Hidden Worlds: Hunting for Quarks in Ordinary Matter 9780691187242, 0691057737, 2002074878

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HIDDEN WORLDS

HIDDEN WORLDS HUNTING FOR QUARKS IN ORDINARY MATTER

TIMOTHY PAUL SMITH

PRINCETON

UNIVERSITY

PRINCETON

AND

PRESS

OXFORD

Copyright © 2003 by Princeton University Press Published by Princeton University Press, 4 1 William Street, Princeton, New Jersey 08540 I n the U n i t e d Kingdom: Princeton University Press, 3 Market Place, Woodstock, Oxfordshire OX20 1SY A l l Rights Reserved Library o f Congress Cataloging-in-Publication Data Smith, T i m o t h y Paul, 1960Hidden worlds : hunting for quarks i n ordinary matter / T i m o t h y Paul Smith, p. cm. Includes index. I S B N 0-691-05773-7 (acid-free paper) 1. Quarks. I . Title. QC793.5.Q252 S65 2003 539.7'2167—dc21 2002074878 British Library o f Congress Data is available This book has been composed i n Galliard and Futura Condensed Regular Printed on acid-free paper.«> www.pupress.princeton.edu Printed i n the U n i t e d States o f America 1 3 5 7 9

10

8 6 4 2

- Kristina AND H E R INFINITE PATIENCE W I T H M Y M A N Y PROJECTS

-CONTENTS LIST OF FIGURES

ix

ACKNOWLEDGMENTS

xi

-CHAPTER ONE Hidden Worlds: The Search for Quarks in Ordinary Matter -CHAPTER TWOThe Rise and Fall (for the right reasons) and Rise Again o f the Quark Hypothesis 15 -CHAPTER THREE The Players and the Stage

33

-CHAPTER FOUR The Nature o f the Evidence -CHAPTER FIVEMeasuring a Rainbow

51

68

-CHAPTER SIX Particle Taxonomy and Quark Soup

90

-CHAPTER SEVEN The Shape of Things 109 -CHAPTER EIGHTThree Quarks Plus 131 -CHAPTER NINE Digging a Little Deeper

148

-CHAPTER TEN A New Age o f Exploration w i t h i n the Hidden World GLOSSARY INDEX

165 175

161

1

- FIGURES -

2.1 2.2 2.3 2.4 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4.1 4.2 4.3 4.4 4.5 5.1 5.2 5.3 5.4 5.5 6.1 6.2 6.3 6.4 6.5

A n early photographie plate s h o w i n g strange decay. Particles arranged by charge and mass. Particles arranged by isospin and strangeness, the eight-fold way. The first observation o f the O" particle. The nucleon-nucleon force. The Feynman diagram for a p i o n exchange. A baseball sequence o f play. A baseball Feynman diagram. A baseball Feynman diagram for a more complex play. Feynman diagram o f a p i o n exchange between t w o protons. Water: Van der Waals force. C o l o r V a n der Waals—quark flip-flop. Feynman diagram for quark flip-flop. Spectrum as histograms o f counts. Spin up and spin d o w n . C o m b i n a t i o n o f t w o spin | quarks. T h e decay o f the A particle. The cross section o f the A particle. The accelerator at the Thomas Jefferson N a t i o n a l Accelerator Facility. The t w o H i g h Resolution Spectrometers i n H a l l A . H a l l C, w i t h t w o spectrometers. The H a l l B C L A S detector unfolded. The cross section o f C L A S w i t h a reconstruction o f an event. Beta decay—the quintessential weak decay. M u o n decay—the simplest o f all weak decays. T h e allowed weak decays. L a m b d a decay. The life o f a 71°.

X

7.1

FIGURES T h e p o t e n t i a l arrangement o f quarks that depend u p o n spin effects.

7.2

T h e charge d i s t r i b u t i o n w i t h i n a n e u t r o n and a p r o t o n .

7.3

T h e Bates Large Acceptance Spectrometer T o r o i d

7.4

W o r l d ' s data for

7.5

T h e n e u t r o n charge d i s t r i b u t i o n .

7.6

A sketch o f w h a t the three quarks m i g h t l o o k like i n

(BLAST). G\ E

a neutron. 8.1

T h e A , Roper, and other resonances i n the cross section

8.2

A n up and a d o w n quark i n t e r a c t i n g by a g l u o n and w i t h

o f a nucléon. color. 8.3

T w o different pictures o f a t h r e e - q u a r k / t h r e e - g l u o n system.

8.4 8.5

A picture o f a nucléon w i t h a tangle or w e b o f gluons. T w o modes o f oscillation that may drive the Roper resonance.

9.1

A scale o f physical phenomena f r o m humans t o current quarks.

9.2

T w o different electron-photon l o o p diagrams.

9.3

V i e w i n g a hucleon w i t h t w o different resolutions.

