalent to reversal of components 0,1,2, and 3 with a spatial rotation. So to obtain the TP part of CPT, we need only consider reversal of the 0 and 1 components only. Therefore,
we
define
if/x v 0or l,if^v>2, 0, otherwise.
f-1,
^=jI
+
Let the fields transform under CPT
=
=
1,
2)
as
</,(x)^y1tT(0x),'} 4a(x)^Aa(9xl
(12.1.13)
c"(x)-*<f(0x),
J
?"(*)- *c"(fe). Then the theory is CPT invariant. (We
y0T
=
y°*5 and yMt
=
regarded
as
y-matrices
in which
y°
=
y^) Notice that the ghost field, cfl, transforms to itself,
antighost field, ca, complex conjugate fields.
rather than to the
use
even
though
these fields might be
12.2 Proof of renormalizability 12.2.1 Preliminaries
theory provide relations between different mostly relate Green's functions of elementary fields to Green's functions containing the composite fields listed in (12.1.8)—(12.1.11). However, to prove renormalizability, we actually need relations between Green's functions of elementary fields only. Consequently proofs tend to be rather indirect and long. The following references contain a representative selection of the proofs The Ward identities of
a
gauge
Green's functions. However, the identities
12.2 Proof of renormalizability
299
in the literature: 't Hooft (1971a, b), Taylor (1971), Slavnov (1972), 't Hooft & Veltman (1972b), Lee & Zinn-Justin (1972), Becchi, Rouet & Stora (1975),
Lee (1976), Itzykson & Zuber (1980), and
Piguet & Rouet (1981). A great introduced the simplification by discovery by Becchi et al. of their the symmetry. However, proofs still are mostly rather inexplicit. The proof was
given in this section gives all the steps needed to go from the basic Ward identities to the relations between the counterterms. The method
to be
given by Brandt (1976) and Cvitanovic (1977). A point at which proofs became rather inexplicit turns out in this method to be the
follows that many
point at which the operator ¥ail (defined It is
in (12.1.10)) makes its appearance. unobvious operator to use, but its use is essential to completing the that the gauge coupling to matter fields is the same as the self-
an
proof coupling of the gauge field. The proof is by induction on the number, JV, of loops. We assume that all graphs with less than N loops have been successfully renormalized by counterterms of the form implied by the Lagrangian (12.1.1). We also require that Green's functions of the composite operators considered in Theorems 2 to 5 are also finite up to N 1 loops if the indicated counterterms are used. The induction starts with tree graphs, which need no —
counterterms.
Our strategy is
as
follows:
(1) At each order below N loops we have values of the five independent renormalizations Z, Z2, Z3, M0, and X. For each 1PI Green's function with
overall divergence
an
computed
a
value of the overall counterterm is hence
at each order less than N. Partition this counterterm into a
set of counterterms to cancel the overall
graphs
divergences
of the individual
for the Green's function.
(2) Compute N-loop contributions to Z, Z2, Z3, M0, and X by imposing renormalization conditions on certain Green's functions. The Ward identities will be true, but it is not immediate that the many other 1PI graphs are finite. This is done at step (3). (3) Using these Ward identities show that Theorems 1 to 5 hold for N-loop
graphs. technical but important. Its use is that, in order to say that the only divergence of an N-loop graph is the overall divergence, we must have subtracted off its subdivergences. However, for individual graphs the
Step (1) is
imposed by gauge invariance do not hold. Consider, for the two-Joop graph, Fig. 12.2.1. Its self-energy subgraph needs a example, counterterm of the form A {g^k2 Bxkjiv. constraints
—
300
Renormalization of gauge theories
Fig. 12.2.1.
Fig. 12.2.2.
Similarly the subgraph of Fig. 12.2.2 has As
B2k^kv.
will see, the Ward identities
we
a
counterterm
imply
A2g^k2
that the total self-
energy counterterm is obtained from the term £(Z3 l)(d^Av in JS?. It follows that Ax + A2 Bx + B2. However, the relation is -
-
=
the separate graphs, i.e.,
we
have
—
—
dvAJ2
false for
Al^B1, A2^ B2.
At steps (2) and (3), where we discuss the N-loop counterterms, we then know that the only divergences are overall ones. Moreover, we know this without
having different graphs.
to check
on
intricate series
of cancellations
between
But for the purposes of finding the constraints imposed by on the AMoop counterterms, it is convenient to consider
the Ward identities
single overall counterterm for the sum of all AMoop graphs for a given 1 PI Having obtained these constraints, we decompose the N- loop counterterms into individual portions for each graph; this enables us a
Green's function.
to
continue the induction at the next order. If we
use
minimal subtraction, the counterterms
without
by-graph
worrying about
can
the constraints
be obtained graph- imposed by Ward
identities. These constraints will be satisfied automatically. For a
in
general renormalization prescription the Figs. 12.2.1 and 12.2.2 have the form
counterterms to
example, in the subgraphs
ff2k-/^"4) + fl;], ^-^[Vtf-^ + ftj],
^
=
finite and depend on the renormalization prescription. The Ward identities tell us that we can renormalize the divergences by a
where
a\
and
b\
are
transverse counterterm
(a,
+
a2)/(d
-
4)
+
a\ +a'2= (bx
+
b2)/(d
-
4)
+
b[
+
b'2.
Evidently ax + a2 bx + b2, and a\ +a'2 b[ + b2. The first equation always be satisfied, the second must be imposed by choice of renormalization prescription. Minimal subtraction with a\ =a'2 b\ b2 0 always satisfies these equations. =
=
must
=
=
—
12.2.2 Choice We now assume that set
of counterterms
step (1) has been carried out. The next step is to pick
a
of 1PI Green's functions to fix Z, Z2, Z3, M0, and X. This is somewhat
12.2
301
Proof of renormalizability
arbitrary, but our choice will determine the form of the rest of the proof. We following set:
choose the
(1) The fermion self-energy has divergences proportional to p and to These are cancelled by counterterms in Z2 and M0. (2) Z is chosen to cancel the p2 divergence in the ghost self-energy.
(3)
X is chosen to make the
ghost-gluon coupling
finite
part is concerned. (4) Z3 is chosen to cancel the part of the divergence + /cM/cv. energy that is proportional to -
Next
we
check for
of
as
far
the
as
1.
the cabc
gluon's self-
g^vk2
will examine the Green's functions used in Theorems 1 to 5 to
possible divergences
lower order
are
at
N-loop
order. Since all divergences at
cancelled, the possible remaining divergences
are
overall
of Af-loop 1PI Green's functions. These are Green's functions with either elementary external lines or with insertions of the various composite operators we use. The dimension of a 1 PI Green's function must
divergences
non-negative degree of divergence potentially divergent. The contributions to such a Green's function are (a) AMoop basic graphs, (b) graphs with counterterms to cancel be zero or greater in order that it have
a
and thus be
subdivergences, (c) an overall N-loop knowledge of Z, Z2, Z3, M0, and X. We
counterterm derived from
must prove that the
sum
our
of these
contributions is finite.
12.2.3 Graphs with external derivatives There is
a
derivative
on a
ghost line where it exits from an interaction. Thus
the 1PI graphs of Fig. 12.2.3 have negative degree of divergence
Fig.
contributing
to
<0| Tcccc\0}
contributing
to
(0\TccAA\0)
contributing
to
<0|7c accc|0>
contributing
to
<0\T&aftAc\0y
even
12.2.3. Graphs with negative degree of divergence and non-negative dimension.
302
Renormalization of gauge theories
though their dimension is zero. counterterms
are
present,
so
Subdivergences are cancelled and no overall corresponding IPI Green's functions are
the
finite.
argument shows that the ghost self-energy needs counterterm, but only a field-strength renormalization Z. The
same
no mass
12.2.4 Graphs finite by equations of motion Consider Green's functions of (9a with basic fields. These
are
finite. For
example,
<0|T^(x)^(y)|0>
=
Znx<0|Tcfl(x)cb(y)|0> +
=
-
gKXcadc^(0\ Tcd(x)Ac?(x)cb(y)\0> idahd{d)(x
-
y).
The only graphs for the left-hand side with
(12.2.1)
subgraph are of N-loop the form Fig. 12.2.4(a) and (b). The graph (a) has a ghost self-energy made finite by its wave-function counterterm. Graph (b) needs a counterterm in Oa proportional to □ ca. Such a counterterm is graph (c), which has the N- loop contribution to the Z[Jca term in (9a. Finiteness of (12.2.1) shows that this is the correct counterterm. Similarly, the other potentially divergent Green's function of &a, viz., IPI
an
(0\TOacbAc\0) is finite.
(a)
(c)
(b)
Fig. 12.2.4. Overall-divergent graphs for Green's functions of 0
a
12.2.5 Gluon selfenergy We have the Ward 0
=
=
=
identity
(5<0| Td-Aa(x)cb(y)\0)/8lR (l/O<0|T3-^(x)5^5(y)|0>-<0|T^(x)cb(y)|0>
\£-^<0\TA^(x)Ab\y)\0}
+
i&*(x -y\
The second term involves Oa because of our definition of line we used (12.2.1) on Ga9 and remembered that Green's have the derivatives outside the
(12.2.2)
SKAa^.
In the last
functions of d definition. time-ordering, by
A
303
Proof of renormalizability
12.2
Fig.
12.2.5. Gluon self-energy.
The only possible divergences in <0|T,4jJ(x),4j();)|0> are in the AMoop self-energy graphs (Fig. 12.2.5), and they consist of terms proportional to g^M2 or to kjkv. Remember that we used Z3 to cancel any divergence proportional to g^k2 kjiv. Both the remaining divergences are absent, by (12.2.2). Thus no renormalization of the gauge parameter is needed, and no gluon mass is needed. The form of (12.2.2) is just as in QED. It implies that the gluon self-energy is purely transverse, so that the longitudinal part of the propagator is unchanged by higher-order correction. Hence, we can —
write:
ddxei?;t<0|T^^x)^*(0)|0> ^
X I A  ̄\ 111 I 11 >
= —;=
—
&??,
I
q2 + ie |_
1 +
n(<z2) (12.2.3)
12.2.6 BRS transformation of A". Consider the Ward identity 0
=
=
The first
5(0\TAl(x)cb(y)\0}/5XR (l/^)<0|r^(x)a^6(y)|0>
-
<0|TMflW^)|0>.
(12.2.4)
just proved to be finite. The only potentially for the second term are shown in Fig. 12.2.6. Equation divergent graphs shows their sum be to finite. (12.2.4) The c(y)A(z) Green's function (Fig. 12.2.7) of dRA is also possibly divergent (logarithmically). But we have: term we have
d dx"
(0\TdRA"ix)cb(y)Acv(z)\0>
=
=
=
<0\TO°(x)c*(y)Acv(z)\0> -i^(x-y)^<0|^v(z)|0> 0.
t N-lOOp
Z^AMoop
Fig. 12.2.6. Divergent graphs for right- hand side of (12.2.4).
(12.2.5)
^
9
r Fig. 12.2.7. Graphs for 0>.
<0|T<^(x)c1'(>K.W|
304
Renormalization of gauge theories
Applying d/dx* is equivalent to multiplying by k^ in momentum space. Since the divergence is at most logarithmic, (12.2.5) shows that there is no divergence
at all.
12.2.7
Gluon
self-interaction
Now
0
=
=
5<0\TA;(x)Abv(y)?(z)\0}/SlR <0\T5KAl(x)Abv(y)?(z)\0} -<0\TA;(x)dRAb(y)c<(z)\0} -<0\TAl(x)A'Xy)5tp(z)\0> 1
=
where
we
divergent,
used the and
lowest-order
we
finite
previous
have
r)
+-_<0|r^4^|0>,
(12.2.6)
result. The three-gluon vertex is
equal (Z3XZ ̄l possible left-over divergence to
a counterterm
vertex. There is no
—
linearly
1) times the that satisfies
(12.2.6).
Similarly
—<0\TAK(w)A,(x)A^ymz)\Oy=rimte. Here the
only potentially divergent AMoop
1PI
(12.2.7)
subgraphs
are
as
in
Fig. 12.2.8. We have just seen that the divergences in Fig. 12.2.8(6) are cancelled by the counterterm for the triple gluon coupling. Since the divergence in graph (a) is logarithmic, (12.2.7) proves that by the counterterm in the four-gluon interaction.
it is exactly
cancelled
crossings (a)
(b)
Fig. 12.2.8. Potentially divergent graphs for four-point Green's function of gluon.
12.2.8 SRc The only Green's function of 5Rc that could be divergent is
<0|TV"|0>.
(12.2.8)
12.2
But 0
=
=
305
Proof of renormalizability
d(0\Tca(x)cb{y)cc{z)\0} (0\TdRca(x)cb(y)cc(z)\0} -(l/£K0\Tc?(x)d'Ab(y)?(z)\0> + (l/£) < 0| Tca(x)cb(y)d-Ac(z)\0y,
(12.2.9)
finiteness of (12.2.8) follows from finiteness of the ghost-gluon vertex, which is a renormalization condition. so
12.2.9 Quark-gluon interaction,
dip, d\J/;
introduction of 98afl
Consider the Ward identity 0
=
=
d<0\TiP(x)ij,(y)c(z)\0>/dlR <0| Td^clOy <0| T^^c|0> + <0| Ti/,¥(d-A/Z)\0>. -
Finiteness of the last term follows if we can (Note that Now X
dR\j/
is related to
dR\// by the
prove
dRil/
=
—
igRXta\j/ca finite.
CPT transformation of (12.1.13).)
defined by requiring the ghost-gluon vertex to be finite. An proof, which we now give, brings in all the remaining operators was
explicit listed in (12.1.9)—(12.1.11), and in particular &afl. In Fig. 12.2.9 we list all Green's functions still to be proved finite. Observe that dRc d-A/l;9 so that it is finite if the gauge field is. Also Green's or c a c with SR(j) and any number greater than zero of functions of 98 =
a
finite, if the Green's functions of Figs. 12.2.7 to 12.2.9 are (Here dR<f) is the BRS variation of any elementary field <j>) This proves
basic fields finite.
are
Theorems 4 and 5, so it remains to prove finiteness of the Green's functions illustrated in Fig. 12.2.9. The idea behind the proof is to examine the right-most vertex on the
ghost line in Fig 12.2.9(a). This interaction
comes
from the
following
term in the
Lagrangian:
LbcgR(d^ca)cbA^x.
(12.2.10)
(a)
Z^+x%¥&^-, logarithmically divergent, contributing to
<0|T<5Ri/^c|0>
{b)
JL4fcJ¥gg-+-i logarithmically divergent, contributing
<0| T<%aflc\0)
to
Fig. 12.2.9. Green's functions not yet proved finite.
Renormalization
306
The derivative is on the line
parts,
we see
that the vertex
-
of gauge
entering equals
theories
the vertex
graph. By integrating by
0Rc.J d4x[F(3McV" + c°<?d-A'l
(12.2.11)
loop-momentum integrals, so Factoring out the field d^c* on
In the first term the derivative is outside the
degree of divergence is the external line gives the
the
reduced by one. basic vertex
cabccbActl
in the operator
{%a)1.
The
second term in (12.2.11) contains d-A which we shall relate to something else by use of Ward identities. We first prove finiteness of ^a/J, by formalizing the argument leading to
(12.2.11). This is done in
an
unobvious way:
□,<o|raRc-(x)cty)?(z)|o> (l/X){Znz<0|T^Rcac*cc|0> + gRXccde(0\ TVa<*M^d)|0>} + gR<0\TdRc°cb(ccdec'ld-A°)\0} =
-0R^<°|rV-°^(z)|O> =
=
iflM^Ol TC"(x)c?(y)|0>^?(x +igR<0\TSRc°cbdR(ccdec<>c°)\Oy
-
finite
-
z)
i0R<O| TdR<fd-Ab{c a c)c|0>
-0r^<°ItV^W|O>. by the antighost equation of motion. identity plus the nilpotence property
The next-to-last line follows line
uses a
Ward
on
hand side of (12.2.12) is
The last
(12.2.13)
<5R[<Vfl]=0. Now the second term
(12.2.12)
the last line is finite (by Theorem 5), and the left-
finite.
So the last term
divergence is of the possible finiteness of ¥afl follows. uncancelled
on
right is finite. The only Fig. 12.2.10, from which
the
form of
<
Fig. 12.2.10. Only possibly divergent graph for (12.2.12).
12.3 More general theories
SRij/ follow by
Finiteness of
the
same
307
manipulations applied
<0|r,5R#c|0>: □ ,<O|7A<Kx)#(30?(z)|O> =0R<o|rtv(x)#oo|o>a*V-*) -igR<0\T5^3^(0 a c)\0}
to
(12.2.14)
-9K^\TSRiP(x)il}(y)^(z)\0y. 12.3 More general theories
proved renormalizable the simplest gauge theories: gauge group with a single component and with fermion matter fields. More general cases can easily be treated by the same methods. The general result is that renormalizations are needed for each independent coupling in the basic Lagrangian and for the field strength for each irreducible field multiplet. Let us examine some specific generalizations. In the last section
that is, those with
we
a
12.3.1 Bigger gauge group The gauge group can in general be a product of several components: G Y[ni i G{ ? ^U)v- Here each G{ is a simple compact non-abelian group =
=
(like SU(N)\
and there
factor there is
are v
abelian l/(l)'s. For each Gt and for each (7(1)
independent coupling g.m When we perform gauge fixing there will be a multiplet of ghost fields for each component of the gauge group. The proof of renormalizability will need no change. It will relate the an
renormalizations of the couplings within each multiplet. Thus for each of the n + v components of the gauge group there are renormalization factors Xi9Z3ii and Zh for the coupling, the gauge field and the ghost field. In addition there are the usual renormalizations for the matter fields. There
are some
special features
of the abelian
case
which
we
will treat in
Section 12.9.
12.3.2 Scalar matter The part of the
Lagrangian
for
a
scalar field coupled to gauge fields is
D^D^-V(<f>\<f>). Here Fis
DM
a
function of <\> and
(12.3.1)
¥ that is invariant under the gauge group, and
9ovariant derivative. example, consider an SU(2) gauge theory
is the usual
For
in which ¥ is
a
doublet
308
Renormalization
of gauge
under the gauge group. Then the most
theories
general renormalizable form of Kis {1232)
V=m2<p4>+\k(<p4>)2,
while
D^D<j>
=
=
Wave-function and
\d^\2 |3M0|2
IgA'^ffy + g2AlAb^tatb<f> \gAl4?f3"4> + ig2Aa2^<t>.