9.4

A v i r t u a l strange - ant is trange quark pair can decay i n t o different particles.

9.5

I n beta decay we d o n o t conserve quark flavor.

9.6

A v i r t u a l p h o t o n and a v i r t u a l Z°-boson i n electron scattering.

-ACKNOWLEDGMENTS I w o u l d like t o express m y thanks t o the people at the University o f N e w H a m p s h i r e , where I started this b o o k , and at the Thomas Jefferson N a t i o n a l Accelerator Facility, where I was d o i n g m y re­ search at the t i m e . A m o n g t h e m are B i l l Hersman, John Calarco, John Dawson, M a u r i k H o l t r o p , N a t h a n Isgar, and many more w h o g o t me started o n nucléon structure. I w o u l d also like t o t h a n k the people at the Massachusetts I n s t i t u t e o f T e c h n o l o g y Bates Linear Accelerator Center, where I finished this book. These include faculty and co-workers Townsend Z w a r t , Richard M i l n e r , E d B o o t h , June M a t t h e w s , Karen D o w , D o u g Hasell, K e v i n M c l l h a n y , and many more. But i t t o o k more t h a n just learning about physics and experi­ ments t o create this book. I t t o o k the ten thousand questions f r o m students and fellow physicists w h o made me hone m y de­ scription, and even m y understanding, o f the physics o f nucléons and quarks. T h e list is l o n g and includes Ben Yoder, M a r k Szigety, Jeff Vieregg, Lisa G o g g i n , V i t a l i y Z i s k i n , and A b b y G o o d h u e , w h o w o r k directly w i t h me. Also A d r i a n , Peter, A d a m , A a r o n , Chris, Ben, T o n g , Ehsan, D a n , W i l l i a m , Tavi, Nikolas, H a u k e , Tancredi, and the rest o f the army o f curious m i n d s w h o are w o r k i n g o n these experiments. I must also m e n t i o n m y family, w h o supported me t h r o u g h this project. First m y boys, W i l l and R o b i n , w h o watched this b o o k grow, n o t as fast as they grew, often i n the early hours o f Saturdays and vacations. Finally, Kristina, w h o as a physicist read, reread, and reread this b o o k again, and as m y wife encouraged and supported me w i t h infinite patience. M y deepest thanks.

HIDDEN WORLDS

Hidden Worlds: The Search for Quarks in Ordinary Matter

B

Y M O S T accounts, the quest t o understand the basic struc­ ture o f matter has been an old-fashioned success story o f g r o w t h and expansion. Machines have become bigger; c o m ­ puters have g o t t e n faster. Beams o f l i g h t o r particles have become brighter and more p o w e r f u l . Interactions o f elementary particles have become m o r e fleeting, and have given rise t o ever more energetic and m o r e exotic by-products. T h e basic engine d r i v i n g this g r o w t h , the particle accelerator, began as a tabletop i n s t r u ­ ment y o u c o u l d h o l d i n y o u r hand—and was n o more p o w e r f u l t h a n a l i g h t b u l b . By the 1950s, accelerators had g r o w n large enough t o fill a small warehouse, and drew enough power t o r u n a large p r i n t i n g press. N o w they need farmland or rangeland t o accommodate their dimensions, and enough power t o r u n a me­ dium-size city. H i g h e r energies enable experimenters t o "see" finer and finer details, t o probe and analyze matter at smaller and smaller scales. Today the w o r l d ' s most p o w e r f u l particle accelerators are oper­ ated at F e r m i L a b , o n the I l l i n o i s prairie about an hour's drive southwest o f Chicago, and at C E R N ( C o u n c i l Européen p o u r la Recherche Nucléaire—The European O r g a n i z a t i o n for Nuclear Research), the particle physics laboratory, i n a rural suburb o f Geneva just under the Swiss border w i t h France. T h e F e r m i L a b accelerator is a r i n g 4 miles a r o u n d ; inside the r i n g there is plenty

2

CHAPTER 1

o f r o o m t o pasture a herd o f buffalo. A t C E R N , an even larger accelerator is under c o n s t r u c t i o n . W h e n i t comes i n t o service i n 2 0 0 6 , i t w i l l be 16 miles ( 2 7 kilometers) a r o u n d . A t b o t h these laboratories the c o m b i n e d accelerators and de­ tectors are, i n effect, magnificent microscopes t h a t owe t h e i r magnifying powers t o t h e i r ability t o focus energy i n t o electrons o r protons t h a t carry a t r i l l i o n volts. W i t h these energies b o t h machines can resolve details i n the structure o f matter smaller t h a n 10~ meter across—a b i l l i o n t h o f a b i l l i o n t h o f a meter. A n d 18

b o t h machines are examples o f science so b i g and so costly t h a t they stretch the resources o f i n d i v i d u a l sovereign states. C E R N is funded by a European-wide c o n s o r t i u m , and b o t h lab­ oratories are used by a c o l l a b o r a t i o n o f scientists f r o m all over the w o r l d . B u t the h i g h price o f the ability t o probe such details is n o t measured only i n dollars and cents. H i g h energies can magnify, b u t they also carry great destructive power. I t was often said i n the early decades o f high-energy physics t h a t its basic investiga­ tive tactic was m u c h like smashing t w o fine Swiss watches t o ­ gether i n m i d a i r and t h e n t r y i n g t o understand h o w they w o r k e d by l o o k i n g at the fragments. I n fact, the true situation is even worse t h a n that. Particles accelerated by today's state-of-the-art machines collide so v i o l e n t l y that the collision fragments are often s t r i k i n g l y different f r o m o r d i n a r y matter. For many—even most—elementary particle physicists w o r k ­ i n g today, that's just the p o i n t . T h e emphasis i n the past few decades at such places as F e r m i L a b and C E R N has been t o p r o ­ duce some o f the most exotic particles predicted by theory, the " t o p " and the " b o t t o m " mesons. O n a different f r o n t , at B r o o k haven N a t i o n a l L a b o r a t o r y o n L o n g Island, a campaign is under way t o create a " q u a r k - g l u o n plasma," a state o f matter, as some physicists have described i t , " n o t seen since the b i g b a n g . " Such phenomena can be studied only by accelerating, smashing, and, i n effect, heating and squeezing o r d i n a r y matter t o c o n d i t i o n s beyond the edge o f extreme: far beyond the temperatures and

THE SEARCH FOR QUARKS

3

pressures prevailing even i n the cores o f the hottest stars, t o re­ gimes i n w h i c h matter takes o n strange and o u t l a n d i s h forms that d o n o t exist at all i n the universe as we n o w k n o w i t . Yet i n t h a t rush t o re-create such exotic c o n d i t i o n s , and t o study t h e i r implications for the b i r t h , death, and u l t i m a t e struc­ t u r e o f the universe, elementary particle physicists have almost forgotten the w o r l d i n w h i c h we live. I f the i n i t i a l intellectual impulse was t o probe the p r o t o n and the n e u t r o n i n order t o understand t h e i r role i n o r d i n a r y matter, that impulse has v i r t u ­ ally disappeared f r o m the C E R N s , FermiLabs, and Brookhavens o f the w o r l d . To m y m i n d , that's a shame. I d o n ' t live at the d a w n o f t i m e , and I d o n ' t live i n a fantastically h o t and energetic collision. I live i n a w o r l d made up p r i m a r i l y o f electrons, neutrons, and protons. A n d I w a n t t o k n o w h o w they act and interact under " o r d i n a r y " conditions. I am a nuclear physicist, or t o be more precise, a nucléon physicist. " N u c l é o n " is the generic t e r m for n e u t r o n or p r o t o n , the particles that make up the atomic nucleus. M y w o r k and the w o r k o f m y closest colleagues is dedicated t o understanding the physics o f o r d i n a r y nucléons, a layer i n the onionlike organization o f matter that gives rise t o an incredibly r i c h set o f phenomena. Those phenomena are quite literally de­ stroyed a m o n g the debris o f the highest-energy accelerators.

ORDINARY MATTER T h i n k about i t this way: the universe that we understand is more t h a n 99.95 percent neutrons and protons by mass. I t is true that there are t h i n g s we physicists d o n ' t understand, such as the stuff astronomers and cosmologists call dark matter. B u t the range o f t h i n g s we d o understand i n terms o f electrons, neutrons, and protons is astonishing. Stars, those h o t , g l o w i n g beacons i n space, those l i g h t - and life-giving orbs suspended i n the v o i d , are made up o f these three materials. Nebulae, the misty

4

CHAPTER 1

veils o f interstellar gas and dust t h a t stretch like curtains across the galaxy, are made o f the same stuff. Even the most exotic o f stars, the so-called n e u t r o n stars, are made o f the same three ingredients. Closer t o home, the w i n d , the r a i n , and the earth beneath o u r feet are made o f these three b u i l d i n g blocks. Even the substance o f life itself—the b l o o d coursing t h r o u g h o u r veins, the b r a i n and nerve tissue that provide the scaffolding for o u r t h o u g h t s , the deoxyribonucleic acid ( D N A ) that carries the blueprints f r o m w h i c h each o f us is b u i l t — i s made up entirely o f electrons, neutrons, and protons. Books and tables, hands, hearts, and heads, are made o f these three most basic substances. A t one t i m e , n o t so many years ago, the study o f such particles coincided w i t h the frontiers o f high-energy physics. O n e o f the first laboratories I ever w o r k e d w i t h was the N a t i o n a l I n s t i t u u t v o o r Kernfysica en H o g e Energie Fysica ( N I K H E F ) i n Amster­ d a m , T h e Netherlands. Even i f y o u d o n ' t speak D u t c h , y o u can probably understand most o f the name: read "hoge