-
-
(12.3.3)
are used to make the propagator is cf)fd(f)A coupling proved finite as for a fermion. The self- of the coupling 0-field is made finite by a renormalization of the Xft 02 term, which is the only four-point coupling invariant under global SU(2) transformations. One further Green's function, viz., <0| TAA(f)f(f)\0y has a mass
renormalization
finite, and the
potential logarithmic divergence.
=
It is
proved finite by the Ward identity:
+
(l/^)<0|Ta-^^(/) (/)|0>-h<0|Tc^(/)t(/)|0> + <0|Tc^^V|0> + <0|Tc^(/)t^|0>.
(12.3.4)
12.3.3 Spontaneous symmetry breaking Consider the abelian Higgs model as an easy example. is
^basu
=
-i^2v + (3?
"
ieAJjUP + ieA?)<t>
-
Its basic
Lagrangian
|AVtf> -/2/2A2)2- (12.3.5)
The symmetry is spontaneously broken with < 01 </> 10 > =//(A we write </> [f//. + (</>l + i</>2)]/
order. So
^basic
=
Jl:
=
-
i^2v (e2f2/2l2)A2 +
ief/VA^fa + h^\ + \ty\ -/2<A2/4 MM* + 4>\)IA +
^(4>\ + </?l)2/16 + eAli<pid"4>2 + e2A2(<}>2 -
If
we
quantize with
-
the
*J2), in lowest
simple
+
<j>2 + 2/>1/A)/2.
gauge
fixing
(12.3.6)
term
(12-3.7)
**=—¥? A?,
by the discussion of Section 9.2, where we breaking in a non-gauge theory. We first renormalize in the unbroken theory (i.e., with/2 f2). We need wave- then renormalization is covered
treated spontaneous symmetry
-?
—
function renormalizations Z2 and Z3, coupling renormalization A2 -? /£ Y9 and mass renormalization (which is effectively f2-+f2Zm). Since the Faddeev-Popov ghost is a free field, the gauge-coupling renormalization is 1. After spontaneous symmetry breaking the same renormalizations X =
12A Gauge
dependence of counter terms
make the Green's functions finite.
Lagrangian makes use
309
Unfortunately the AJj*¥2 term in the free
the Feynman rules rather messy, so it is convenient to another gauge condition. We will discuss this in Section 12.5.
12.4 Gauge dependence of counterterms To
quantize and renormalize a gauge theory, we choose to fix the gauge in a particular way. It is important to show that physical quantities are independent of this choice. On the other hand, Green's functions of the elementary fields can certainly be gauge dependent. Since it is the Green's functions with which
we
work when renormalizing the theory,
we must
understand their gauge dependence and its effect on the renormalizations. One important class of physical quantities which we will treat is the set of
gauge-invariant operators. Also important is the S- physical states. In a spontaneously broken theory there are particle states that couple to the elementary fields of the theory. Some such states are physical, and we must prove that their S-matrix is gauge independent. But other states, like the Faddeev-Popov ghost, are unphysi- cal. We will not attempt a complete treatment here. Green's functions of
matrix for
When a gauge symmetry is unbroken there may even be no states
elementary fields. Indeed, it is commonly expected that in QCD colored states are confined. Certainly, within perturbation theory there are severe infra-red problems in obtaining the S-matrix for quarks and gluons. However, S-matrix elements of hadrons can be obtained from
coupled
to the
gauge-invariant Green's functions. Consider, for example:
<0| ^K(w)/5A(x)/5/i(>;)/5v(z)|0>,
(12.4.1)
wherej^ is the axial isospin current. Application of the LSZ formalism will give the 5-matrix for %n -? %n scattering. Similarly the gauge-invariant field *l/i*l/j*l/k£ijk is an interpolating field for baryons. Here ^ is a quark field and i its color label. There are two
sorts
of gauge-dependence that we will consider. The first is
change the gauge-fixing function Fa to a different function of the specific application of our general results for this case will be given Section 12.5 for the i^-gauge. The second type of gauge-dependence is a
where
we
fields. A
in
variation of the parameter <!;. Although variation of i by a factor X is equivalent to variation of Fa by a factor I  ̄1/2, there are a number of special simplifications that will lead us to treat this second case first. The
use
of IJRS invariance will make
dependence very
simple.
our
computation
of gauge
310
Renormalization of gauge theories 12.4.1
Change oft
The Green's functions and the renormalization factors in general depend on <!;. We
can use
the action
Green's function with
no
principle
<0|TAT|0>.
explicit dependence
to compute the
Here X denotes any
dependence on ^ of a product of local fields
<!;. Then
on
^<0|rX|0>=ild4K0|r:—{y):X\0> +
^td*y<0\T:F2a(yy.X\0>.
In the first term we use Y to denote any of the
factors. vacuum
As usual, we
expectation
use
the
symbol:
value. Next
:
we use
(12.4.2)
independent renormalization
to
indicate subtraction of the
the form of the BRS
transformations to write
(12.4.3)
(l/Z)F2a=d(caFa)-cadFa9 and substitute for
F\
in
(12.4.2).
We
can use
d(0\TcFX\0} and the
equation of
=
the Ward identity 0
motion
<0\TcadFa(y)X\0y= ̄i^(0\TcJX/dca(y)\0y a
to
give
|<0|TX|0>=iE|JdV0|y:i^|0> -^fd4>;<0|T:caFa:^|0>
+
(Nc/20<0|TX|0>. (12.4.4)
Here
Nc is
the number of factors of the
Suppose first that
we
choose to
use
the
ghost field
in X.
same counterterms
for all values
particular value, say £ elementary fields will not be finite except at this special value. But if we take a Green's function of gauge-in variant operators then (12.4.4) indicates that the Green's function is gauge independent, for such operators contain no ghosts. of £
as
at
some
=
0. Then Green's functions of
12.4
Gauge dependence of counterterms
Next consider a Green's function of
<0|7X|0>
=
elementary
311
fields:
<0|Tn^W)|0>.
(12.4.5)
i=l
are adjusted to keep Green's functions finite for (12.4.4) we find the ^-dependence of the counterterms
The renormalizations in <£ any value of f. Then in
by
requiring
the dYd^
\df dY'' term to be the sum of the counterterms needed to cancel the
divergences of
-:caFa:/(2a We consider the ultra-violet
divergences
of
<0\T:caFa(y):dX\0}.
(12.4.6)
Divergences in (12.4.6) interaction vertices
or
occur either when y coincides with some set of when it coincides with one of the x/s, which are the
positions of fields in X. We can renormalize Green's functions of :caFa: with elementary fields by adding to it an operator :D(y):. Since we choose X to be a product of elementary fields, the only remaining divergences in (12.4.6) are when y coincides with the position x of a BRS varied field d(f)(x) in SX. To renormalize this divergence we need counterterms to (12.4.6) of the form
<0|r:D(jO:W|0>+ £ aw(y *,)<0| TAT lO)^.^,,,, -
(12.4.7) The counterterm when y coincides with -
iE<p(x)d(y
—
x), there
normalization factor
-
are
possibly
d<j)(x)
has been written
as
derivatives of the ^-function, and the
i will be convenient later.
By use of Ward identities and equations of motion,
we can
write (12.4.7)
as
-<0|T:5i)(y):X|0>-X<0l7,:£^)^/^(y):X|0>.
(12.4.8)
This is the most general form of counterterm needed to keep the Green's function finite as £ varies. It is even the correct form for the change in counterterms caused by a change in the form of Fa9 as we will see in Section 12.4.4. To
proceed
any further we must do
which counterterms
to determine
occur.
restrict attention to the gauge condition Fa d-Aa. The only illustrated Green's functions by their elementary of:caFa:are
We now
divergent
actually
power-counting =
of gauge
Renormalization
312
theories
XT0" Fig. 12.4.1. Lowest-order graph for
Fig.
lowest-order
graph
has
a
cases
in
(0\TcA:caEa:\0)
Figs.
12.4.1 and 12.4.2. The
divergence
Since
integrate
over
to
only contribution caFa.
:caFa: with
12.4.3. Lowest-order graphs for
divergence proportional to fa as The
=
all y, the
operator D is proportional
Fig.
of the first
factor of the derivative at the ghost interaction. The (d^cJA* derivative caFa.
counterterm for Fig. 12.4.2 is proportional to we
graph for
12.4.2. Lowest-order
<0|rc:c.F.:|0>.
with
—
to the counterterm
BRS variation of
a BRS-varied field 6<j> is in the graphs of Fig. 12.4.3. So
logarithmic we
have
a
field.
and
£0oc0.
This
corresponds to a variation with £ of the wave-function renormalizations with g0 and M0 fixed. For example, dZ,d&
d£ dZ2 A
complication arises since
dZ3d& d£ dZ3
lainZ,
Hil/SS/Si)/ + il/dS/dij/)
2
d£
we
know £ is not renormalized:
ldlnZ, 2
;(S±F2a/2£)
l5A<
d£
+ F2J2Q). mnZ-3(A-gdS/SA d£
2
Putting
all this work together,
we
|<?-s(1+S
(12.4.10)
find
d*y(0\T:caFa(y):SX\0) ,
dlnZ 2
-<o|r*|o><;jv,-^
,
+
NA8lnZ3 ft
rain(zzn i
CL This is just the
(12.4.9)
ft
2
(12.4.11)
counterterm structure we need. The operator
caFa is
12.4
Gauge dependence of counter terms
313
multiplicatively renormalized by theory
no
a factor 1 + £d In Z3/d£. (In an abelian renormalization is needed since ca is a free field; so Z3 is
independent of £.) The other terms (where iV0 is the number of external <\> fields of X) come from renormalizations of 1PI graphs including both caFa and
a
6<j).
Z3/3^ occurs in two places. This comes from assuming that gauge-fixing term is d-A2/(2^) with no extra F2/(2£) renormalization factor. We proved this from the Ward identity: Note that d In
the
0
=
=
=
—
=
—
d(0\Tca(x)Fb(y)\0} (l/?<0\TFa(x)Fb(y)\Oy + (0\TcadFb\0} (\/ZK0\TFa(x)Fb(y)\O>-id(x-y)dabi
(12.4.12)
where the last line follows from the ghost equation of motion. Since Fa d-Aa, its Green's functions are finite, and so £ is finite. We can use (12.4.11) to prove gauge in variance of the S-matrix by picking out the residue when we go to the particle pole for each external line. If we use the correct, <!;-dependent counterterms in j£f, then a gauge- =
invariant operator like
(G°v)2
=
(d^Al
-
dvA; + QoZ¥cabcA\A<y
is actually <!;-dependent; from (12.4.11)
we see
that is
(12.4.13)
proportional
to
Z^i.
But the bare operator
(^(0)/iv) is gauge
=
independent.
(^i
 ̄ ̄
(0)v
°v^(0)fi + 9oCabc^{0)n^{0)v)
Both operators have UV
divergences,
so
that they
must be renormalized.
12.4.2 Change of Fa Let
us
assume
that Fa
interpolate between same
method
as
depends
two different
on
a
parameter
gauge-fixing
k.
This allows
us
to
conditions. We follow the
for the ^-dependence. The change in the
Lagrangian
is
d^=Y——--Fd-^ ̄-^ Ca dY 3k 3k
ydK
f
a
dK
So die
<0|TX|0>
=
ixgfd4y<0|T:^(y):AT|0> +
f
8F
i\d'y<0\T:ca-^:8X\0>.
(12.4.15)
314
Renormalization
of gauge
The counterterms for the second term have
in the
theories
exactly the same form as we met
Green's function, i.e., the form (12.4.8). Until the form of Fa is specified we can make no further statement about the necessary counterterms. We will examine an example of this in Section 12.5.
^-dependence
of
a
Green's function of gauge- invariant operators and omit any /c-dependence of the counterterms in <£. 0. Then the Green's functions are gauge-independent: <9<0| TX\0}/6k In the most general
case
we can
take
a
=
12.5
R^-gauge
mixing between the gauge field and the scalar field in broken spontaneously gauge theories one can use the i^-gauge devised by't To eliminate the
Hooft (1971b), Fujikawa, Lee & Sanda (1972), and Yao (1973). Let us use the Higgs model of (12.3.5) as an example. The i^-gauge is defined by using the
gauge-fixing Lagrangian J2V
We
use
the parameter
and the gauges
we
-
=
k to
(1/2£)(<M
interpolate
-
(12.5.1)
K^m<j>2)2.
between the
used in Section 12.2, where
k
i^-gauge, where
=
0. Also,
interacting Faddeev-Popov ghost is needed, even theory (which is the only case we will explicitly treat): An
j£?gc
=
d^cd^c
—
The BRS variations of the fields
6<f)2
—
=
(m
K^m2cc
—
=
dc
=
=
1,
ef/L
an
abelian
(12.5.2)
} F/£9\
(12.5.3)
are:
+
e^Jc,
(d'A-£Km(t)2)/£
=
0.
Observe first of all that the extra so
couplings relative to the gauge k
=
0 are
that the renormalizations of the dimensionless
super-renormalizable, couplings can remain unchanged. all
in
k
efynK<f>xcc.
Hi=e<l>2c, dc
m
=
These
are X
(which equals unity in
an
abelian theory), Y, Z2, and Z3. To determine what further renormalizations are needed, we use the method of Section 12.4.2, with dF/dK £m(/>2. =
Thus
we
—
need the counterterms to -ifm
\d*y(0\T:c<f)2(y):5X\0y.
(12.5.4)
The only divergent graphs contain c</>2, the BRS variation 6<f>u and other external lines. They have the form of
Fig. 12.5.1,
which is
no
logarithmi-
12.5
*■
^4>iV
315
Rr gauge 7^1
=
eC(t>2
Fig. 12.5.1. Divergent graphs for (12.5.4).
cally divergent. Notice that interactions on the ghost line would make the graph convergent. The counterterm to (12.5.4) therefore has the form
cfdV<0|T^/^1W|0> =
iC
(12.5.5)
d4KO|r:5S/501O'):A'|O>.
This counterterm is generated by
a
shift of <j>1 in the
Se(4>1+mlei4>2iA,cic)^£e(4>1
+
Lagrangian:
(12.5.6)
Bm/e9<f)29A9c9c)
with
?^ e
As far
as
=
(12.5.7)
C.
ok
renormalization is concerned, the effect of using the new gauge an ultra-violet divergence in the vacuum
condition is to generate
Although this might appear strange, it is permitted to happen since the field is not gauge invariant. Gauge-in variant Green's functions are unchanged, of course, as is the S- matrix. The ghost field, 02, and the longitudinal part of A^ all couple to unphysical states. In this abelian theory, F^ is a gauge-invariant field which expectation value of the scalar field.
couples to the transverse part of the gluon, while
ZmZ2^4>
=
=
ZmZ2l(4>1
+
ZmZ2(Bm/e)2
Bm/e)2 +
+
tf\
^ [2(ftn/e)ZmZ2]
+
?
?
■
is gauge invariant and renormalized, and couples to the (f)1 -particle. The Lagrangian can be written out in terms of the fields, to exhibit the interactions. It is rather fearsome-looking. We set k 1 and find =
£e
=
-iZ3F2^ (l/2£)d-A2 + \m2B2Z2Al + dcdc -
-
m2Blcc + AJd^2m(Z2B
-
1)
iZ2(d(t>Ud(t>22)-i(t>2m2^e-2(3B2Y-Zm) -h<t>Wte + \A2e-\B2Y-Zm)-] -^A2y(^ + ^)2-^2me-15y01(^ + (/)22) -iA2m3e"3B(B2y-ZJ01 + eZ2A¥xH2 +\e2Z2Al{ct>2 + <j>22) +
+
emBZ2<\>xAl
-
im<\>xcc.
(12.5.8)
Renormalization of gauge theories
316 The
same
methods
complications
can
be
applied
theory with extra Fujikawa, Lee & Sanda
to a non-abelian
in the structure of the vertices
-
see
(1972). 12.6 Renormalization of gauge-invariant operators
applications such as to deep-inelastic scattering we need the operator product expansion of gauge-invariant operators. It is natural to assume that only gauge-invariant operators appear in the expansion. Moreover, in using the operator product expansion, we need the anomalous dimensions In
of the operators. To compute these, we need the expression for the renormalized operators in terms of the bare operators. Again, it is natural to that only gauge-invariant operators are needed. Both assumptions, taken literally, are false (Dixon & Taylor (1974), and Kluberg-Stern & Zuber (1975)). We will first treat the renormalization
assume
problem, where certain gauge-variant operators mix with gauge-invariant operators. As we will see, the gauge-variant operators vanish in physical matrix elements (Joglekar & Lee (1976), Joglekar (1977a and b)). We will simplify many parts of the proof by working in coordinate space. Then we will apply the same methods to find the operators that appear in the operator
product expansion. ordinary global symmetry (like
In the case of an
Lorentz invariance
or
isospin), operator that is invariant under the symmetry mixes only with invariant operators. In the case of a gauge theory, we break the invariance an
of the action
by gauge-fixing, leaving only
a
BRS invariance. But
invariant operator even mixes with operators that How can this be?
To
answer
this question,
we
consider
an
are not
a gauge- BRS invariant.
operator 0 that is invariant
under some given transformation. Let C be the sum of its counterterms, and assume
the action is invariant. Then the Green's functions
under the transformation, o
=
=
=
Now C is defined finite. For
<5<o|ta:(0
+
c)|o>
io\Tsx{o
+
c)\oy
+
io\TXS(o
(0\TdX(V
+
C)\0}
+
(0\TX 5C\0>.
so
are
invariant
so
+
Q\oy (12-61)
that Green's functions of 0 + C with the variation of an
elementary fields elementary field is
ordinary symmetry, elementary field, so the first term on the right of (12.6.1) is finite. Hence the other term, an arbitrary Green's function of SC, is finite. If we use
are
again
an
an
minimal subtraction, the counterterms are powers of 1/e, so SC is finite only if it is zero. Thus counterterms to an invariant operator are invariant.