énergie

fysica" phonetically and y o u w i l l hear "high-energy physics." N o t so obvious is "kern-fysica," w h i c h corresponds t o the E n ­ glish phrase "nuclear physics." S t i l l , whether one uses the t e r m "nuclear" or " k e r n e l , " the w o r d is meant t o emphasize the role o f the nucleus at the very heart o f the a t o m . That's an i m p o r t a n t clue t o understanding w h a t stirs the soul o f the nuclear and the high-energy physicist. W h e n the w o r d "nucleus" was coined i n 1 9 1 2 , i t was viewed as the " a t o m " had been before i t : as the u l t i m a t e , indivisible, fundamental particle o f matter. T h e nucleus stood at the core or kernel o f the a t o m , the sun about w h i c h the planetary electrons o r b i t e d . Nuclear physics, therefore, was essentially a quest t o discover and describe the most basic b u i l d i n g blocks o f the universe. W h e n physicists discovered that the nucleus itself was divisible i n t o nucléons and had structure, and that those nucléons also had internal struc­ t u r e , the "dream o f a final t h e o r y " o f u l t i m a t e particles had t o be abandoned w i t h i n the d o m a i n o f nuclear physics. T h a t dream

THE SEARCH FOR QUARKS

5

was passed o n t o high-energy physics, a discipline that, by its very name, n o longer defined itself by assumptions about where the u l t i m a t e particles w o u l d be f o u n d . T h e N I K H E F accelerator c o u l d accelerate electrons t o ener­ gies o f 7 7 0 m i l l i o n electron volts ( M e V ) . A 7 7 0 M e V electron beam can probe matter at a scale slightly smaller t h a n a f e r m i , or 10"

15

meter. T h a t is just p o w e r f u l e n o u g h t o resolve structure i n

the atomic nucleus itself; y o u c o u l d say t h a t N I K H E F marks the start o f nucléon physics. A n o t h e r laboratory that I visited recently is called D E S Y (pro­ nounced "daisy"), the Deutsches E l e k t r o n e n - S y n c h r o t r o n , i n H a m b u r g , Germany. DESY's beam energy is 30 b i l l i o n electron volts (Giga electron volts [ G e V ] ) , 4 0 times m o r e energetic t h a n N I K H E F ' s , w h i c h also makes its resolution 4 0 times finer. The trade-off is that DESY's "field o f v i e w " is t o o small t o be o f m u c h use for the study o f nucléons. I t c o u l d reasonably be argued that D E S Y marks the energetic upper l i m i t o f nucléon physics. T h e beam energy is so h i g h that w h a t y o u see are the "bare" constit­ uents o f nucléons. These constituents interact so v i o l e n t l y that the nucléons themselves n o longer m a i n t a i n t h e i r identities, b u t become transformed instead i n t o new and exotic particles. I n ­ deed, most physicists at D E S Y identify themselves as high-energy physicists, n o t nuclear or nucléon physicists at all. B e y o n d DESY, the high-energy frontier has m o v e d even far­ ther o n , t o C E R N , t o F e r m i L a b , t o Brookhaven, and elsewhere. N I K H E F and other accelerators o f its class have l o n g since been displaced as record holders for h i g h energy, just as they had dis­ placed t h e i r predecessors. B u t the c o n t i n u a l historical advance o f the high-energy frontier hardly means t h a t the " c a p t u r e d " territories have been subdued, m u c h less f u l l y mapped o r colo­ nized. T h e physicists r u s h i n g ahead w i t h plans for ever m o r e p o w e r f u l accelerators, for p r o b i n g ever m o r e deeply i n t o the u l t i ­ mate b u i l d i n g blocks o f matter, have seldom stopped t o f u l l y p l u m b the structures that they f o u n d along the way. Yet nucléons represent a level i n the organization o f matter having exceptional

6

CHAPTER I

stability, unique i n the universe. M a n y physicists, and I am one o f t h e m , w a n t t o k n o w as m u c h as possible about electrons, neu­ trons, and protons.

WHAT MAKES A NUCLÉON? Just w h a t d o we physicists already k n o w about these three p a r t i ­ cles? I n the case o f the electron, the summary can be quite brief. Electrons stand stark and i n n o c e n t before us. W h a t we see is es­ sentially w h a t we get: infinitesimally small particles, each w i t h one u n i t o f negative electric charge ( - 1 ) , a spin o f i , and a mass o f 9 x 10" kilogram—some 2 , 0 0 0 times lighter t h a n either the 31

p r o t o n or the n e u t r o n . A n d that is all. We can say n o m o r e about the physical proper­ ties o f the electron because i t appears that there is n o t h i n g m o r e t o say. I d o n ' t mean t o be dismissive about the electron o r its physics. Electrons are u n d o u b t e d l y useful t o twenty-first-century h u m a n k i n d , w h e n we push or p u l l t h e m t h r o u g h wires i n the f o r m o f electricity. T h e y are fundamental, o f course, t o the archi­ tecture o f matter: the n u m b e r o f electrons and t h e i r orbital pat­ terns i n an a t o m or a molecule are w h a t give rise t o all o f chemis­ try. Finally, the self-interaction o f the electron is at the core o f one o f the greatest theoretical successes o f the t w e n t i e t h century: q u a n t u m electrodynamics, or Q E D . Q E D serves as a p r o t o t y p e o n w h i c h theories o f other particles (the quarks) are modeled. A n d Q E D has generated some o f the most precise, experimen­ tally confirmed predictions i n all o f physics. B u t the electron itself appears t o be t r u l y elementary. Elec­ trons seem t o have persisted unchanged since the dawn o f t i m e , and they are likely t o r e m a i n as they are, i m m o r t a l , u n t i l the final sunset o f the universe. There is n o evidence that a n y t h i n g exists inside the electron; there are n o " u l t i m a t e electrons" r a t t l i n g a r o u n d inside its shell. I n all experiments ever p e r f o r m e d i t really and t r u l y appears t o be p o i n t l i k e .

THE SEARCH FOR QUARKS Not

7

so the nucléons: the p r o t o n and the n e u t r o n . A h u n d r e d

thousand times smaller t h a n the smallest a t o m , b o t h the p r o t o n and the n e u t r o n are measured i n fermis. Yet as small as that m i g h t be, there is a w o r l d h i d d e n inside each one. A t first glance that w o r l d appears t o be exceedingly simple. Protons and neutrons are each made up o f exactly three particles k n o w n as quarks. Whatever quarks really are, just t h i n k o f t h e m for the m o m e n t as three balls r a t t l i n g a r o u n d inside o f the sphere we call a p r o t o n or a n e u t r o n . There are t w o c o m m o n kinds o f quark, k n o w n as the up quark and the d o w n quark. T h e p r o t o n is made up o f t w o up quarks and one d o w n quark. T h e n e u t r o n reverses those numbers: i t is made up o f one up quark and t w o d o w n quarks. T h e fact that the p r o t o n and the n e u t r o n each have three quarks gives rise t o s t r i k i n g similarities i n t h e i r masses, sizes, and inter­ actions. B u t that single difference, an extra up quark i n the p r o ­ ton, an extra d o w n quark i n the n e u t r o n , also accounts for t h e i r unique characteristics: t h e i r differing electric charge, t h e i r decay patterns, and the details o f h o w they couple. T h e genesis o f such p r o f o u n d differences merely o u t o f v a r y i n g combinations o f simple parts should n o t be t o o surprising. Sci­ ence has vast experience at larger scales w i t h objects whose dis­ t i n c t i v e properties arise o u t o f the number, identity, and arrange­ m e n t o f t h e i r parts. T h e familiar shorthand for specifying molecules takes tacit advantage o f the fact that i t is often e n o u g h just t o enumerate their elemental components: H 0 — w a t e r — i s 2

two

parts hydrogen and one part oxygen. O f course there are

cases i n w h i c h one set o f atomic ingredients (such as C H 6

1 2

0 ) 6

can give rise t o t w o or more d i s t i n c t b u t related molecules ( i n this case, glucose and fructose). Chemists call t h e m isomers. W h e n we ratchet up the magnification and view the w o r l d o n the atomic level, the very i d e n t i t y o f an atom—the d e f i n i t i o n o f its elemental type—depends solely o n the n u m b e r o f protons i n its nucleus ( w h i c h is equal t o the n u m b e r o f electrons o r b i t i n g that nucleus, i n the electrically neutral a t o m ) . H e l i u m is h e l i u m because i t has t w o protons and t w o electrons. C a r b o n is carbon because i t has six protons and six electrons. Q u a n t u m mechanics

8 - CHAPTER 1 dictates h o w those six electrons i n the carbon a t o m arrange themselves physically, and t h a t arrangement i n t u r n ordains the chemistry o f carbon: w h a t i t w i l l b i n d t o , w i t h w h a t s t r e n g t h , i n w h a t configurations. T h e concept o f isomer has an analogue i n atomic physics as w e l l : a given element can come i n several forms k n o w n as isotopes, chemically almost indistinguishable f r o m one another, b u t different nonetheless i n the n u m b e r o f neutrons t h a t share real estate i n the nucleus w i t h the protons. I n sum, i t seems entirely natural t h a t the properties o f the p r o ­ t o n and the n e u t r o n themselves arise f r o m the n u m b e r a n d ar­ rangement o f the quarks t h a t make t h e m u p . O n e o f the c h i e f burdens o f this b o o k is t o show h o w t h e i r properties and config­ urations give rise t o a " q u a r k c h e m i s t r y " o f remarkable complex­ ity—just as the configurations o f atoms i n molecules and the con­ figurations

o f electrons i n atoms give rise t o o r d i n a r y chemistry.