12.6 Renormalization But when 6 represents
of gauge-invariant operators
317
BRS transformation the variation of
a
an
elementary field is composite. So finiteness of(0\TSX(G + C)\Q} does not follow from finiteness of the elementary Green's functions of 0 + C. Hence the counterterms to a gauge-invariant operator need not be BRS invariant, let alone gauge invariant. This argument also shows us how to handle the problem. Let X be a (t)1(x1)...(f)N(xN) of local fields. Then the only divergences
product
< 01T5X ((9 (y) the position of
+
of
C(y)) 10 > that lack a counterterm are when y coincides with
a BRS-varied operator <5</>f(x.) in SX. Hence, provided y is not equal to any xi9 (0\TSX((9{y) + C(y))|0> is finite. From (12.6.1) we then see that <0| TXdC\0} is then finite. But if we use minimal subtraction this
means
that it is
zero.
Hence
an
arbitrary
Green's function
SC with elementary fields is zero. Thus, as counterterms to a gauge-invariant operator
an are
<0| TX5C\Q}
operator, <5BRSC
=
of
0: tne
BRS invariant.
try verifying this theorem by explicit calculations (e.g. Kluberg-Stern & Zuber (1975)). These apparently contradict the theorem. However, the non-invariant counterterms must correspond to operators that vanish by the equations of motion. As usual, the treatment of One
can
derivatives of fields in covariant perturbation theory implicitly generates commutator terms. After Fourier transformation into momentum space the non-invariant operators are not manifestly zero.
actually interested in are gauge-invariant operators rather BRS-invariant merely operators, for it is only the gauge-invariant that have physical significance independently of the method of operators gauge-fixing. So we must show how it is that the gauge-variant operators that mix with (9 do not enter into physical quantities. Let 0u...,0n be the set of gauge-invariant operators that mix with (9. These are the operators which have the same transformations under global symmetries (e.g. Lorentz, isospin) as 0, and whose dimension is at most that of (9. Choose this set so that it is linearly independent (after use of the equations of motion). There are three other classes of operators that mix What
we are
than
with (!): Class A These operators, Ai9
At
=
are
the BRS variation of
operator:
5At
Class B These operators, Class C
Any
Bi9 vanish by the equations of motion.
other operators that mix with (9, and that
combinations of
At\ B(%
are not
linear
and #.'s.
nilpotence of BRS transformations vanishing by the equations of motion.
The
some
ensures that
SAt
=
0 up to terms
318
Renormalization
of gauge
theories
The most obvious classes of BRS-invariant operators are the gauge- invariant operators 0t and the BRS transformation of operators bA{. The first result to prove is that there are no others; i.e., there are no operators of
class C. The second result is that the renormalization matrix expressing the renormalized operators [#], \A\ and \B\ in terms of unrenormalized
triangular:
M [A]
^7 =
M
i
\
7
7
0
Zaa
Zab\
0
0
\°\
operators is
A
ZBB\ w
It is easy to prove this second result. The only operators that
counterterms to
(12.6.2) can
be
class B operator must themselves vanish by the equations of motion, i.e., they are of class B. A Green's function of a class A operator SA.
a
be written
can
as
<k0\T5Al{y)X\0y=-(0\TAt5X\0). Thus if the
counterterm to
A{
is
C(At)
(12.6.3)
then
<0|T[^ + CM.)]<5X|0> is finite if y equals
SC(At)9
of the
xt\ Hence the vanishing by the equations
none
modulo terms
counterterm to
SA( is
of motion. That is, the
counterterms to At SA- are class A and class B only. The proof that there are no class C operators is somewhat complicated. It =
essentially a mathematical exercise in homology theory. We refer the reader to Joglekar (1977a, b) and Joglekar & Lee (1976) for proofs. The importance of these results is as follows: both class A and class B
is
operators vanish in physical matrix elements. For class B this is because of the equations of motion. For class A we obtain the matrix element by the
LSZ reduction formula from a Green's function < 01TA tX10 >. But there is
Ward
a
identity
<0|T<M>|0>= -<0|7\3.(5A:|0>.
(12.6.4)
Whenever X is gauge invariant we get zero. But we may also use elementary field operators in X. A term in 6X with a varied field operator 6<f>(x) has no physical particle pole for this line. This can be seen by examining the possible Feynman graphs, and observing that 6<j> contains a ghost field. If the renormalization matrix in (12.6.2) were not triangular then renormalized operators of classes A and B would be non-zero on-shell, even though the unrenormalized operators are zero. The triangularity ensures that for
physical
matrix elements
\P\ Thus
we can
disregard
we =
have
ZecQ.
the non-invariant operators.
(12.6.5)
319
12.7 Renormalization-group equation 12.6.1 Caveat
practical calculations with Feynman graphs one must beware of taking (12.6.5) too glibly. Consider the calculation of [G^] in QCD. The only gauge-invariant operator to mix with it is {J/ij/, so we can write (12.6.5) as In
(&}) (*? ZA(G^\
(12.6.6)
=
The Z21-coefficient is
zero
because
{j/i//
has lower dimension than G2.
Fig. 12.6.1. One-loop graph for We compute the
There
are
one-loop
term in
proportional
graph
of Fig. 12.6.1.
0vvP2
 ̄
Ga2x
to
Tensors that vanish on-shell are, for
simplify
from the
various tensor structures for the counterterms. The
counterterm is
To
Zn
<0|T^[G;Jv]|0>.
example,
9Hv(P + <l)2
Pflv
the calculations it is
 ̄
(P
tempting
+
<l\(P + ?)v
to set q
=
0. But then these
equal and it is not possible to separate the three coefficients. If one sets p2 0 (p + q)2 so that p and q are on-shell, and then multiplies the graph by polarization vectors e^e,v satisfying e-p 0 e'-(p + q\ then one has taken a physical matrix element and only the three structures
are
=
=
=
=
Ga2v
counterterm survives.
The calculations of
Kluberg-Stern
momentum, and they have to go to
12.7
some
& Zuber
effort to
(1975)
overcome
are at zero
the above
problems.
Renormalization-group equation
are derived in gauge theories just in as any other theory. One feature that is easy to overlook when making calculations is the variation of the gauge-fixing parameter under a
Renormalization-group equations
renormalization group transformation. We consider the theory (12.1.1) and let ^Nc.Nf.tfg ^e the renormalized Green's function with Ng external gluons, Nf fermions, Nf antifermions, Nc ghosts, and Nc antighosts. The corresponding bare Green's function is (7(0)
_
fNcyNfy]Sg/2rT
Renormalization of gauge theories
320
and it is RG invariant:
-l"£ Here
/?,
?S-'""'5=-?S>cM-
+
yM and 6 are obtained
by requiring
#0,
(12.7.1)
M0, and q0
=
Z3£ to
be RG
invariant. In the minimal subtraction scheme we have
g0
=
V2-d/2g\l+ fa^)/(4-^r L
n=\
(12.7.2) n= 1
2. Here
=
i+
I c,n(3,a/(4-rfr.
Zf stands for Z2, Z3, or Z. p
=
=
The
renormalization-group coefficients are
(d/2-2)g0/(dg0/dg) (d/2-2)g + ±gdaJdg,
yM=pdlnZM/dg=-lgdbl/dg^ y.
d
=
=
(12.7.3)
pd\nZi/dg= -\gecixldg, y3=
-jgdc3A/dg.
Similar formulae hold in any other subtraction scheme. Observe that, by the results of Section 12.4, g0 and ZM are independent of £ so that p and yM are independent of £. But Z2, Z3 and Z depend on ^, so that y2, y39 y and d also depend on <!;. As a special case, in an abelian theory, n2 ̄d/2gZ3112, so that Z3, y3 and 6 are independent of <!;. #0 =
The functions
renormalization-group equations
are
0-[fi^Ncy
+
N(y2+\Ngy3)G
&Zj£ Since the anomalous dimensions
equation
for renormalized Green's
then
are a
little
Landau gauge £
=
complicated.
are
+
Ncy + Nfy2
^-dependent,
computed /?
*Jty3 )G.
(12.7.4)
the solutions of the RG
The most convenient gauge to
use
is the
0, for then £ does not vary when a renormalization group
transformation is made. Tarasov Vladimirov to
+
three-loop order in this theory.
& Zharkov
(1980) have
12.8 12.8
Consider
321
Operator-product expansion
Operator-product expansion
the Green's function
M(y;xl9...9xN)
=
(0\Tj(y)j(0)X\0}.
(12.8.1)
where j is a renormalized gauge-invariant operator, and X is a product of local fields at points xl9...9xN. We have the Wilson expansion
M(>;;x1,...,xN)^XCI.(>;)<0|r[^(0)]X|0>.
(12.8.2)
i
all renormalized operators of the appropriate dimension. This expansion is proved in a gauge theory the same way as in any other
The
sum
is
over
theory.
application to deep-inelastic scattering in Chapter 13, we will take be a product of two gauge-invariant operators and use the LSZ
In the
X to
reduction formula to obtain
a
matrix element:
W=<P\Tj(y)j(0)\P> EQ<P|[^0)]|P>.
(12.8.3)
=
i
We would like to show that only gauge-invariant operators need be included in (12.8.3). Now the proof of the operator-product expansion (in Chapter 10) treats the right-hand side of the expansion in the same way as
renormalization counterterms. So the method normalization of
gauge-invariant operators
applied applies to
we
also
to the
re-
the Wilson
The operators G{ are either gauge invariant, are the BRS variation of something, or vanish by the equations of motion. Then if we
expansion. keep
y
=£ 0 in (12.8.3) (as is the
case
invariant operators. The renormalized operator j has,
in
applications)
according
we
only
to Section
need gauge-
12.6, the form
8r* + B,
(12.8.4)
where 7GI is gauge invariant and B vanishes Hence if we take a matrix element of j(y)j(0)9
by the equation of motion. like (12.8.3), (or if we take a
J=Jgi
gauge-invariant terms,
so
Green's function
+
ofj(y)j(0))9 then we can drop the
A and B
that
<P\Tj(y)j(0)\P>
=
<P\TjGl(y)jGl(0)\P>.
(12.8.5)
We now follow our proof in Section 12.6, starting with the Ward identity 0
=
=
Here
we
ShRS(0\Tjiy)j(0)X\0} <0\Tj(y)j(0)5X\0}.
assumed that the positions y
9
xifl9 and 0
(12.8.6) are
all distinct,
so
that
322
Renormalization of gauge theories
by the origin,
equations of motion. But if y so that:
<0\Tj(y)j(0)dX\0}
-
=
-?
0, then
none
of the
xifl9s
are at
the
XC^)<0|7[^.(0)]^|0> -
Xc,0)<o|r[^r,.(0)]x|o>.
(12.8.7)
i
But the left-hand side is zero, by (12.8.6), while the right-hand side is its leading behavior as y-> 0 and is therefore also zero. If an arbitrary Green's
function of
an
operator is zero, then the operator itself is zero, i.e.,
XQ [>',.]
=0.
(12.8.8)
i
Hence the operators needed in the expansion are those that are BRS invariant. As in Section 12.6, this means they are either gauge invariant or
of classes A and B. In physical matrix elements like (12.8.3)
we can
therefore
restrict the operators to be gauge invariant. The practical use of this result is to compute the Wilson coefficients by taking the state |P> to be a state of one on-shell quark or gluon. The
infrared divergences are regulated by going to d > 4. The Wilson coefficients and the gauge-invariant renormalization counterterms of the tt/s can be unambiguously obtained provided care is taken to separate the IR from the UV divergences (both of which appear
as
poles
at d
=
4).
12.9 Abelian theories: with and without photon
mass
Consider QED with a possible mass term for the photon. The Lagrangian can be expressed in terms of unrenormalized or renormalized fields:
=
Here
we
have
a
-
single
iZ3Flv + k<Z,Al + ZJW vector field A
Z>>
=
=
,
and
we
-
M0)f
(12.9.1)
let
(^-ie04>A (f^-ieoZ^>.
(12.9.3)
A^ the photon. Without a mass term for the photon, Lagrangian (12.9.1) is invariant under the gauge transformation We will call
A? A? + d^io,  ̄j > ?A-+tAexp(ie0Z.W ij} ij) exp( ie0Z\!2a)).)
the
->?
-*
—
(12-9-4)
12.9 Abelian theories: with and without With
a mass term
photon
323
mass
be
the Lagrangian is not invariant, but the theory as we will see.
can
non-abelian theory with
is not
consistently treated, In
contrast,
a
consistent. For example, there
are
gluon
a
unphysical
states
-
term
mass
the
Faddeev-Popov
which violate the spin-statistics theorem, and the negative metric gluon states. In the massless theory these cancel in sums over intermediate states, but in a massive non-abelian theory they do not cancel. These
ghosts
-
problems do not
occur
in the abelian theory with
a
massive photon.
12.9.1 BRS treatment of massive photon Even with we use
a mass term
for the photon, the Lagrangian is BRS invariant, if fixing. We write
the standard gauge <£
=
j£'.nv
-
(l/2£)d-A2
+
d^c
-
(12.9.5)
£m2Z3cc.
and define BRS variations
SA^
=
aMc, Sij/ (5c
=
=
ie0ZV2oA> 5¥ 0,
-
ie0Z^c9
dc= -d-A/l
Then if is BRS invariant, aside from the Faddeev-Popov ghost is
=
an
(12.9.6)
irrelevant total divergence. Since
free field, it can be omitted without affecting physics (except gravity). We may use the general methods of Section 12.2, with the result that the theory is renormalizable. Since the ghost is a free field, the renormalization a
any
factors X and Z
are
both unity. Hence
(12.9.7)
e0=eRZ;1'2
e is gauge independent. (If we use minimal write eR ja2 ̄d/2e.) We now find that e0A{?] eRA^ and that the covariant derivative and gauge transformations simplify:
and the renormalization of
subtraction
we
=
=
(12.9.8)
(d^ieRA^, xjj^^xjj.
D^
=
Using this method we also find that ml finite
mass.
Then the
Lagrangian
^=
the
1m2, so that the ghost field has
(12.9.10)
Z2iA(if? + e4- M0)ip,
ghost ignored, since it is
counterterm
ZJ
is
-iZ3Flv + ¥m2Al-(l/2£)d-A2 +
with
=
(12.9.9)
for the photon
a
free field. Note that there is
no
mass.
Similar methods apply in theories where the gauge group is
a
product of a
324
Renormalization
of gauge
U(l) factors.
non-abelian group and one or more
theory
of weak and
gauge group
12.9.2
electromagnetic 5(7(2) ?(1).
Elementary
treatment
theories
Weinberg-Salam simple case with
The
interactions is
a
-
of abelian theory with photon
mass
sophisticated than necessary for the the theory by elementary methods. First we write the free photon propagator for the perturbation theory of the The BRS methods abelian theory,
are
so we
much
will
more
now treat
Lagrangian (12.9.10): i Duv Mv
=
 ̄n
2
(
?
m
=
0, then
we
+ Guv y"v
j
r_ 9^
cannot
remove
m2 + ie
If
 ̄
k2-m2+i£K ,
' kukjm2) " v/
i
k k
t^—:f-|—- m2k2-£m2+ie j
(i-g)Mv im2 + ie
[
(12.9.11)
k2- the
gauge-fixing
term, for then the
propagator does not exist. (This is the same as taking the m is non-zero, then the propagator exists when the
ik^kjk2m29
is removed. However it behaves like
limit ^ oo.) But if gauge-fixing term
rather than
-?
1/k2.
So the
has worse divergences theory with m 0 and £ manifestly renormalizable. Even so, physical quantities are independent of £ and the theory is renormalizable if £ is finite, as we will see. Hence enough cancellations are present as £ that the theory remains renormalizable if =
=
oo
-?
we
than usual and is not
oo
only compute physical quantities.
Our treatment makes extensive
use
of the equations of motion:
AS =
=
Z3m2A^ + Z3(D^"
=
0,
-
d?d-A)
+
d*d-A/£
+
e0Zj/2#ytyZ2
AS
<?+
=
|^
=
Z2¥(
-
i 0-
M0)
=
0.
(12.9.12)
We have not assumed the relation (12.9.7). gauge-field equation of motion gives 0
Here,
we
=
(n/Z
+
Taking
m2)d'A.
the divergence of the
(12.9.13)
have used the invariance of the theory under the
transformations, which
are
(12.9.4) with
global
constant co. The Noether current
12.9 Abelian theories: with and without photon for this in variance is the
electromagnetic
current:
(12.9.14)
jr=-e0zy2Z2¥W. This is conserved, by the electron's equation of motion. The result (12.9.13) that d-A is a free field is important for three
(1)
It would otherwise be difficult to
interpret
the
which has
(2)
The
325
mass
reasons:
longitudinal part of A^
unphysical properties. <!;-dependent part of the propagator
is confined to the kjiv term, which only contributes to d-A. Then the ^-dependence decouples from physics if d-A is free.
(3) Similarly the bad ultra-violet behavior when ^
-?
oo
is decoupled from
physics. To make these statements
We will prove
(1)
Ward identities,
(2)
m2,
(3)
no counterterms are
=
precise we will examine the Green's functions.
directly:
Z^lm2,
so
that
no mass counterterm
is needed for the photon,
needed for the d-A2 term,
eRZz 1/2, with eR finite, but not Z3 depends on ^, (5) Z2 (6) the S-matrix is independent of <!;.
(4) e0
=
We will also compute the subtraction is used, and
we
exact
^-dependence of Z2 when minimal ^-dependence of the pole.
will compute the exact
residue of the electron's propagator
12.9.3 Ward identities Green's functions of &A9
<£^ and S£^ are non-zero, since the derivatives in
are implicitly taken outside the time-ordering. To obtain identity corresponding to (12.9.13), we use identities like
these operators the Ward
<0|T^(x)X|0>=i<0|7^^|0>.
Now
(Ult
+
m2)d-A
=
d^All
-
iej^i + ieJS?^.