T h e concepts o f isomer and isotope, for instance, have an ana­ logue i n the w o r l d o f quarks, as we shall see w i t h such particles as "as" and "Ropers."

I N VITRO VERSUS I N V I V O B u t before e x p l o r i n g those details, the very idea o f quark chemis­ t r y can shed l i g h t , I t h i n k , o n a curious question t h a t afflicts nucléon physics w i t h far m o r e confusion t h a n seems necessary. T h e question goes t o the heart o f the discipline: just w h a t is n u ­ cléon physics? I once posed this question t o a n u m b e r o f m y col­ leagues d u r i n g l u n c h at a conference at a laboratory t h a t special­ izes i n w h a t I call nucléon physics, the Jefferson L a b , i n N e w p o r t N e w s , V i r g i n i a . A n d I was surprised t o find t h a t even m y use o f the phrase "nucléon physics" was controversial. Some people w a n t e d t o call i t "intermediate-energy physics," a name m o t i ­ vated p a r t l y by the kinds o f accelerators the w o r k relies o n and p a r t l y by the r e c o g n i t i o n t h a t the study o f the quarks inside n u ­ cléons bridges the study o f the role o f nucléons i n the a t o m ( n u ­ clear physics) and the study o f high-energy physics proper.

THE SEARCH FOR QUARKS

9

Yet most o f the physicists at o u r l u n c h saw w h a t we nucléon physicists d o as less o f a b r a n c h i n g away f r o m nuclear physics t h a n as an extension o f i t — a n d an obvious extension at that. Fur­ ther m u d d y i n g o u r discussion is the way physicists identify t h e m ­ selves: w h o is a nucléon physicist and w h o is a high-energy physi­ cist? T w o experimentalists w o r k i n g side by side o n the same experiment m i g h t identify themselves as "nuclear" o r " h i g h energy" physicists, n o t o n the basis o f the experiment at hand, but

rather o n the basis o f t h e i r o w n experiences i n graduate

school. Physicists w h o had been p a r t o f a high-energy research g r o u p i n graduate school m i g h t wear t h a t tag for an entire career. Likewise, physicists whose advisors called t h e m "nuclear" c o u l d carry t h a t label i n t o retirement. Perhaps i t is n o t surprising that the labeling o f physicists is a social, hence largely accidental, p h e n o m e n o n . B u t the labeling o f the subdisciplines o f physics need n o t be the p r o d u c t o f historical accident as w e l l . Part o f the confusion about nucléon physics stems f r o m the historical shift I n o t e d earlier i n the boundaries ofhigh-energy physics. A 1 G e V electron accelerator i n the 1960s clearly belonged t o the high-energy c o m m u n i t y , whereas at pres­ ent i t is the province o f the nuclear physicist. W h a t t h a t early convergence i n labeling concealed was that the t w o fields have named themselves according t o different criteria. Nuclear and nucléon physics are named after the subjects they study; h i g h energy physics is named after the machines and detectors i t uses. I f nucléon physics was once "high-energy" physics, b u t the h i g h energy frontier has n o w m o v e d o n , doesn't i t f o l l o w that nucléon physics has n o w become "intermediate-energy" physics? T h e answer is n o . B u t let me t r y t o clarify the situation by m o v i n g f r o m a chemical analogy t o a biological one. I n biology, laboratory workers often remove and p u r i f y a cell line t o study the cells " i n v i t r o , " that is, i n a test tube. ( " I n v i t r o " literally means " i n glass.") I n v i t r o studies can give "clean," u n a m b i g u ­ ous results. A n y variables—temperature, r a d i a t i o n , chemical concentrations—that m i g h t affect the results can be scrupulously c o n t r o l l e d . B u t n o biologist w o u l d j u m p t o the conclusion that