(12.9.15)
(12.9.16)
*
We choose e0 eRZ^1/2, m^ Z J m2. This gives a Lagrangian of the form (12.9.10). There are no counterterms for A2 and d-A2, and the counterterm 1 for ^^. We will prove for {J/4^ is proportional to the counterterm Z2 =
=
—
later that this is correct. But for the moment we will choose to have
Lagrangian
our
in this form. Our gambit is that if extra counterterms should be
326
Renormalization
needed, then
they will
of gauge
theories
not be available and the Green's functions will
diverge. (12.9.16)
From
(nx/Z =
+
m2K0\Td'A(x)X\0y
i-^(0\TAX/AA^(x)\Oy +
=
it follows that
eR(0\TiI/(x)AX/A{l/(x)\0} -eR(0\Ti//(x)AX/Ail/(x)\Oy
i|£ ̄£T<0|T^|0>,
(12.9.17)
where the
gauge variation is computed according to (12.9.4). A convenient way to write these results is as follows:
(1) Let 0 be
(2)
Let
a
(5(field)
free scalar boson field of =
mass
£1/2m.
gauge variation of 'field' with parameter 0, i.e.,
Sa^=drf, 6\jj
dij/
(3)
=
ieR(f)\l/,
=
—
\eR<f>^j.
Then (12.9.17) is
(i/ZKO\Td-Ax\oy The field <j> is
=
<o\T<f>(x)sx\oy.
similar to the Faddeev-Popov ghost except for
(12.9.18)
being a boson
(this will be important later). In a non-abelian theory the nilpotence of the BRS transformation is crucial to proving Ward identities and is proved using the anticommutation of c with itself. A field introduced in the same way has to be the ghost field and must be to be a boson.
a
fermion. In (12.9.18)
we can
as
0
choose 0
12.9.4 Counterterms proportional to A2 and d-A2 Let
us
apply (12.9.17)
to the case X
=
Av{y):
(aj^m2)-^<0\TA^(x)Av(y)\0y -i-JL^x-y). This equation implies that d-A2
no
counterterms
proportional
to
(12.9.19) A2
or
to
needed. For suppose otherwise. Then consider the lowest order in which there is a divergence in the photon propagator. The divergence comes are
from the insertion of
a
divergent self-energy graph
in the free photon
propagator. The divergence has the form of Fig. 12.9.1, where the cross denotes the counterterm to this divergence. Power-counting indicates that
12.9 Abelian theories: with and without
photon
327
mass
Fig. 12.9.1. Counterterm to photon's self-energy.
only divergences are proportional to kjkv and g^v. A divergence g^k2 kjcv is cancelled by the wave-function renormalization Z3 for the photon. Insertion of the extra counterterms in the propagator gives a divergent
the —
contribution to the left-hand side of (12.9.19). But the left-hand side is finite. So there are in fact no divergences. Hence m\ Z^ lm2. It also follows that =
photon self-energy is transverse. Applying(12.9.17)toA: ^M(^v(z)andto^K(w)^A(>;)^v(z)showsthatno counterterms cubic or quartic in A are needed. the
=
12.9.5 Relation between e0 and
Apply (12.9.17)
to X
(D^ =
Z3
<A0#(z):
=
+
m2)^<0|T^(x)^|0>
e(0\ rMO0(z)|O> [8(x -y)- S(x
-
(12.9.20)
z)].
We assumed e0 Z^ l/2eR. If this is not the correct counterterm, then let the \l/-\]/-A 1 PI vertex first diverge at AMoops. Then the left-hand side of (12.9.20) =
diverges
at order
make the
e2iN+1i
right-hand
while there is
a counterterm
Z2
—
1 available to
side finite. Hence the left-hand side does not
diverge.
12.9.6 Gauge dependence The gauge variation of
Green's function
a
<0| TX\0}
is
^olralo-^j d4x<o|r[a-^2(x)-<o|a-^2|o>]x|o> ^jd4x<0|T^X|0> .
8 In Z,
+ i-
=
^-
d4x < 01T8 A(x)<j>(x)5X 10 > ■
-N,<0\TX\0>dhiZ2/dZ
=^|d4x<0|T[?/,2(x)-<0|</.2|0>]l2X|0> -iv^,<o|TA:|o>ainz2/a^
(12.9.21)
Here N^ is the number of i/f fields in X (which equals the number of ¥ fields).
328
Renormalization
of gauge
d
i
<0|7V4?/1J0>=
—
ft
=
theories
lowest order
;
j
^tx
( ) 51nZ2 ^J
^
^<0|T^/i|0>= ^^-+-^-^ {) ainZ.
+
ln{
Fig. 12.9.2. Examples of (12.9.21).
The
disappearance
of the factor
\ in
the last equality is due to the identical <O|02|O> appears, rather than 02,
nature of the two <j> fields. Also, <j>2 because the pure vacuum graphs never
—
occur on
the left-hand side. (A better
derivation uses two different fields <j> t and <j> 2 for each application ofthe Ward identities.) Notice that we cannot derive this equation if <j> is a fermion, for
then (f>2 the
non-abelian theory, the derivation must be replaced by complicated one which leads to the weaker (12.4.4). The graphical
=
more
0. So, in
a
(12.9.21) for a few simple cases is shown in Fig. 12.9.2. From (12.9.21) it is immediate that all Green's functions of gauge-in variant
structure of
fields
^-independent. only graphs with UV divergences are those with the simple loop (Fig. 12.9.3) attached to a ¥ or a \j/ vertex together with those with the c In Z2/d£ factor. We therefore find that are
The
dlnZ,
-
"£ .
+
,
—
l—nt
(12-9.22)
(2nY(k2-im2)2
is UV finite. Hence
ainZ22 d£
e2
/£m2 V'2"2
167i2jr(2-d/2)(7—j \4w
\
_
.
+ finite.
)
Fig. 12.9.3. All UV divergent graphs contributing
to
(12.9.21) contain this loop.
12.9 Abelian theories: with and without photon If
we use
329
mass
minimal subtraction then
Z2=Z2(£ =0)exp
(12.9.23)
%n2(d-4)_
^-dependence of the S-matrix, we recall the LSZ formula. It consider the corresponding Green's function, and pick out the
To obtain the
tells
us to
poles in its external momenta. Since we use transverse polarization (e-k 0) for photons this picks out graphs with the loop Fig. 12.9.3 and the d In Z2/d£ =
terms. Then
n
S-
Fermions
From
physical
Fig.
(^p*)
masses
photons
<0| Ti/^|0>
12.9.2 for
(t^)<0|rX|0>.
0
and for
(12.9.24)
<0| T^v|0>, we see that the
and the residue of the photon pole
are
^-independent, while
the gauge dependence of z2 exactly cancels the gauge dependence of the particle pole coefficient of <0(7X|0>.
Wecancomputeexplicitlythe^-dependenceoftheresidue,z2,ofthepoleof
the fermion propagator, if
we use
^
=
=
i.e., z2
=
z2(£
=
minimal subtraction:
lim (12.9.22)
^?[y ln{^)] +
0) exp i
^p \y
Since £0 ^Z3, e\ Z^le2, and ml £m2 in (12.9.25) are RG invariant. =
—
12.9.7
=
-
1 + In
Z^m2,
(-^ v47r/r
the combinations
(12.9.25)
e2£
and
Renormalization-group equation
Using our knowledge of the renormalization of e0, £0, and ml we find the RG equation for a Green's function of Nf\J/'s, N {iji% and NA A's to be
0=(4+pi y?MiL+y>m2ib ^|+N^+>N^)G'  ̄
 ̄
(12.9.26)
with P
=
iey3 + (d/2-2)e.
If we use minimal subtraction then the only ^-dependent coefficient is y2, and
330
Renormalization
from
(12.9.23)
we
of gauge
theories
find that 72
=
y2?
=
(12.9.27)
0) + ^2/8tt2.
Theresults(12.9.23),(12.9.25),and(12.9.27)werederivedbyLautrup(1976) and
by Collins (1975a). 12.10
Unitary
The unitary gauge is the limit £
operators
-?
gauge for massive oo.
photon
Since the S-matrix and gauge-invariant
<!;-independent, this limit exists for them. But for gauge-variant there are severe ultra-violet divergences. Thus the limits £-? oo,
are
operators d -? 4 do not commute.
regulated theory. The resulting UV divergences by extra counterterms beyond those that we have already considered. Since the S-matrix is gauge-independent, all We may take £->oo first, in the at d
=
4 may be cancelled
these counterterms must vanish by the equations of motion.
13 Anomalies
A characteristic feature of relativistic quantum field theories is that symmetries of the classical theory are not always present after quantization.
We do not
mean
non-invariant
here the spontaneous breaking that is characterized by a and by the presence of the Goldstone bosons. Rather
vacuum
situation where there is
we mean a
no
conserved current for the symmetry
despite the absence of any terms in the action that appear to break the symmetry. Such breaking of a symmetry is called anomalous. If the classical action is invariant, then theorem gives symmetry
us
a
breaking.
a
naive
application
of Noether's
conserved current. That is, there is no anomalous What prevents the argument from being correct is the
presence of UV divergences. The current is a composite operator, i.e., a product of elementary fields at the same point, and to define it, some kind of
regularization and renormalization is needed. The renormalization may invalidate the equations used to prove Noether's theorem. For simplicity, we will consider only global symmetries, as opposed to
local,
or
gauge,
symmetries.
The
simplest
cases
of
global symmetries
were
considered in Chapter 9. These could be treated by using an ultra-violet regulator that preserved the symmetry. The proof of Noether's theorem can then be made in the cut-off counterterms are needed.
theory. We showed that only symmetric Consequently the symmetry remains good after the
cut-off is removed. However, not all symmetries can be preserved after regularization. The case which we will treat in this chapter is that of chiral symmetries. These are transformations that act independently on the left- and right-handed components of Dirac fields. These are particularly interesting because sometimes the anomalous breaking of chiral symmetries cancels. Indeed
theorem, first proved by Adler & Bardeen (1969), that if anomalous breaking of a chiral symmetry is zero to one-loop order then it is
there is
zero to
a
all orders.
Our treatment will
valid in the
use
dimensional
physical space-time
regularization. Chiral symmetry is 4, but not when d =£ 4. The
dimension d
=
anomaly in, say, an equation of current conservation will be an operator with
332
Anomalies
in effect
a
4, where d is the space-time 4, were it not for the existence of
coefficient proportional to d
dimension. This would vanish
at d
=
—
ultraviolet divergences which allow the anomaly operator to have a pole at d 4 so that a non-zero anomaly results at d 4. We will see explicitly how this =
=
works. There
key issues.
are two
The first is to derive
simple forms
for the
anomaly. Of these the most dramatic is the Adler-Bardeen theorem that tells us that in some cases there is complete cancellation of the anomaly. The second issue is to derive the results in a form that is applicable to the physical theory, i.e., after renormalization and removal of the cut-off. Even though a particular derivation depends on the choice of a particular
regularization scheme,
the final results must be
Note that to prove existence of an
independent
anomaly, it is not
of this choice.
sufficient to say that
symmetry in question is broken in the regulated theory. The breaking may go away after removal of the cut-off. For example, if one uses a lattice the
cut-off, then Poincare invariance is lost. However,
one must
prove that
Poincare invariance is restored in the renormalized continuum limit, if the theory is to agree with real world
phenomena. Aside from the case of chiral transformations, there are a number of other important situations where there are anomalies. One of the simplest is that of dilatations. These
space-time: x"->axm. A classical Lagrangian is scale-invariant if it contains no dimensional parameters, like a mass scale. But to cut-off ultra-violet divergences we necessarily introduce a mass scale. The symmetry is necessarily broken in the regulated theory and the question arises of whether the symmetry remains broken after the answer
scale transformations
theory
on
is renormalized and the cut-off is removed. This
is, in general, yes, if the theory has interactions. The
there is, in fact, when
are
we
a mass
scale hidden in the renormalized
reason
theory,
is that
as we saw
discussed dimensional transmutation and the renormalization
group, in Chapter 7. Detailed treatments can be found in the literature (see Collins (1976), Brown (1980) and references therein). The simple form for the Ward identity is known as the Callan-Symanzik equation (Callan (1970) and
Symanzik (1970b)). The information contained in the Callan-Symanzik equation is in fact also contained in the renormalization-group equation that we studied in Chapter 7. Other situations which we will not treat include the following: chiral gauge
Piguet & Rouet (1981)), conformal transformations (Sarkar (1974)) and supersymmetries (Piguet & Rouet (1981), Clark, Piguet & Sibold (1979, 1980), and Piguet & Sibold (1982a, b, c)). theories (see Costa et al (1977), Bandelloni et al. (1980),
13.1 Chiral
transformations
333
13.1 Chiral transformations
QCD with two flavors of quark: up and down. (We could flavors, but no essentially new ideas would be needed.) The Lagrangian is (2.11.7). If the quarks were all massless then the classical theory is invariant under the following transformations of the quark fields: We will consider
have
more
i/,
-
exp
{i[|(l
-
y5)K + com + i(l
+
y5)K + <*")] ty.
(13.1.1)
Here the matrices ta are the generators of the isospin group acting on the flavor indices. The transformations (13.1.1) form a group that we will call
U(1)L? U(\)R? SU(2)L?SU(2)R. The symmetry of the QCD Lagrangian under these transformations is broken by mass terms for the quarks. Since the of the u and d quarks are small, the chiral symmetries are only weakly broken. These symmetries and their breaking were understood well before the
masses
advent of QCD-see, for example, Treiman, Gross & Jackiw (1972).
symmetries in the light of QCD can be found in Marciano & Pagels (1978) and in Llewellyn-Smith (1980). Thus it is unnecessary to go into details here. What we will emphasize is how the Treatments of chiral
potential
for anomalies arises.
Since these transformations involve y5, they
spin space-time,
the
we
imposition of
can
an
in
coupled to symmetries of
some sense
to the
expect trouble when the theory is regulated, for
ultra-violet cut-off must alter the
Notice that there is a
transformations have coL
correct. The
are
theory. Since spin is related
structure of the
space-time structure. involving y5; these subgroup them the treatment of Chapter 9 is
U(1)?SU(2) coR. For
=
corresponding
Noether currents
ra
=
not
are
(13.1.2)
Z¥y?ty.
The conserved charge derived from j*1 is the conserved quark number, while the transformations generated by fa are just ordinary isospin
transformations. For the other generators of chiral transformations, let
us
define axial
currents
y£5 In four dimensions
y^y5
=
=
—
y 5yM,
so
that we could have written /y5 in place
of the commutator in (13.1.3). However, because
regularization, currents are hermitian.
we must use
(13.1.3)
^l[^y5]rV. we
will
the form (13.1.3) in order to
use
dimensional
ensure
that the
334
Anomalies
To define the
this
we
will
use
theory we must regulate its ultra-violet divergences, and for dimensional regularization. We will
theory is not invariant under the transformations
see
that the regulated
generated by
the axial
currents;^ and;^a. This allows the possibility of an anomaly. When thecut-off is removed we will find that we can arrange for the non-singlet currents)^ to be without anomaly.
Although we will not demonstrate it, it is true that the
singlet current necessarily has an anomalous divergence. Our remarks above were addressed to the case that all the quarks mass less.
generalize our discussion. The vector singlet current /* is the quark number i.e., 1/3 of baryon number so this remains an
so we now
current for exact
-
-
symmetry. The SU(2) symmetry given by the vector currents is broken
by quark
mass
differences:
(13.1.4)
fy'J =-¥[*-, Mty, where M is the quark small
are
But in the real world, there are quark mass terms in the Lagrangian,
we
have
an
mass
matrix. Since the masses of the u and d quarks are
approximate isospin symmetry of strong interactions.
the theory given in Chapter 9, the currents;^ and/,
finite, since the breaking is from The axial symmetries
mass terms
as
(Symanzik (1970a)).
broken by the anomaly
are
By
defined by (13.1.2), are
as
well
as
by the quark
mass terms:
dj%
=
—
2\\jjy5M\l/
+
anomaly,
Wa5=-ih5{t\M}il,. The
and
(13.1.5)
approximate rf-quark light enough give SU(2) ? SU(2) symmetry. The axial part appears to be spontaneously broken as well as explicitly broken. The abnormally light mass of the pion is usually w-
taken to
mean
Marciano &
to
masses are
that it would be
a
us an
Goldstone boson if mu & Leutwyler (1982) for
=
Pagels (1978) and Gasser
md
=
more
0. See
details.
13.2 Definition of 7 s In order to formulate consistently the dimensional
regularization of theories
with fermions we had to define an infinite set of matrices yM, {jx we saw
in
Chapter 4, they satisfied
the
{/jWI. Our task is
now to
find
a
=
0,1,2,...). As
algebra (13.2.1)
trl=4.
generalization of the matrix which at d
=
4 is called
y5. Its four dimensional definition is ys
=
iy<y y.
(13.2.2)
13.2 Definition
335
ofy5
and it satisfies the anticommutation relation
{y*,y5}=0 (rf
=
4).
(13.2.3)
It would be natural to assume that this relation (13.2.3) can be maintained for arbitrary values of d.
show, when
now
we
Unfortunately
inconsistency arises, as we will trace of y5 with a product of the
an
wish to compute the
ordinary yM's. The ultimate result will be that stated
in Section 4.6, where
we
used the definition (13.2.2) of y5 for all values of d. Then y5 has mixed commutation and anticommutation relations, (4.6.3). We now demonstrate the inconsistency in trace calculations, starting with
try5: dtr(y5)
=
=
=
=
In the first and last lines
tr(yMy5yM) -tr(y5yM/) (13.2.4)
-dtv(y5).
used
we
y,y"
tr(y5y?yj
=
Hy^}
=
g^=d.
In the second line we used cyclicity of a trace, and in the third line we assumed
(13.2.3). From (13.2.4)
we see
that tr y5
=
0 except at d
apply dimensional regularization we wish to obtain meromorphic function of d. Hence we must have try5 Similarly
=
=
=
=
=
Here
we
same
a =
we
result that is
a
0 for all d.
tr7A75V/v7A -tr75/V/v7A 2Ktr ysyJx + 2#v tr 75 Va d tr y5y^yy 2tr75{7?,7v} + (4 d)try5y^yv (4-d)try5Vv. -
-
 ̄
-
used the result tr 75
Now, the
0. Now when
=
technique
=
we find that (d be used to prove that
0. Hence
can
(4-^tr(757KWv)
—
=
0.
(13.2.5)
2) tr 757M7V is zero. (13-2-6)
4 this equation permits the usual non-zero trace of y5 with four other Dirac matrices. However, if the trace is to be meromorphic in d9 equation (13.2.6) shows that it must be zero at d 4, and we can therefore not obtain
At d
=
=
normal physics at d section.
=
4. This is the inconsistency referred to at the start of this
336
Anomalies
We are therefore forced to drop one of the hypotheses that led to (13.2.6). Candidate hypotheses for removal include:
(1) the anticommutation relation (13.2.3) ('t Hooft & Veltman (1972a)), (2) the use of dimensional regularization for fermion loops (Bardeen (1972), and Chanowitz, Furman & Hinchliffe (1979)).
These last authors have shown how to calculate with a totally anticommuting ys. Bardeen chooses to use a regulator other than dimensional regularization for all fermion loops. On the other hand, Chanowitz et al. regulate fermion loops with an even number of y5's dimensionally. Their procedure is useful for low-order graphs, since the Ward identities are preserved for graphs without fermion loops. However, we then lose the use of dimensional regularization as a complete regulator; the theorems that we derived in Section 6.6 and in Chapter 12 no longer apply. The details of higher-order calculations by this method have not been spelled out. Therefore let
us
follow 't Hooft & Veltman (1972a) and Breitenlohner & change the anticommutation relation. In fact, we may
Maison (1977a) and
the definition (13.2.2) for all values of d9 just as stated in Section 4.6. Our definition is, of course, not completely Lorentz covariant, since the first four dimensions are picked out as special. But this is not an overwhelming use
objection, for
actual
our
physics
is confined to these dimensions. An
important advantage of the definition is that it gives of y5. We are therefore guaranteed consistency. One notational inconvenience arises. We have \i
=
0,1,2,.... Usually
we
only
refer
explicitly
a set
construction
of matrices yM for
to the first
four; the
rest are
collectively. But there is / with ja 5. It is not the same as y5 by (13.2.2). However, we bow to standard usage and use y5 to denote
referred to
defined
a concrete
=
the matrix defined in (13.2.2). Confusion should be rare. The y5 so defined has mixed commutation and anticommutation relations
that follow from (13.2.2) and the properties of /. These were stated in (4.6.3). The derivation of (13.2.4)-( 13.2.6) now fails, because y5 does not anticom- mute with all of the
trace of
an
y^s. We can derive the correct relations easily. Since the
odd number of y^'s is zero, tr (/y5)
=
we
have
tr{y5yKyY)
0,
=
0,
(13.2.7)
etc.
We may read off the trace of y5 with an even number of /'s from (4.5.13) and its relatives with
more than
four
y-matrices. Thus
trW tr(?5yV)
=
=
°'l o.J
we
have
(13.2.8)
13.2 Definition
ofy5
337
Finally ^(y5yKy,y,yv)
=
(13.2.9)
^K^
where eK^v is given by (4.6.2). The relations (4.6.3) mean that thechiral transformations (13.l.l)no longer
generate a symmetry if d is not equal to 4. To discuss the resulting problems, the following notation is useful (Breitenlohner & Maison (1977a)): ^
\guv9
=
if n and
K
v are
4
or
larger,]
j
otherwise;
(1321°)
V"=g"vVv. Here V is any vector. Then g^ is a projector onto the unphysical dimensions.
Thus, for example,
{V,y5}= I We may also define
projections
W,y5}=2f?5 2?5f.J =
onto the four
\g"v if ix and v are ^0 otherwise; W
The
physical dimensions:
less than 4 J >
J
g>"Vv.
=
following results
are
elementary,
but will prove useful:
?fi=(6-<*)f, ffi=(4-<of, ffy?=-2f, 4f Tfiu =
Let
define a>fl
us
?
=
+
(13.2.12)
(13.2.13)
-
?
<w£)/2 and a>"5
=
(- a£
+
a£)/2.
Then the
variation of if under the chiral transformations (13.1.1) is b<£
=
i#(2o>2y5 + a)"[M,f] + a>a5y5{M,rfl})i^ -2i}>y5p(co05 + o/5tfl)"A- (13.2.14)
Hence the
divergences
SJia
=
=
dji
=
of the axial currents are
ihsP* M)* + fast?* -i¥y5{t°,M}^ + fa5{t°,P}il,/2, 2#y5Aty + #y5^. -
-
(13.2.15) (13.2.16)
The second term in each equation can potentially give an anomaly when we letd^4.
Anomalies
338 13.3 There
are a
Properties
of axial currents
number of somewhat different situations in which axial currents
appear. The original papers on chiral anomalies primarily addressed anomalies in Ward identities of the chiral currents of strong interactions. More general
have since been worked out, with corresponding generalizations of the Adler-Bardeen theorem. In this section we will list the various
cases
cases
and state what is known. 13.3.1 Non-anomalous currents
The following properties apply, for example, to the non-singlet axial currents in
QCD:
(1) If there is no anomaly to one-loop order (as for the current)^), then there is no anomaly to all orders. The Ward identities of the current with elementary fields have no anomalies. (2) Under the same condition as in (1) there is no anomaly in the two-current Ward identities.
(3) Under the same condition there is an anomaly in a three-current Ward identity (like the one for dK(0\ TjKa5(x)j£(y)jvc(z)\0y). However, the only non-zero term in the anomaly is the one-loop contribution. The theorem that the
complete anomaly in these cases is determined
one-loop value is due to Adler & Bardeen (1969). The lack of anomalies in the Ward identities of
elementary fields is essential if the currents
one
current
generate the
are to
by the
with
correct
transformation law for the fields. These transformations imply commutation
relations for the currents. Since these commutators
are
also given
by
the
Ward identities with two currents, the two-current identities must be
anomaly-free. No such consistency requirement applies to the three-current Ward identity. The value of its anomaly is related to the decay rate for n° 2y, and -?
the lack of higher-order corrections enables
successful prediction to be made easily. (See Adler (1970); for reviews from the point of view of QCD, see Marciano & Pagels (1978) and
a
Llewellyn-Smith (1980).)
13.3.2 Anomalous currents The
singlet
current y 5 has an anomalous
fy'S
=
divergence.
C(g)GlvGa^ + mass
where G"v is the gluon field strength
g?Mv
=
terms,
tensor and
fii*vKAG?
It has the form
(13.3.1)
G"v is its dual: (13.3.2)
13.4 Ward identity for bare axial current
339
The Adler-Bardeen theorem asserts that the coefficient of C{g) is equal to its
one-loop
value
However,
the operators^ and GG need renormalization, and there is not
C(g)=Nng2/(32n2). an
obvious natural renormalization condition. So the value of C is susceptible to change by redefinition of the renormalization prescription. We will not treat this case here.
13.3.3 Chiral gauge theories
Weinberg-Salam theory of weak interactions have a gauged chiral symmetry. It is essential that there be no anomaly, for otherwise the theory is not renormalizable and loses other important properties (Gross & Jackiw (1972) and Korthals Altes & Perrottet (1972)). A generalization of the Adler-Bardeen theorem is that there is no anomaly to any order of perturbation theory if there is none to one-loop order. Proofs have been given by Becchi, Rouet & Stora (1976), and by Costa et al. (1977). Theories like the
13.3.4 Supersymmetric theories Supersymmetric theories have potential anomalies similar to the chiral completely general treatment has not yet been given, but many cases have been treated see Piguet & Rouet (1981), Piguet & particular Sibold (1982a, b, c), Clark, Piguet & Sibold (1979,1980), and Jones & Leveille (1982). anomalies. A
-
13.4 Ward identity for bare axial current Without
use
of the
equations
of motion the
divergence
of the
non-singlet
current)^ is
dJi5=&2fa5ta(ty-M0)il, + hc + iZ2<A{M0, f}y5il, + &¥?{% y5ty ^L The first
+
^
+
equation
term we will call the
03A1)
^nonv
of motion term. When inserted in
Green's function with elementary fields it
gives
<o\TDaejx)i\AU^yi)U^j)\o> i
J
=E#*-:vxo|Tn^n*n*io>,?,--,,., i
a
340
Anomalies
The variations of the fields
i. (We set cor
—
=
a>l
=
just their chiral transformations multiplied by
are
of5
in (13.1.1) to obtain these transformations.) This
equation (13.4.2) has the expected right-hand side for the Ward identity in the absence of anomalies. The
mass term on
the right of (13.4.1) is expected; it is the non-anomalous
breaking. The anomaly term is D°nom. If we insert it in a Green's function with no divergences of any kind, then we may set d 4. The result must be zero =
Danom vanishes if only the first four y-matrices are used (for then 0). But if the Green's function has an overall divergence, or if it has a divergent subgraph that contains the vertex for Danom, then the result may be because
{p, y 5}
=
non-zero at
d
=
4,
as we
will
see.
—<0|T^5(x)n^n,/'n,/'l0>
=
The full Ward identity reads
right-hand
side of
(13.4.2) +
mass term
+<o|rr>aflnoinwn^n'Ar['/;io>-
(13.4.3)
Recall that in the case of a symmetry such as the isospin SU(2) of QCD that has
anomaly, we used its Ward identity to prove the current finite. The only possible counterterm for the current is proportional to itself, so finiteness of the divergence of the current, d 7, implies finiteness of the current itself. The Ward identities imply that the divergence of the current is finite. no
However, for the axial currents the extra term in (13.4.3) prevents this argument from
being
made.
13.4.1 Renormalization of operators in Ward identities Our aim will be to construct
a
finite current j%a5 that at d
=
4 satisfies
a non-
anomalous Ward identity:
A<0|T7L5(x)n^n^n^l°>=right-hand side of (13.4.2). (13.4.4) The first step is to observe that the only counterterm to fa5 is itself. No other operators have the correct dimension and quantum numbers. So we can
define
a
minimally
subtracted operator
=
12Z5Z2{p[y",y5]t^.
(13.4.5)
Throughout this section we will use square brackets to indicate minimal subtraction. So the renormalization factor Z5 has the form
Z5
=
1 -I-
poles
at d
=
4.
13.4 Ward To obtain the
identity for bare axial
operator jRa59
we
341
current
will later show that
we
have to invoke
a
further finite renormalization. We will show that the Ward
by
the
minimally
subtracted
identity (13.4.4) is not correct ttjRa5 is replaced
[ja5].
renormalization to obtain a Ward Jro5
=
=
where
z5(g)
is
a
Rather
we must
make
a
further finite
identity without anomalies. Thus we have
Z5Ua5j
(13A6)
iz5Z5Z2(A[/,y5]^,
finite factor.
The anomaly operator I>anom is a dimension four scalar quantity, so there are several operators with which it can mix. It is proportional to the e-tensor
AK?fiV. Since eKAfiV appears nowhere in the Lagrangian, the tensor A*;/iV is invariant under all Lorentz transformations. Given these restrictions, a complete list of operators that times a fourth rank tensor, which we will call
mix with D°nom is
^5,^?om^{M0,rfl}y5(A. involving only ghost and gluon fields can be constructed such that it mixes with D°nom. The linearity in eKXflv implies the presence of four factors of vector objects (derivatives or ,4-fields). Therefore the coefficient of No operator
the operator is independent of mass, by our usual results. Gauge invariance of a restricted set of gauge variant counterterms (see Chapter 12) of which have low enough dimension to appear. Since ja5 is even under
^anom allows none
so is D°nom. Therefore the only allowed counterterm to proportional quark mass has a flavor factor {M,ta} rather than the commutator [M, **]; this gives us the operator DaM that appears in (13.4.1). We can therefore write the minimally subtracted operator corresponding
charge
to
conjugation,
[Daflnom]
as
WnonJ
=
ZaITanom
+
Za5d-ja5
The equation of motion operator is finite
+
by itself
(13.4.7)
ZamDaM.
-
see
(13.4.2)
-
so we
have
(13.4.8)
[^m]=^m.
Note that the definition of Datm includes some counterterms, but these are manufactured from the wave-function, mass, and coupling renormalizations
in the Lagrangian. We also need the renormalization of the
lDaM ̄\
=
mass
operator
Z5MD°M.
It is somewhat uQobvious that the renormalization is
(13.4.9) multiplicative.
The
easiest method is to examine the y-matrix structure of self-energy graphs with
Anomalies
342 an
insertion of
¥tay5\lj
or
\j/y5ij/.
multiplicatively renormalizable
These operators can then be shown to be with a common factor.
of (13.4.6)—(13.4.9) we can express the equation (13.4.1), for the divergence of the bare axial current, in terms of renormalized operators to
By
use
find
[D*em]=Z-l(l-Za5Z^)d^fa5-] -
[DaM]Z^(l
+
ZaMZa *)
-
Z-1
[^nom].
(13.4.10)
on the right are all linearly independent, so the linear combination of them can equal the finite left-hand side is for
The renormalized operators
only
way a
the coefficients to be finite. Since
we use
ZaM
za5
=
=
minimal subtraction this implies:
i-z5.
We therefore have the renormalized Ward
^<0|T[;^]n^n^n^l°>
(13.4.11)
Z5M-l,} identity:
=r-hs. of (13A2)
+<o|T(ra+[^nom])n^n^n^io>, (13.4.12)
which apparently still has an anomaly. Before showing how the anomaly in disappears, let us examine some low-order graphs.
fact
13.5
One-loop calculations
The tree approximation for the two-point Green'sfunctionoffD^] is given by Fig.
13.5.1. To
save
algebra we will set quark masses to zero. The graph's
value is
l^2(-W p)y5ta%. P
(13.5.1)
+
P
If we let the external momenta be
physical, i.e., in the first four dimensions,
then this vanishes. The vertex for D%nom has the property we define as evanescence: it vanishes when the cut-offis removed and we go to the physical
renormalized theory. We will formulate
later, when
we
a
precise definition
of evanescence
have understood the subtleties associated with
inserting
the
vertex inside loops. a
Danom >
p
X
) p
Fig. 13.5.1. Tree approximation for two-point Green's function of [££nom].
One-loop calculations
13.5
343
P'
p
to
(b)
(c)
Fig. 13.5.2. Graphs up to order g2 for two-point Green's function of;£5. Next let us examine the graphs for the two-point function of the current;'^, as
given in Fig.
13.5.2. Graph (a) has the value
ysrvS^. Taking the divergence is the
=
When
we set
lowest-order
p
=
case
p'
-
=
same as
y5t°iplp2
0,
multiplying by i(p'
—
p)M. The result is
i(f/p'2)y5f + (13.5.1).
to get the four-dimensional
result,
(13.5.3) we
obtain the
of the chiral Ward
identity. Fig. 13.5.2(b). Its
The next order graph is propagators
-
(13-5.2)
value, with the external
amputated, is \4-d
r"=76^(27l/i)
k\p'
+
k)\p + k)2
This is evidently divergent. It is easy to calculate the pole at d
pole (r26)
=
[
?
=
X)
4:
g2fCF f 1 pole < -^-j- —^y^yy
g2tacF
l^yy5(d-6)2J
;-32^-pole)4-rf
C¥f,y?y*.
%n2A-d Here
we
(13.5.5)
have twice used the result that
yKfy¥f
=
=
=
yKfysT + yKfysyK 2f?5
-
if f ys
(6-dWy5.
(13.5.6)
344
Anomalies
There
is
a
^2^7^75ffl,A>
in
implicit
graph
counterterm
the
definition
w^tri trie quark wave-function renormalization
Z2
=
g2CF
=
0(g4).
(13.5.7)
graph Fig. 13.5.2(c) therefore cancels
the UV
lr l-S%n2{4-d) ?
The resulting counterterm
j%5 given by
+
divergence of graph (b), leaving a finite result. No additional renormalization is needed :
Z5 Let
us
next
take the
(13.5.8)
l+O(04).
=
of (13.5.5) plus its counterterm, by
divergence
multiplying by i(p' p)^ It is left as an exercise for the reader to verify that the Ward identity (13.4.3) holds at this order. What we will do is examine the —
graphs
g2
of order
for the Green's function of Dlnom. These
Fig. 13.5.3. Note that the definition includes
=
The \[fAy5\jj
gives rise
term P
Z2i0y5^ to the
-
a
are
listed in
covariant derivative:
2ig0Z2^0y5ta^
(13.5.9)
graphs (b) and (c).
p'
-Tzr-
x^r
t7
k
(a)
(b)
(c)
(d)
Fig. 13.5.3. Graphs of order g2 for two-point Green's function of [fi°nom]- Graph (<j) equals r3a
t**i-yW 4* Crf2f(2^)*-" 16*4 J
+
=
W
ig2cF
| dx| ^-3-^(2^)*-" Jo Jo x
-
Mfi
+
t + 2h^ + ft)y
k2{p>+k)2{p + k)2
dyx
{r(3-d/2)Dd!2-3yv[t'(l -y)-*x]tf'(l -2y) + p\\ -2x)] ir(2
rf/2)^'2 2yv[2(/>'(l -y)- tx)yKy5f -
-
+
2yKfy5W-x)-fy)
+
7K(PV-2y) + kl-2x))y5yKy},
(13.5.10)
where D=
If it
were
not
-p2x(l-x)-p'2y(l-y) + 2p-p'xy.
that there is
an
(13.5.11)
ultra-violet divergence given by the
345
13.5 One-loop calculations
d/2)9 we could let p and p' be four-dimensional, and then set d 4 to obtain zero. (Note that yKyK yKyK d 4.) The pole prevents this argument from being made. First of all, notice that the pole is T(2
=
—
=
=
—
po's(r,.)--^IIffpoie|5--jH.(^ _^V£,!£±? 87T 4
The manipulations
following
y^sf
=
=
=
(d
—
yKyKfy5
are easy to
+
do incorrectly, so let us
iv/^f
yKyKfy5 (10-d)f<%.
we
(13.5.12,
result
yKfy5yK
split yK 4)-dimensional piece y*.
In the first line
fe!vV
d
-
the Dirac matrices
on
be careful. We need the
+
-
+
if y5
(13.5.13)
four-dimensional piece y* and a Then in the second line we used the
into
a
or anticommutators of yK and yK with yM and y5. The graphs of Fig. 13.5.3(6) and (c) may be evaluated similarly. The sum of the pole terms for all three graphs is
commutators
poie(r3a
+
r3b
+
r3C)=
*
g^2
,
4_rf
(13.5.14)
which is cancelled by thecounterterm Fig. 13.5.3(d). Thisisin agreement with our general result (13.4.11).
ready to compute the value at d 4 of the sum of the graphs of Fig. 13.5.3. Considerable simplification occurs. Since p and p' are now zero, the term in (13.5.10) that multiplies T(3 d/2) vanishes. Similarly the last The term multiplying T(2 zero. d/2) gives remaining two terms have a We
are now
=
-
—
factor yKyK
=
d-4, which cancels the pole
r3flU^=p.