10

CHAPTER 1

some result observed i n a test tube or p e t r i dish implies that the same results w o u l d necessarily u n f o l d " i n v i v o . " ( " I n v i v o " means " i n l i f e , " that is, i n the l i v i n g organism.) T h e effect o f a d r u g o n cells i n v i t r o , for instance, m i g h t be quite different f r o m its effect o n those same cells i n t h e i r natural habitat, the b o d y o f the organism f r o m w h i c h they come. T h e behavior o f cells i n v i v o is often so complex and so different f r o m t h e i r behav­ ior i n v i t r o that i n v i v o study is a complementary discipline i n its own right. Once I was at a c o m p u t a t i o n a l workshop at F e r m i L a b w h e n I was asked w h a t distinguished the studies o f protons, neutrons, and t h e i r quark constituents that we pursue f r o m the studies t h a t high-energy physicists make o f the same particles. M y answer was " i n v i v o versus i n v i t r o . " T h e p r o t o n and n e u t r o n physics we study is the study o f quarks i n v i v o , i n t h e i r natural home w i t h i n the b o d y o f a nucléon. H i g h - e n e r g y physics looks at quarks i n a m o r e rarefied, extra-nucleon e n v i r o n m e n t . I t turns o u t t h a t na­ ture forbids the observation o f a single quark, b u t y o u can infer a great deal about the nature o f quarks f r o m the fragments they f o r m . T o pursue the Swiss w a t c h analogy, nucléon physics studies the w a t c h by listening t o its ticks, and, w i t h o u t o p e n i n g i t , tries t o infer w h a t is g o i n g o n inside. So I propose the f o l l o w i n g w o r k i n g d e f i n i t i o n o f the differ­ ences between nucléon and high-energy physics: High-energy physics is the study o f quarks, gluons, bosons, and other exotic, fundamental, and elementary particles o f nature, whereas nuclear/nucleon physics is the study o f these particles as they combine to form normal matter; as they combine w i t h i n the hidden world buried inside protons and neutrons.

CONFINEMENT N o matter h o w y o u study quarks, one o f t h e i r most distinctive properties makes investigating t h e m a peculiar and difficult exer­ cise: quarks are always h i d d e n , b u r i e d deep w i t h i n some larger

THE SEARCH FOR QUARKS - 11 particle. N o one has ever been able t o isolate a single quark. I t is not just t h a t we have n o t been clever enough t o b u i l d a " p r o t o n smasher," or some better machine or experiment. Rather, nature has c o n t r i v e d its laws i n such a way that n o t only have we never seen an isolated quark—but we never w i l l ! T h a t is the r o o t o f a great deal o f frustration, and at the same t i m e the source o f the scientific challenge that we explore i n this book. So h o w d o we proceed? S h o u l d n ' t this "non-observability" lead t o a k i n d o f intellectual crisis? Doesn't the f o r w a r d move­ m e n t o f science have t o be fueled by observation? Yet—it bears repeating—we will never observe quarks. I suspect that a staunch positivist w o u l d have a field day w i t h the quark hypothesis. We cannot see t h e m , or even detect t h e m , yet we k n o w a great deal about t h e m . T o overcome that l i m i t a t i o n we w i l l have t o exam­ ine and r e t h i n k w h a t is meant by g o o d evidence. I n fact, a special emphasis o f this b o o k is t o consider h o w we k n o w w h a t we claim to know. A l o t o f books about contemporary science r e p o r t the end products o f a generation o f research. A b o o k o n quarks w o u l d give great prominence t o the h a r d - w o n list o f their six k n o w n "flavors." B u t for me the road t o these results is at least as inter­ esting as the results themselves. T h e issue is one o f personal taste and appreciation, b u t let me t r y t o explain i t this way: I admire the great beauty o f a medieval cathedral such the D o m , i n Graz, Austria, or York Minster, i n n o r t h e r n E n g l a n d . B u t w h e n I t h i n k about h o w these v a u l t i n g expressions o f the h u m a n spirit were handcrafted i n a preindustrial age, w i t h o u t girders o f i r o n or steel, the cathedrals are transformed t o m y sensibilities f r o m the merely beautiful t o the t r u l y magnificent. So h o w can we k n o w a n y t h i n g about s o m e t h i n g we cannot even see or sense? Briefly, the s o l u t i o n is t o make t h e o r y and experiment w o r k so closely together that they become inter­ leaved. Given a t h e o r y o f quarks, h o w m i g h t a p r o t o n be b u i l t out o f t h e m , and what, under various testable conditions, w o u l d we see? T h e n , w h e n we d o see s o m e t h i n g that r o u g h l y matches those expectations i n o u r experiments, we p r o m o t e and finetune the theories that predicted i t . These general procedures

12

CHAPTER 1

h o l d for any attempt t o reconcile t h e o r y w i t h experiment. Yet o u r experiments that l o o k at the way quarks c o m b i n e t o b u i l d particles are n o t like the ones conducted at the high-energy labo­ ratories such as the Stanford Linear Accelerator Center ( S L A C ) i n California, or F e r m i L a b , or C E R N . Instead o f h u r l i n g p a r t i ­ cles at each other w i t h such f u r y and energy t h a t they are de­ stroyed, we tickle and excite the particles. I n that way we study the quarks " i n v i v o . "