=
to leave a finite result:
o=10L^'-^5-
(13-5.15)
Similarly T3b and r3c give (r3fc
+
r3JU4,^^0
Effectively Fig. 13.5.3
d-ja5. fc
can
p
to
^tv-^,
g2CF/(An2)
times
(13.5.16) the
vertex
for
why the result should be of this form. loop integration, the vertex for D%nom vanishes when
It is easy to understand
Without =
sums
=
=
get
p'
the =
0. When we include the integration over the components of k, we
a non-zero
value for the
graphs
even
if p
=
p'
=
0. However, the
Anomalies
346
property of the basic vertex implies that it has effectively a factor d-4. We only get a finite result by multiplying by an ultra-violet evanescence
4 is of a local operator. divergence so the effect at d A general theory of evanescent operators can be worked out. The results simply generalize what we have learnt from examples: =
-
(1) We define an evanescent
graph when
tree
we set
d
vertex as one
=
that is finite and that vanishes in
a
4 and when all momenta and polarizations are
four-dimensional. (2) A Green's function d
or a
graph or an operator is evanescent if it is finite at polarizations are
4 and if it vanishes when its external momenta and
=
four-dimensional. (3) Consider
a
graph containing
an evanescent
vertex.
If the graph is
completely finite then it is evanescent. ('Completely finite' means that the graph and all its subgraphs have negative degree of divergence.)
(4)
A renormalized operator
[£]
whose basic vertex is evanescent has the
following expansion:
[£] The
sum
is
over
=
£C£K[F]
+ evanescent
(13.5.17)
operators.
v
operators V whose basic vertices
are non-evanescent.
The only operators that are needed are the ones that according to the usual power-counting and symmetry requirements will mix with E under renormalization. The general proof is left as an exercise to the
reader. 13.6 Non-singlet axial current has
no
anomaly
13.6.1 Reduction of anomaly The only operator that can appear on the right-hand side of (13.5.17) for the case
E
=
Dlnom
is
d-\_ja5].
The restrictions
are
that it be pseudoscalar,
iso vector, gauge invariant and of dimension at most four. (If we had non-zero
quark
masses, then the operator
Wnom]
=
[£>£,]
could also
C(g)d'lJi5]
appear.)
+ evanescent,
So
we
have
(13.6.1)
which, when substituted into the renormalized identity (13.4.12), gives
^<o|T(i-Q[;s5]n^n^n*io> =
r.h.s. of (13.4.2) + evanescent.
(13.6.2)
13.6
So
we
Non-singlet axial
current has no
347
anomaly
should define the renormalized current i{.3
=
=
(i-c)[A] (l-C)Z;1ra5i
(13.6.3)
which is (13.4.6) with z5 1 C. For the physical four-dimensional theory this implies that j^a5 has Ward identities with no anomaly, viz. (13.4.4), as we =
—
wished to prove. From our calculations of Fig. 13.5.3 C
=
we see
(13.6.4)
g2CF/(4n2) + 0(g4).
general theory of evanescent idea, however, is simple. in the physical theory is when a
proof has been long and involves
Our
that
a
operators summarized in Section 13.5. The basic
The only way
an
anomaly
can appear
divergence cancels an effective factor of d 4 for the evanescence of an anomaly. The anomaly in the four-dimensional theory is a local operator, —
and the only possible operators are those which power-counting would allow as counterterms to
d-j.
In the case of our iso vector current, the only such operator is d -j itself. So
finite renormalization (13.6.3)
serves to
eliminate the anomaly at d
13.6.2 Renormalized current has
no
us
fx— right-hand side
=
djj.
right-hand side {Z
a
A.
anomalous dimension
apply the renormalization-group operator /xd/dju identity (13.4.4). For the right-hand side we get Let
—
to the Ward
anomalous dimensions of fields}.
To get the same result for the left-hand side, the current j£a5 must have zero
anomalous dimension (when d
=
4):
dja This is a sensible result: the current j^a5 is a physical object, and it should not on how we parametrize the theory by a renormalized coupling. Useful consequences follow, for the minimally subtracted current does have an anomalous dimension:
depend
nd[U/dn=-y5[ja5l
(13.6.6)
where
y5(g)
=
=
nd\n(Z5)/dn
[(2-d/2)g + p(g)]d\n(Z5)/dg.
(13.6.7)
348
Anomalies
Now, the coefficient C is
presence of
function of g; it is dimensionless and
a
masses cannot
depend
factor Z5. We also define it to
on
have no cut-off dependence,
between renormalized operators at d
even
in the
them, just like the renormalization =
since it is a factor
4.
We therefore have 0
=
=
A*d/afl5/d^
-P(dC/dg)[ja5-]-(\-C)y5[ja5l
that
so
/W(l-q/30
=
(13.6.8)
y5(l-C).
It follows that 1-C
where
used
we
as a
expansion starting
expj exp
=
boundary condition
g2.
at order
convergent, the order
<13-6-9)
dg' dg'y5(gVp{0')h the fact that C has
In order that the
a
integral
perturbation in (13.6.9) be
g2 term in y5 must vanish. (13.6.7) it must be that Z5 has no order g2 term; this
So from the definition
by explicit calculation. Moreover, we loop value of C, so that
we
know
y5
=
-
—
=
(13.6.4) the one-
A,g*C¥l{2n2) + 0(g%
where the one-loop term in /? is A tg3. Hence of the leading divergence in Z5:
z3
know from
i-, Z¥\, 4n2(4-d)
+
(13.6.10)
(13.6.10) gives us a prediction
o{g6).
(13.6.11)
The reader is invited to check this by Feynman graph calculations. We may use the techniques of Chapter 7 to sum the divergences. We find the full renormalization factor of y'^a5 to be if1
^ J
=
(i_c)z5-1
a5
=
=
=
exp{J9od,W)[^-
y5(g%d/2-2)g'
dg' exp\ a"* /?(9')[(<*/2-W+ /?(<?')]. l +
->1
Evidently
(d/2-2)g' + p(g').
(rf-4)O[ln(02/(4-<*))] (13.6.12)
as<^4.
the Noether current is finite in the
complete theory.
13 J Three-current Ward
identity; the triangle anomaly
13.7 Three-current
identity;
Ward
13.7.1 General form
349
the triangle anomaly
of anomaly
We consider the Green's function D&(P,P')
jd*xdVp-x+i"'-y<0| Tj*a5(0)j&x)j;(y)\0>.
=
(13.7.1)
Only connected graphs contribute; Lorentz in variance forces the vacuum expectation value of a current to be zero. The currents are all renormalized currents, so all subdivergences are cancelled by counterterms, and the only possible infinity in (13.7.1) is an overall divergence. In fact there is divergence, as we will now show.
4+k *
r^ *
k
\^k J>'
no
overall
+
^
^
Fig. 13.7.1. Lowest-order graph for (13.7.1). Individual graphs for (13.7.1) have a linear divergence, as can be seen from, say, Fig. 13.7.1. Any divergence must be linear in external momenta and
proportional
to the e-tensor. The
only possibility is
^[a{d)pK
+
(13.7.2)
b{d)p'Kl
There is also the constraint of conservation of the vector current. This is expressed by constructing
a
Ward
identity in the dimensionally regularized
theory. Consider
PM0AMV=-ifd'^^ By
use
of the result
i^Z^y^ty and the
(13.7.3)
=
Z2{j}ta{ip
equations of motion,
p^
=
we
-
M0)\jj
-
Z¥(
-
ilj)
-
M0)fty
find
jdW+''>-<0| T/ifl5(0)Z2<A[Y, r>]/<AW|0> dVij,''y<O|TZ20[^f*]y^;^)|O>.
+
(13.7.4)
In these equations we assumed that the currents;^ andyj are conserved. Each of the terms in (13.7.4) is
a
Green's function of a
vector
and
a
pseudovector
350
Anomalies
current
Parity invariance forces
them to be zero,
PfiD^v
Similarly
pfvD^v
so
=
0.
(13.7.5a)
=
0.
(13.7.56)
The counterterm (13.7.2) must therefore give zero when multiplied by p^ or p'v. This forces the whole counterterm to be zero; the Green's function (13.7.1) is finite
as
it stands.
In the regulated theory the axial current is not conserved, so we cannot 0 by the same manipulations. Indeed prove the Ward identity (p + p\DXflv =
we
have
(P +
P')xD^v
=
commutator terms +
d4xd4yei(px
+ p'y)
x
x<0\TEa(0)rb(x)rc(y)\0}. (13.7.6) Here E is the evanescent operator in E
=
=
(13.6.2):
lDlBom]-C8Ua5] [^nom]-C(l-C)-1a7fia5.
The commutator terms in
(13.7.6) vanish,
as
(13.7.7)
in the Ward
identity
for the
vector currents. Hence, finiteness of the left-hand side implies that the Green's function of £ with/£ and) is finite. Even though graphs for it are quadratically *
divergent, the divergences cancel. Now, E is an evanescent operator. This means that its Green's functions with elementary fields vanish in the four-dimensional theory. The general theory of evanescent operators, which we summarized at the end of Section 13.5, then tells us that the only way that the right-hand side of (13.7.6) will fail to vanish is for E to be part of a graph or subgraph with overall degree
of divergence at least zero. Now, the definition of E has ensured that these subgraphs are all evanescent. Hence we are left with the complete graphs. So we have (at d 4) =
(p
The
tensor
+
p,hD^
structure
dimensionless at d
=
4,
is so
A(g)e^pp^Eabc -i^jip+p'np-py1**.
=
the
it
can
only one possible. The coefficient A is only be a function of g. Note that the right-
hand side of (13.7.8) obeys vector current conservation,
P,(P + P\D^ as
should be.
(l3-7-8)
=
0,
so
that
13.7 Three-current Ward
351
identity; the triangle anomaly
13.7.2 One-loop value The lowest-order value of A is easily computed from the graphs of Fig. 13.7.1. a
"va"P
.
P
* = 3
f d"/c
tr
?J (2n)d +
[tf ¥' + 2|)ys(/> + JQy'ME -
k\p
+
k)2(p'
-
/>')]
k)2
-
charge conjugate.
(13.7.9)
The factor 3 is the number of quark colors. To evaluate this, notice that
/r(YV) 0\ 1 =
(13-7.10)
Since eKXfiV is restricted to the first four dimensions, it follows that the trace of y5 with four Dirac matrices is zero if one of the matrices is a y:
tr(y5fyVV) Let
us
the
commute
whenever possible. tr
-p'
+
ft in (13.7.9) p'
We will also set p
=
=
now
an
left, and
to the =
use
(13.7.11)
0. The result is
use
(13.7.12)
of (13.7.11). The charge conjugate
equal contribution.
have
^<WV This is now
(13.7.11)
-tr[2ty5U> + my'lP"f'] + 0 -tr[2|)-5#TVy]+0 -te[2fttys^y] + 0-
The terms indicated by '0' vanish by We
=
0.
[tf -}' + 2%)y5(j> + JWM -f + m
=
term gives
=
=
3
'd'k
tr[#y5#*y]
}{2nfk2{p'-kf{p
(13.7.13)
kf
+
only a logarithmically divergent integral. After use of Feynman
parameters the standard result (4.4.14) gives
A^apP'P
"£J>.
1-JC
dy±T(2
-
dfl)
x
o
tr[i/75/W] [
-
P
*(1 -x)- p'2y(l -y) + Ipp'xy]
+0(d-4)
-"g^W^"- The
evanescence
of the vertex has effectively given
(13.7.14) a
factor of d
—
4 which
Anomalies
352 cancels the UV pole to leave
a
finite result
(13.7.15)
A=-3/{in2) + 0(g2). This result for the anomaly in the axial Ward identity for (13.7.1) found by Adler (1969), and Bell & Jackiw (1969).
was
first
13.7.3 Higher orders There are, in fact, (Adler & Bardeen (1969)) no higher-order corrections to the
anomalyfor(13.7.1).WewillfollowtheproofduetoZee(1972).Thebasicidea is simple. Each of the currents in (13.7.1) is RG invariant, and there is
no
overall counterterm. Therefore this Green's function is invariant when
we
RG transformation. The anomaly must therefore be invariant also. But the anomaly coefficient A (g) depends on the coupling g and on no
make
an
other parameter of the
theory.
We
can
change g arbitrarily by changing the
mass /x. Hence A is independent of g. This proof may easily be written out. Renormalization-group invariance of D*j£ is the equation
renormalization
"dp* Hence
^AgKbc^Pj'p Since
jud/d^u
=
\idjc\i
+
fid/dg,
=
(p
independent of
g, so it
A
=
V^^iC
=
°-
this gives fidA/dg
Then A is
+ p
-
=
0.
equals its lowest-order
value:
(13.7.16)
3/(8tt2) exactly.
This is a very striking result. The proof we have given is very simple, but the reader should not suppose it is not
a
deep result. The whole
power of the
renormalization apparatus is needed for its derivation. We first had to show that there is no anomaly in the divergence of the axial current. Then we had to show that there was no counterterm needed to make
Definite. These results
involved considerable cancelation of UV infinities. Since the anomaly in d-ja5
disappears when the U V cut-off is removed, it can affect a Ward identity only by being enhanced by a UV infinity which has not made its appearance earlier. Thus the anomaly is associated with a UV pole implicit in the Feynman graphs. It is precisely for this reason that it must have the dependence on the
13.7 Three-current Ward identity; the triangle anomaly
353
parameters of the theory and on the external momenta that is characteristic of a renormalization counterterm. In particular, it is polynomial in the
momenta and masses of the degree determined by U V power-counting. Once this is clear, the most general possible form of the anomaly is (13.7.8). The final step to show that A is independent of g is trivial. An important phenomenological consequence of the anomaly is a calculation of the decay rate for n° -> 2y (see Marciano & Pagels (1978) and
Llewellyn-Smith (1980)). The amplitude is proportional to the number of quark colors, so the decay rate is proportional to the square of this number. The measured rate in fact agrees with the standard theory that there are three colors.
14
Deep-inelastic scattering In this chapter we will show how the operator-product expansion can be used
to compute the cross-section for calculation is fairly
deep-inelastic scattering. Since the this straightforward, process is one of the classic tests of
quantum chromodynamics (QCD). The process is the scattering of a lepton of momentum P momentum pM, with the
only observed particle in
on a
the final state
hadron of
being the
lepton: / + N -? /' +
practice hadrons in the target
In
one uses a
there
are
anything.
(14.0.1)
beam of electrons, muons,
are
nucleons. There
experimental data,
the
cases
-?
e
+
->
fx +
are a
or
neutrinos, and the
number of cases for which
being distinguished by
the types of
lepton involved: e +
N
jx + N
The basic
v +
N -?
v +
N
->
v
+
(e
anything,
anything, anything,
or
jj)
+
f
(14.0.2)
anything. J
measuring the cross-section for these processes is to (quarks and gluons) of the hadron. we have a of a Suppose scattering lepton on a small-sized constituent, and that the momentum transfer is large, so that the scattering happens over a small time-scale. The weak and electromagnetic interactions have a small coupling, so to a good approximation the lepton does not interact again. reason for
study the fundamental constituents
Moreover the interactions that turn the final state, involving the struck quark, happen on a much longer time-scale. So these interactions do
into hadrons
not interfere with the basic Born graph.
Hence, we should be able to calculate
the cross-section for the process (14.0.1) rather simply in terms of the cross- section for lepton-quark scattering (Fig. 14.0.1). The approximation in which final-state interactions of the hadrons are ignored is called the impulse
approximation. over
We can use it because
all hadronic final states. 354
we
choose to
sum
the cross-section
355
14.1 Kinematics, etc.
Fig. 14.0.1. Parton model for deep-inelastic scattering. A more mathematical formulation of the same idea is the parton model,
in Section 14.2. There are, however, many weak points in the intuitive discussion just given, and we must remedy them. In the remainder of
explained
this chapter we will compute the cross-section from the theory of strong interactions (QCD). The parton model will in fact give a qualitatively correct
approximation to the real cross-section. Our intuitive argument shows why deep-inelastic scattering is simple enough to permit calculations. Our treatment in this chapter is based on material to be found, among other places, in Gross (1976) and Treiman, Jackiw & Gross (1972). 14.1 Kinematics, etc. We will compute the cross-section to lowest order in weak and electromagnetic interactions. Then the amplitudes contributing to the process (14.0.1) have the form of Fig. 14.1.1 where a boson is exchanged between the lepton and the hadron. The boson can be a photon, a
Wot a
Z. At high enough
energy, it is also necessary to include Higgs boson exchange. Higher-order weak and electromagnetic corrections do not need to be included, except for
photon effects. We will ignore the soft photon corrections here, and will concentrate on understanding the strong-interaction corrections. soft
/
Fig. 14.1.1. Amplitude for
,
?ot>* process
contributing
to
deep-inelastic scattering.
We first review the kinematics of the process. The
two
independent
Lorentz invariants of the hadron system are chosen to be
Q2=-q%, v
=
(14.1.1)
p-q,
where q* is the momentum transfer from the leptons. The
mass
of the final
hadron state X is then
ml
=
(p
+
q)2
=
m£ + 2v
-
Q2.
(14.1.2)
356
Deep-inelastic scattering
In the
laboratory frame, where the initial hadron is at rest, we can express Q2
and v in terms of the initialand final lepton energies E and £', and of the lepton
scattering angle
6:
neglected lepton inequalities hold We have
<22
=
v
=
2££'(1
-
cos
0).
(14.1.3)
mN(E-E').
masses
with respect to E and £'. The
following
(14.L4)
v>Q2/2>0.
The region we will investigate is where both Q2 and m\ get large in a fixed ratio. We define the Bjorken scaling variable x Q2/(2v). Then we let Q2 get with fixed. the x This is called large Bjorken limit or the deep-inelastic region. =
In this
region the missing
mass mx
m2
=
is large:
(14.1.5)
Q2(l/x-l\
(where we neglect m^ compared with Q2). In order that m\ be positive we must have 0 <
x <
equivalent variable to x that is The cross-section is given by E'
dc
d3//
applies only if x is not at its endpoints. An sometimes used is co 1/x.
1. The Bjorken limit
1
=
dc
InE'dE'dcosd n
4ME
^ZK'1//plO<^|;riiv>D^)|2^4K-^-^) x
4k) ^^^^^^^^^^(A^ /ept
j**d
(14.1.6)
exchanged vector boson couples, gw is its coupling, and Dx\q) is its propagator. The lepton tensor LKA is easily computed in the tree approximation for Here
and
are
the currents to which the
LKX The hadron
Wltv(p,q)
tensor is
=
=
<l\f;?(0)\l'><l'\j)^ (0)|/>.
equal
(14.1.7)
to
^-YJ<p\jtl(Oy\X>(X\jv(0)\PXW#*)(P"x "P" 47T x
4ttJ dVr^p|;>)Vv(0)|p>,
-
<?")
(14.1.8)
where the normalization is the standard convention.
Deep-inelastic scattering is the region where Q gets large with x fixed. This treated in Chapter 10. There we took all fixed ratio, so that Q^> co with Q/p-q fixed,
is not the short-distance limit
we
components of q" to infinity in
a
357
14.1 Kinematics, etc. i.e., x is proportional tog. This is not in the physical region 0 <
inelastic scattering. Luckily
x <
1 for deep-
relation (Christ, Hasslacher & Mueller (1972)) to relate the deep-inelastic limit of W^ to the we
can
use
a
dispersion
short-distance limit of the time-ordered matrix element
T,v
=
[d V?<P|
77>)VV(0)|0>.
(14.1.9)
perform this analysis in Section 14.3. decompose W^ into tensors with scalar coefficients. We will assume that the hadron is unpolarized, i.e., that we average over its spin states. Then the most general form of W^ is We will
Let
us now
Ww
=
Wd
0,v +
 ̄
Wjl2) + W2(Plt
-
q,v/q2)(pv
-i^3W?V/(2M2)+ W^qJM2 + WJ/(2M2) + W5(P??, -iW6(pfa The scalar coefficients W.(Q2,v)
are
-
-
qvv/q2)
q,pv)/(2M2).
(14.1.10)
called the structure functions. The
normalizations are such that they are dimensionless. A number of properties
follow from symmetries and basic quantum mechanics (Treiman, Jackiw, & Gross (1972)): (1) Each Wi is real. (2) Time reversal invariance of strong interactions implies that W6 In the
of
case
conserved,
so
purely electromagnetic
a
that
qllWllv
electromagnetic current is invariant. So W3 0.
=
process, the current
0. Hence W4= W5
=
=
jh^d
0.
is
0. Moreover the
a pure vector, and strong interactions are
parity
=
only conserved if quark masses are zero. In that case the only non-zero structure functions are Wx, W2 and W3. It turns out that when the masses are non-zero the contributions of W± and W5 to the cross-section are suppressed by a factor of order m^JQ2, where ml and mq are lepton and quark masses. We will discuss this further in In the case of neutrino scattering, the currents
are
Section 14.8. When
we
compute the structure functions in the Bjorken limit,
Q2
-?
oo, x
fixed, we will find that they behave as a certain power of Q2 times logarithms.
The power is (Q2)0 for
Wl9
l/g2for W2and W3iznd l/<24for W4and W5. So it
anticipate these results and define scaling structure functions FifaQ2) which depend only logarithmically on Q2. The standard is convenient to
definitions are:
F1(x,Q2)=Wl(Q2iv Q2/(2x))i Fi(xiQ2) vWi/M29 for i 2 and 3, F.(x9 Q2) v2 WJM\ for i 4 and 5. =
'
=
=
=
=
")
>
J
(14.1.11)
Deep-inelastic scattering
358
14.2
Parton model
discuss the true theoretical predictions for deep-inelastic let scattering, us discuss the parton model (Bjorken & Paschos (1969)). This is Before
we
the simplest model for the process, and contains the essence of the correct physics. One considers the initial state hadron in the overall center-of-mass for the whole scattering. Now time dilation slows down processes in the hadron so that they typically occur on a rather long time-scale T of order Q/m^. Then the scattering happens on a much shorter time-scale T ̄ 1/Q. This implies that the hadron may be regarded as an assembly of non- interacting point-likeconstituents. These are what were originally (Feynman (1972)) called partons. (We now identify them as quarks and gluons.) Since the hadron in this frame is ultra-relativistic, we must regard the partons as massless and as each moving parallel to the hadron with a certain fraction z of its momentum. The structure functions of the hadron are obtained
partons'
structure functions in tree
approximation
by computing the
and by then summing
all partons weighted by their number density. Letfa/N(z)dz be the number of partons of type a in hadron N with fraction z
over
to z +
dz of its momentum. Then, for example, the electromagnetic structure
functions
are:
2F7n(*)
=
* ̄1*Tl(*) +
The relation F2
=
heavy quark
2xF1
(14.2.1)
terms.
spin-^ of the quarks (Callan independent of Q2. This is the
is characteristic of the
& Gross (1969)). Notice that Fj and F2 are property known as scaling. Experimentally,
Fl
and F2 obey this property
approximately. It is
common
in the literature to
use
the terms 'structure function' and
because of the parton model relation between them (14.2.1), which is also approximately true in QCD-see Section 14.7. However, it is important to distinguish the two terms. Structure 'quark distribution'
functions
are
interchangeably,
coefficients of certain tensors in
a current
correlation function.
be defined for other processes, e.g., the Drell-Yan process (Lam & They Tung (1978)). On the other hand the quark distribution functions are exactly can
what their
name
implies: probability distributions
of
quarks
in
a
hadron.
The parton model is correct in a super-renormalizable theory (Drell,
Levy
and Yan (1969,1970a, b, c)). However, in a renormalizable theory like QCD, there are processes inside a hadron that happen significantly on all time scales
143 Dispersion relations and moments
359
assumption of the parton model does not hold. cause the problems allows the to come into operator-product expansion play to solve the problem. There are two approaches, essentially equivalent for deep-inelastic scattering:
down to zero. Then the basic
However, the fact that it is short times that
(1)
Use a dispersion relation to show that moments ofthe structure functions
(i.e., M
FiAQ)
=
(14.2.2)
Axx^-'F^Q2))
be directly computed by the Wilson expansion. This approach was initiated by Christ, Hasslacher & Mueller (1972), and it is the one we will can
use.
(2) One
can
generalize
the derivation of the
operator-product expansion
(Amati, Petronzio & Veneziano (1978), Ellis et al. (1979), Libby & Sterman (1978), Stirling (1978)). For deep-inelastic scattering, the result is equivalent to the first method without the taking of moments.
14.3 Dispersion relations and moments Consider the time-ordered Green's function
TMV defined by (14.1.9). It can be
expanded in scalar structure functions Tl9T29T3i T4, T5, T6,just like W^. We will only need Tl9 T2i and T3. The operator-product expansion derived in Chapter 10 can be applied to T^ when Q2 and v get large with Q2/v2 fixed. As we have seen, this is not the scaling region, for we have x-> oo instead of x fixed. However, we will relate T^v to W^ by a dispersion relation. Then we will
W^v in the physical region can be obtained from the operator-product expansion for T^v. If Q2 is fixed and positive then each T- is analytic in the v-plane. There are cuts going out to infinity from the thresholds v ± Q2/2. See Fig. 14.3.1. see
that information
on
=
(This
is
a
Fig.
standard
14.3.1.
property.
Analyticity of
W
It
can
be
proved
and contour to derive
by expressing
(14.3.1).
360
Deep-inelastic scattering
<N | Tj^yVjlO)| N > in terms of W^ and noting that The discontinuities across the cut are:
T^P,q)\\^ TJP,q)\;;'£ is
W^
W^v
=
] > 0), J
^WJptq), (if v>0).
.
,
where
=
W^ is zero if | v|< Q2/2.)
4tt Wv?(p,q),
(if
v <
(14.3.1)
replaced by its hermitian conjugate /:
with ;'
K=^*yeiqy<p\My)jM\p>- For the
electromagnetic
hermitian,
so
By
Cauchy's
neutral-current processes the current is
W^ W^. W^v is for charged-current neutrino the for antineutrino scattering. structure functions W^ gives
that
scattering then
or
(14.3.2)
But if
=
theorem
we
have
where C is any contour enclosing v, as shown in Fig. 14.3.1. We will be able to compute the
Ti(Q2, v) in
T/s in the short-distance limit v/Q2
a
power series in 1/x
Tr=
vT,/M2=
=
Tln(Q2)x-\
t
T,.n(Q2)x-" (i
n = 0
expand
2v/Q2:
t
n=0
0. So suppose we
-?
=
2or3).
(14.3.4)
(Weexpandvr./M2(ifns2or3)inanalogywith(14.1.11).)Thenfrom(14.3.3) i
r dvve2 ,
where
a.
=
0 if i
=
convergence at | v' |
1 and =
a.
=
,,
1 if i
=
,
2
i
T,(22,v'Mv7M2r,
or
we
n
is large enough to give
oo, then we can deform the contour and pick up only the
contribution from the discontinuity of T{
Finally
3. If
(14.3.5)
across
the cuts:
write the right-hand side in terms of the scaling functions
F;(x,22): 7y<22)= =
-a -
f1dx'x'"-i[Fi(x',e2)+(- ir+"^',G2)]
2i\_FtJQ2) + (-
1)"+" W)]-
(14.3.7)
14.4 Expansion
for
scalar current
361
This is the dispersion relation referred to earlier. It relates the power series expansion of 7^ about v 0 to the moments of the F's, which are defined by =
PiAQ2)=
fdx^-'F^G2).
J
(14.3.8)
o
applies if the integral is convergent. For small it of n values enough diverges. If vai Wi behaves like vp as v -? oo, then we have convergence only if n is greater than p. Now the limit v -? oo at fixed Q2 is a The relation (14.3.7) only
Regge limit (elastic scattering of a virtual boson off a hadron at energy m \). So there is fori
=
a
1 standard expectation (Treiman, Jackiw & Gross (1972)) that p Ofori 2. This is equivalent to FvF2/x9F3 all behaving =
1 or 3 and p
=
=
1/x behavior for quark distributions, and is roughly what is measured experimentally. Our theoretical predictions will give all the terms in the series expansions of the 7)'s, e.g. (14.3.4). Those coefficients for which (14.3.7) does not apply will like 1/x as x -? 0. In the parton model this would correspond to a
not have any direct
implications
14.4 To
for
Expansion
deep-inelastic cross-sections.
for scalar current
explain without a slew of indices the method for computing moments of functions, let us first work out the case where ;M is replaced by a
structure
hermitian scalar operator;. For example,; might be ZZmqtqi9 appropriate to the coupling of a scalar boson to a particular flavor i of quark. We have a
single scalar
structure function:
F(x9Q2)=W(viQ2)
=
(l/4n)
dAy^<p\j(y)j(0)\Py9
(14.4.1)
while the time-ordered function is: T(v, Q2)
=
IdV?
y(p\ Tj(y)j(0)\p}.
(14.4.2)
(Notice that we choose j to be a renormalized operator. As is the case for the
cf)2 operator for a scalar field, the renormalization factor for qq is the same as the mass renormalization factor, so [qq ̄\ ZZmqq Zmg0g0, where g0 is the =
=
bare
quark field.) dispersion-relation argument applied
The
T(Q2/(2x\Q2)=
t
n = 0
to the series
Tn(Q)x-?
(14.4.3)
gives UQ)
=
 ̄
2i[Fn(Q) + (
-
1)"F?(G)].
(14.4.4)
362
Deep-inelastic scattering
Since the current is hermitian, r(Q) nW) Regge
theory suggests
T
=
have F(x, Q)
{-4iF?(e)' \ 0,
1/x
 ̄
we
as x -?
0,
=
F(x, Q)9
so
that
if"iseven> if n is odd. so
this
[(14 4 5)'
equation is
valid
only if n is
bigger than 1. We now apply the operator-product expansion. The results of Chapter 10
apply in the Now
limit
Q2
-?
oo
with
THx-*
v2/Q2 fixed, i.e., with x
Q2
(14.4.5) is
not
Q.
(14-4-6)
0 term are non-leading by a power a have reliable only prediction for n 0. But the relation
that it would appear that all but the n
of
we
constant times
Q-"[TH(2v/Qrl
=
so
and that
=
=
=
expected
to hold unless n > 1.
Fig.
14.4.1. T(v, Q).
remedy this problem, we must find an object for which x ̄nTn contains This is done by making a partial wave Q in the t-channel. That is, we treat T(v, Q) (Fig. 14.4.1) as we decomposition would treat a scattering amplitude, and decompose it in terms of angular To
the leading-power behavior as
-?
oo.
momenta:
<p|r/GO./(0)|0>=
J
£ =
0
(plVjlpyM^yq/q2).
(14.4.7)
Here<p| Vj\p} is the reduced matrix element of some operator 7JflflofspinJ,
appropriate polynomials in yq/q2. (The operator V3 is not necessarily local.) Now we perform an operator-product expansion of T7'(y);(0), keeping the leading-power behavior for each spin:
and the M3
are
<p\Tj(y)j(0)\p>
 ̄
-
2i
£ (- i)\pK^JpyCJa(y2)y^...y^
J,a
(14.4.8) Here the operator
(9^
^
is
a
different operators of the same
local operator of spin J. The label V denotes spin. Only the symmetric part of the operator
is relevant, and in order that it be of definite
spin, it
must be trace less. Since
14.4 Expansion
363
scalar current
for
the hadron is unpolarized the matrix element has the form
(14.4.9) <pKl..Jp> <p\&Ja\p>(p?r..p,j -traces), (p\(9Ja\p} is the reduced matrix element-a scalar quantity. The =
where
adjusted for later convenience. leading y^O is obtained by using operators of the minimum possible dimension. In QCD these are normalizations in The
have been
power of y as
^J(^...plj ?i9
(14.4.8)
=
= u
21
 ̄JqytliDfl2... iDHj q, symmetrized minus traces,
-23"JGUiViD ...iD?
Gl
,
symmetrized minus
traces.
(14.4.10) In accordance with the results of Section 12.8
we
have kept only gauge-
invariant operators. (We use D^ to denote the co variant derivative, and q to quark of flavor q. Sums over color indices are implicit in
denote the field of a
(14.4.10). Only hermitian operators are needed.) Our usual power-counting arguments imply that the behavior of the scalar coefficient
CJa(y2)
in (14.4.8) is
(^2)"dim(C) times
logarithms,
with dim(C)
=
=
2dim(;)-dim(0£ J+ J
2dim0)-dim[<p|^[p>/<p|p>].
The dimension minus the spin of
(14.4.11)
operator is evidently the important quantity here; it is called the twist of the operator. The leading twist is two, for the operators of (14.4.10), and for them C(y)  ̄y ̄* modulo logarithms. an
Fourier transformation of (14.4.8)
now
T(v,g2)= -2i£ <p\VJa\p>CJ*(Q)x-J J,a
gives +
non-leading
powers of ft (14.4.12)
where
we
define
e?-<mwjdv?c?
(.4.4,3,
Perturbative calculations of T(v, Q2) will give us CJa in (14.4.13). We can then obtain moments of F(x, Q) from the dispersion relation:
2Fj(Q)=Z<p\&Ja\p>CJ%Q)[l a
if J is greater than
one.
+
0(l/Q)l
(14.4.14)
364
Deep-inelastic scattering 14.5 Calculation of Wilson coefficients
There are two parts to the calculations. The first part is to do low-order calculations of the Wilson coefficients
where
now explicitly indicated the dependence on all parameters. corrections have logarithms of Q//z, so the second part of the Higher-order calculation is to compute the anomalous dimensions and then to do a we
have
renormalization-group transformation
to set \i to be of order
Q. Thus
we
write
2FJLQ)
I <p\eJtt\p>wCJa\g(Q),i)MatAg(Q),Q/ri-
=
(14.5.1)
a,a'
The subscript {jx) on the matrix element denotes renormalization with unit of The matrix M is obtained
by solving the renormalization group and for the CJa is C, equation (g(Q\ 1) well approximated by its lowest-order mass ju.
term in perturbation theory. Measurements of deep-inelastic scattering at one value of Q are enough to give <p|0Jfl|p>, and then (14.5.1) predicts the moments of the structure functions at other values of Q.
14.5.1 Lowest-order Wilson coefficients The Wilson coefficients
independent of the target, so we may calculate by a quark state. In tree approximation we have the expansion sketched in Fig. 14.5.1. The scalar 'current'; is the renormalized operator ZZJq^ [^g,], for a particular quark flavor i. Since the Wilson coefficients are independent of mass, we set quark masses to zero. Then from the graphs of Fig. 14.5.1 we find are
them with the hadron state replaced =
itr[>(J + ftl 2 (p + q)2
r=
=
+
4)] (p-q)2
i_ tr W 2
-
1/x2 ',, 2. '1-1/x2
(14.5.2)
-2i,
The factor \ comes from averaging over the spin of the quark. For a quark of any other flavor than i,
p + q
or
for
a
gluon,
we
have T= 0 to this order.
p-q
Fig. 14.5.1. Wilson expansion of
<0|T//|0>
to lowest order.
of Wilson coefficients
14.5 Calculation
To compute the Wilson coefficients
operators defined by (14.4.10). In
we
also need the matrix elements ofthe
quark
a
365
state with flavor i we have
<p^i;'...,Jpo=i^tr(^i^2...^) =
(14.5.3)
2dirp^...pMji
in tree approximation, with the label i' denoting a quark flavor or a gluon. For a
gluon
state and
gluon operator
<H°^...,.,N>=2/v-/v All other matrix elements elements
are zero.
Hence the non-zero reduced matrix
all equal to 2. Comparison of (14.5.2)-(14.5.4) with the
(14.4.12)
(14-5-4)
are
operator-product expansion
shows that
0{g2\
CJi
=
1 +
CJi'
=
0{g2\
if
if
J > 1 and is even,
i'j= U
or
iff is
a
gluon.
"] I
J
(14.5.5)
14.5.2 Anomalous dimensions The anomalous dimension of the operator ZZ^^ is ym:
^(ZZmqq) and
we
let
yJaa\g)
=
ym(g)ZZmqqi
(14.5.6)
be the anomalous dimension matrix of the (9Jais:
^Ja
=
Iyaa(g)eJa'-
(14.5.7)
(Operators ofdifferent spin do not mix.) As shown in Section 10.5 in a simpler case, the Wilson expansion then implies a renormalization-group equation for the Wilson coefficients:
d
d
CJa(9, QM
=
-
X ya.aCJa' + ymCJa a'
(14.5.8)
This equation would be trivial to solve were it not that it is a matrix equation. The lowest-order counterterms for the operators are generated by the graphs of Fig. 14.5.2. It is evident that the different operators mix. To solve
diagonalize the anomalous dimension matrix at order g2. The first step is to recall that the counterterms are independent of quark masses. So the renormalizations respect the SU(nn) symmetry of the flavor space. Therefore let us now choose a new basis for the twist-2
the RG
equation
operators.
we must
366
Deep-inelastic scattering
"A
+
'*\
+
AAA AA _rs
CJg
Fig. 14.5.2. Lowest-order divergences of the operators in the Wilson expansion of <0|T/y|0>.
There is
a
multiplet
of non-singlet operators:
GJ,?s,a
where the /lfl's There
are two
are
the nfl
x
=
2i
-^yMiLDM2... ^
(14.5.9)
singlet operators: 0", defined by &'s=
(14.4.10),'
f
q*\
£
flav,i The
;.>,
n{l matrices that generate the flavor symmetry.
renormalizations
(14.5.10)
are
[0JS]
\[e>JNS'\
=
U21 \
z22
0
0
|U? 1,
(14.5.11)
I
(14.5.12)
0
from which the anomalous dimension matrix 7u
7i2
0
?22 0
^=|?21
0
14.6 OPE for vector and axial currents is obtained
367
by
Z^^ A calculation of the
divergences
i?|-Zar
=
of the
(14.5.13) of Fig. 14.5.2
graphs
gives (Gross
(1976)) r,2
I1
2
=
-
+
1)+\?2*J'
12
Vn=^T2 8tt y?
J(J
J(J
-
J
12
1)
(J
+
l)(J + 2)
+
1
2
1
2nfl (J2 + J + 2) .zi 4jt2 J(J + l)(J + 2)'
r;2._il<4±i±*
(14.5,4,
14.5.3 Solution of RG equation
-
non-singlet
The Wilson coefficients of the non-singlet operators evolve
tt^CJNS(g(n), Qln) An
approximate solution
can
=
(
be found
-
very
yis + ym)CJNS.
by taking gives
simply:
(14.5.15)
the one-loop
approximation for the anomalous dimensions. This
CJNS(9(?\Q/li)
=
 ̄ln?2/A)
CJNS(9(Qll)\\Mfi/A)]
(14.5.16)
y^ 7 and y^ denote the coefficients of g2/4n2 in yJNS(g) and ym{g\ and At is the coefficient of g3/(4n2)inp(g). We may replace CJNS{g{Q)9 l)byits
where -
value in tree approximation. The accuracy of (14.5.16) may be systematically
improved by taking more terms in the perturbation expansions of /?, yNS9 ymi and C The singlet coefficients may be obtained by diagonalizing the 2 x 2 matrix of anomalous dimensions. Then there are two linear combinations of singlet
coefficients that have simple behavior like (14.5.16).
14.6 OPE for vector and axial currents We will now apply the operator-product expansion to the structure functions of T for a weak >or
electromagnetic
current. The
argument is
a
simple
368
Deep-inelastic scattering
generalization of the treatment in Sections scattering with a scalar current. We have
14.4 and 14.5 of
deep-inelastic
dV*,yX<H^...JP>(-i)Jx
rMV(p,*) ̄-2i
i_2
x>"-H-^-^CM vdy2 oy"3yy
-^???^^-^)(^-^-?W) -i/J.../'V^C?(y2)|-
(146.1)
acting on the Cffl,s are arranged to be in combinations with divergence, to correspond to the condition q* 7^v 0. Fourier transformation as at (14.4.13), with suitable factors of Q2, gives
The derivatives zero
=
^v--2il<p|^j?x J,a
*{(-0,,v+v?vA?2)* ̄JCia(e) + (l/v)(p? q?v/q2)(pv qvv/q2)x<  ̄JC{°{Q) i<wV(l/2v)x-JC^(0}. -
 ̄
-
(14.6.2)
The set of leading twist operators is the same as in Section 14.4, and we have arranged normalizations so that the C/s are dimensionless in leading twist. Hence the moments of the structure functions satisfy
FltJ(Q) + (- 1)JFM(0= £Cf(0<p|^"|p>+correction, a
F2,j-1(Q) + (-1)JF2.j-i(Q) f
=
lCJ2"(QKp\&Ja\p) + concction, a
3,j(6) + (- l)J 1^3,j(e)= Z Cia(Q)<P\?Ja\P> +
+ correction.
(14.6.3) These equations are valid for J times logarithms. 14.6.1
Wilson
>
1, and the corrections
coefficients
-
electromagnetic
are
of order
1/Q2
case
To compute the lowest-order Wilson coefficient we consider deep-inelastic scattering pn a quark target. The graphs are the same as in the case of a scalar
current, Fig. 14.5.1, except that the current operator is
now
the elec-
14.7 Parton interpretation
tromagnetic t-1
of Wilson expansion
369
current. We find
(lowest order)
-4K^M*£&]} Xe2A 9 ) (- 0?v
=
V
+
Expanding about 1/x
0
=
Vfiv
U>,
-
<7?<7v/<Z2h),1/X2
+<
,2
liilvll
qj>/q2)(py
-
-
1
<?vv/<?2)
<>. --^- 1/x
v
—
1
(14.6.4)
gives
£
r1=-2ie2
(i/x)2"+2
+
(l/x)2n+1
+
o(02),
n=0
vT2/M2=
,2 -4ie2 y £
n
The Wilson coefficients [ficients
 ̄
are
=
therefore therefor
 ̄
 ̄
j
(14.6.5)
O(02).
n /^2n+i _l n^2
0
,
,
if J is
even
and > 2,
C3=0, C^ The relation
=
(14.6.6)
0+O(^2).
C2 2Ct corresponds to the Callan-Gross relation F2 =2xFt in the parton model. Since the renormalization-group equation is the same
for
both
C\a
corrections
=
CJ2a9 of 0(g(Q)2). and
the Callan-Gross relation is true in QCD with These corrections
are
from the Wilson coefficient,
and have been calculated. See Buras (1981) for an up-to-date list of references.
The renormalization-group equations except that ym is replaced by conserved current is zero.
14.7 Parton
zero
are
the
same as
in the scalar case,
since the anomalous dimension of
interpretation
of Wilson
a
expansion
The
use of moments in comparing theory and experiment is not very convenient, since cross-sections are needed outside the range in which they are measured. A more convenient form can be derived in which an expansion
is obtained for the structure functions themselves. We will just summarize the results. More details
can
again
be found in Buras
It is sufficient to examine the case of
a
(1981).
scalar current for which the
expansion is F(x, Q)
=
£
p.
-falN(z,n)Ca(z/x;Q/n,g(n)).
(14.7.1)
370
Deep-inelastic scattering
The sum is over all species of parton, i.e., flavors of quark and
antiquark, and gluon.ThegeneralizedWilsoncoefficientCaiseffectivelyastructurefunction deep-inelastic scattering of a parton state, while falN(z, n) is a parton distribution. Lowest-order calculations reproduce the parton-model result,
for
so
that
Cq
C-=eq2S(z/x-l)/2y
=
Cg
0.
=
The renormalization group
A*;p/U(*>A0
(14.7.2)
equation
=
E
for / has the form
-jybaWx,g)fbiN(z>i*)-
This is called the Altarelli-Parisi (1977) equation
(14-7-3)
it was first derived by these authors in leading logarithmic approximation from an heuristic argument. Later derivations (Collins & Soper (1982a), and Curci, Furmanski
& Petronzio (1980))
-
complete. Integro-differential equations like (14.7.3) are not particularly easy to work with. One mathematical simplification that can be made is to take moments, with the result that the operator-product expansion of are more
Sections 14.5 and 14.6 is recovered. This will enable
us to see
that the two
essentially equivalent. However, the mathematically more complicated method using convolutions, as in (14.7.3), gives more physical insight, can be extended to other processes (Buras (1981)), and can be used without knowing structure functions at small x. To see the equivalence of the two methods, let us define the following methods
are
moments:
/&fo)= C?
=
y£>= Then (14.7.1)
fdzz"-1/,,^), P Wz)(xlzrlCa{z,x-,QltL,(m, {
d(x/z)(x/zr-lyba(z,x;g(fx)).
(14.7.4)
implies Fn(Q)
=
TS?Ui*)C"\g,
Qlfr
(14.7.5)
a
This expansion has the the
same as
same
form
as
(14.4.14). In fact, moments of the/'s are
the matrix elements of the twist-2 operators:
(N\0Ja\N}
=
P dxxJ ̄ Va/jvM +fm(*)l
Jo
<14-7-6)
14.8
W4
and
371
W5
a contribution from each type of parton and antiparton. This equation can be proved (Collins & Soper (1982a), and Curci, Furmanski & Petronzio (1980)) provided only that the same renormalization prescription
where there is
(e.g., minimal subtraction) is used for both the operators and for the parton distributions.
14.8 W4 and W5 So far
we
have
ignored F4
and
F5. They
are zero
conserved. But the weak-interaction currents
if the current/ in
are not
W^ is
conserved if quark
masses are non-zero:
^OA/a
-
?5u?=^([>aa] =
Here m is the quark
mass
-
y5{m^a}w (14.8.1)
iDa/2.
matrix. It follows that W4 and W5 are non-zero in we should regard the ratio m/Q as setting the
neutrino scattering. However, scale for their effect
the cross-section (Jaffe & Llewellyn-Smith (1973) and Llewellyn-Smith (1972)). They therefore give a small contribution to the deep-inelastic cross-section. Since W4 and W5 are therefore non-leading in on
the Bjorken limit, they are difficult to compute directly. A convenient technique to compute W4 and W5 is to consider
q?W^v
=
(q2W4 + p-qW5/2)qv/M2 + q2pvW5/2M2.
(14.8.2)
We have the operator formula
Wv so wecan
7M(y)Z)(0).
=
Jd *yeiq'y<p\Uy)D(0)\p},
(14.8.3)
compute W4and W5 by making an operator-product expansion for
Since there is an explicit factor of quark mass
m
in the expression for
Z), this operator behaves as a dimension 3 rather than a dimension 4 operator.
Q2. Since,in(14.1.10),thetensorsmultiplyingW/4andPV5havea^or^fvinthem,a
This suppresses the Wilson coefficients for W4 and W5 by
a
power of
similar suppression occurs because the lepton masses are much less than Q, as is seen by contracting q^ or qv with the lepton tensor (14.1.7). The result is that
W4 and W5 make
a negligible contribution to the cross-section at large Q. A detailed treatment of W4 and W5 within QCD can be made but has not yet been published. It generalizes the results of Jaffe & Llewellyn-Smith
(1973), who worked within the parton model.
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Index
e-expansion 62 <j>3 theory 36 one-loop calculations 38 A parameter of QCD 188 Adler-Bardeen theorem 338 Altarelli-Parisi equation 370 anomalies 331 Adler-Bardeen theorem 338 not in non-singlet axial current 346 one-loop calculation 342 triangle 349 anomalously broken symmetries 18, 331 asymptotic freedom 187 bare parameters definition 10 renormalization group calculation 209 dimensional regularization 217 basic graph 90 basic Lagrangian 89 definition 11 BPH 53 see also renormalization prescriptions BPHZ 53, 133 see also renormalization prescriptions broken symmetries 18 anomalous 18 dynamically 19 explicitly 18 spontaneously 18 not at d 2 26 =
perturbation theory 26, 247 renormalization 247, 314 BRS, see gauge theories canonical momentum field 5 causality 6 charge conjugation symmetry, for gauge theories 35 chiral symmetry 17, 333 and anomalies 338 commutation relations canonical equal-time 5 from Green's functions 6
composite operators
138
definition 146 equations of motion 155, 158 mixing under renormalization 149 properties 152 renormalization, example 142 tensor operators 150 twist 363 Ward identities 160 see also operator product expansion continuum limit 8 see also renormalization coordinate space, renormalization in 59, 277 cosmological term, and renormalization 118 counterterm Lagrangian 11, 89 cut-off dimensional regularization 13, 53, 62 for infra-red 56 lattice 8 Pauli-Villars 12 see also individual entries decoupling theorem 222 examples 226 exceptions 223 manifest decoupling 230 proof 233 renormalization group 240 deep inelastic scattering 354 Altarelli-Parisi equation 370 calculation of Wilson coefficients 364 operator product expansion 361 partons and operator product expansion
369 W± and W5 357, 371 degree of divergence 45 differentiation with respect to parameters in Lagrangian 163 dimensional regularization 13, 53, 62 axioms 64 definition 67 convergence 68 Dirac matrices in 83, 334 list of integrals 81 properties 73
378
Index
reduced to ordinary integration 67 uniqueness 65 dimensional transmutation 203 Dirac matrices y5 at 4^4 86, 334 definition in d dimensions 83
dynamically broken symmetries 19
unbroken, invariance of counterterms 245 Goldstone bosons, see Nambu-Goldstone bosons Goldstone model 247 Grassmann algebra 27 Green's functions
momentum space 15
definition 6 equations of motion 13 functional integral 7 generating functional 7
operator 5
momentum space 15
equations of motion Green's function 6,13
Euclidean
space-time 8.
evanescent operators 346
explicit symmetry breaking 18 factorization theorems 1, 187 Faddeev-Popov ghost, see gauge theories
Feynman rules for gauge theories 34 see also perturbation theory fixed points 190 forest formula 109 3-loop example 106 functional integral for bosons 6 for fermions 27 Monte-Carlo calculation 13
gauge dependence of counterterms 309, 327 gauge invariant operators 316 gauge theories 28 BRS invariance 32
Hamiltonian, field theory 5 infra-red behavior, and renormalization group 187, 191 Landau ghost 196 large mass expansion 222 large momentum behavior of Green's functions 185 lattice 8 leading logarithms 193 non-renormalization group 196 list of integrals 81 low-energy behavior 191 mass-shell renormalization prescription 48 massive photon, renormalization 324 minimal subtraction 56, 135 see also renormalization prescriptions Monte-Carlo 13
charge conjugation symmetry 35 Faddeev-Popov ghost 30 Feynman rules 34 gauge invariance v. gauge independence 31
Nambu-Goldstone bosons 21, 252 Noether's theorem 18 non-renormalizability 49
gauge-fixing 30 operator product expansion 321 proof of renormalizability 298 quantization 29
normal products,
renormalization 293 renormalization group 319 Slavnov-Taylor identities 32 Gell-Mann-Low formula 22 General Relativity as example of gauge theory 17 cosmological term and renormalization 118 generating functional for Green's functions 7 global symmetries renormalization 244 spontaneously broken, invariance of counterterms 247
in real physics 122 non-renormalization of currents 162 see
composite operators
operator product expansion 257 and parton model 369 and renormalization group 274 deep inelastic scattering 361 examples 139,258 gauge theories 321 in coordinate- and momentum-space 257 proof 266 to define composite operators 258 overlapping divergence 99 oversubtraction 130 on 1 PR graphs 131
partition function, functional integral as 8 parton model 355, 358, 369 Pauli-Villars cut-off 12
379
Index perturbation theory 21
saddle point expansion 26 Feynman rules for gauge theories 34 maximum accuracy with asymptotic freedom 189 not convergent 24, 189 perturbative QCD 1 Poincare symmetry 17 propagator 22 short-distance behavior 278 thermal 280 as
QCD A parameter 188 asymptotic freedom 188 Feynman rules 34 perturbative 1 see also gauge
theories
QED gauge dependence of counterterms 327 in external field 291 renormalization 293, 322 renormalization group 329 Ward identities 326 see also gauge theories see QCD quantum electrodynamics, see QED quantum field theory analog of statistical mechanics 8 axioms 6 review 4 standard properties 6 standard theories 36 see also, <£3 theory; QCD; QED quantum mechanics
quantum chromodynamics,
as
field theory at d
/^-gauge
314
regulator,
see
=
1 6
reconstruction theorem 6
cut-off
renormalizability 49, 116 renormalization 9, 88 and continuum limit 9 axial current 340 coordinate space 59, 277 examples 281 examples 38 for massless theories 59 forest formula 109 gauge dependence of counterterms 309, 327 gauge invariant operators 316 gauge theories 293 with symmetry breaking 308, 314 in external field 291 locality of counterterms 97, 125 non-zero temperature 286
oversubtraction 130 prescriptions 52 QED 293, 322 with photon mass 323 renormalization group 50, 168 A parameter 188 asymptotic freedom 187 change of renormalization prescription 200 coefficients 180 composite operators 219 computation of bare parameters 209 dimensional regularization 217 cut-off procedure, dependence on 206 decoupling theorem 240 dimensional transmutation 203 equation 183 examples 50, 172 fixed points 190 gauge theories 319 infra-red behavior 187, 191 Landau ghost 196 large momentum behavior 185 leading logarithms 193 minimal subtraction 56, 181 several couplings 197 solution of RG equation 184 renormalization prescriptions 52 BPH 53 BPHZ 53, 133 mass-shell 48 minimal subtraction 135 at one loop 56
spontaneously broken symmetries 253 renormalized field 10 renormalized mass, and physical mass 51
Slavnov-Taylor identities 32 spontaneously broken symmetries 18 absence of IR divergences 252 choice of renormalization prescription 253 renormalization 247 renormalization of current 251 super-renormalizability, definition 118 supersymmetries 17, 339 symmetries 15, 244 BRS, see gauge theories charge conjugation, see gauge theories gauge 28 Ward identities 19, 160 see also broken symmetries; global symmetries thermodynamic limit of functional integral 8 triangle anomaly 349
380 twist 363 ultra-violet regulators 12 see- also cut-off; dimensional regularization unitary gauge 324, 330 Ward identities 16, 19, 160 axial current 339. 349 OED 326
Index renormalization 246-256 Weinberg's theorem 125 Wick rotation 8, 39 Wightman axioms 6 Wilson expansion, see operator product expansion Yang-Mills theory, theories
see gauge