T H E SECRET LIVES OF QUARKS T h e laboratories t h a t measure the shape and excitations o f p r o ­ tons and neutrons " i n v i v o " are o f modest p r o p o r t i o n s , c o m ­ pared w i t h high-energy facilities. T h e energy o f the electron beam at Jefferson L a b can reach r o u g h l y six b i l l i o n electron volts. M I T - B a t e s L a b , n o r t h o f B o s t o n , has an accelerator capable o f delivering particles whose energy reaches a b i l l i o n electron volts, 1,000 times smaller t h a n the " T e v a t r o n " at F e r m i L a b . B u t these laboratories d o have u n i q u e features. They deliver h i g h currents w i t h h i g h precision, and they can manipulate the alignment and o r i e n t a t i o n o f the spins o f the electrons, protons, and neutrons. By using these so-called p o l a r i z a t i o n tools they can disentangle the nuclear effects f r o m the quark effects, n u d g i n g o u t the secrets o f the p r o t o n and the structure o f the n e u t r o n . " A r i d d l e wrapped i n a mystery inside o f an enigma." W i n s t o n C h u r c h i l l ' s w o r d s c o u l d apply t o quarks as w e l l they d i d t o the o l d Soviet U n i o n . W h a t are these most l i l l i p u t i a n particles that lie forever h i d d e n f r o m o u r gaze? W h a t are we t r y i n g t o measure i n o u r nucléon physics laboratories? O n e curious p r o p e r t y o f p r o ­ tons and neutrons is that they vibrate at certain fixed, resonant frequencies; they have a n u m b e r o f natural "harmonics." We w i l l talk about h o w t o " r i n g " a p r o t o n — t h a t is, h i t i t h a r d e n o u g h to make i t resonate. T h a t very concept brings us back t o the inter­ play between theory and experiment, w h i c h is so i m p o r t a n t for unraveling the riddles o f the quark. A g o o d t h e o r y should explain

THE SEARCH FOR QUARKS - 13 how

h a r d we need t o strike a p r o t o n t o get i t t o r i n g . A g o o d

t h e o r y should be able t o predict the magnitude, shape, and en­ ergy o f a resonance. I t should also explain w h y such resonances exist, and t h e n go a b i t further,and predict w h a t n o one has ever seen. A t the same t i m e , a g o o d experiment should be able t o dis­ t i n g u i s h among an entire spectrum o f candidate theories, i d e n t i ­ fying the g o o d ones and rejecting the ones that diverge f r o m the data. O n occasion an experiment should even show us s o m e t h i n g the theorists had never t h o u g h t o f m o d e l i n g w i t h their theories. Sometimes an observation is completely unexpected. Resonances are just one test, only one o f the k i n d s o f clues we have t o solve o u r mystery. We can also ask, W h a t is the shape o f a n e u t r o n o r a proton? A r e the quarks inside t h e m r i g i d l y fixed i n space, or d o they float about freely? O r perhaps the t r u t h is s o m e t h i n g i n between: d o the up quarks t e n d t o congregate i n one r e g i o n o f the nucléon and the d o w n quarks i n another? H o w d o these quarks o r b i t and swirl a r o u n d each other? Three quarks, sometimes e x h i b i t i n g a resonance, sometimes p e r f o r m i n g a q u a n t u m mechanical dance—is that all there is i n ­ side a p r o t o n or a neutron? Yes and n o . W h e n we peek just be­ neath the surface o f the p r o t o n , all we see is this simple choreog­ raphy o f three quarks. B u t o n closer inspection s o m e t h i n g else is g o i n g o n . There seem t o be emissaries dashing back and f o r t h between the stately quarks. T h e intermediate particles are called gluons. T h e y make the quarks aware o f one another, and, as their name implies, they b i n d the quarks together. These gluonic emis­ saries can add t h e i r o w n jigs t o the dance, g i v i n g rise—perhaps— to a new resonance, a n e w harmonic " r i n g . " After w a t c h i n g the b a l l r o o m floor for a w h i l e , we start t o n o ­ tice other dancers. Wasn't there another couple o f quarks swirl­ ing a r o u n d each other o f f i n a corner, just for the briefest m o ­ ment? T h e y seemed t o pop i n t o existence and t h e n vanished again. Yet somehow they d i d i t w i t h o u t defying t h a t cardinal rule: three quarks and three quarks only o n the dance floor. To understand h o w we can see this q u a n t u m choreography, this stately waltz, we need t o discuss three major tools o f the

14

CHAPTER 1

trade. H o w can accelerators push electrons nearly t o the speed o f light? ( O n l y t h e n d o they have e n o u g h energy t o r i n g or tickle a nucléon.) C u r i o u s l y e n o u g h , these fascinating machines are b u i l t essentially o u t o f microwaves and magnets. H o w d o we de­ tect particles such as protons and neutrons, w h i c h are a q u a d r i l l i o n t h o f a meter across, or electrons, w h i c h are even smaller, w i t h l i t t l e more t h a n charged wires and plastic that glows? A n d h o w can we find o u r way t h r o u g h a terabyte o f data (a terabyte is a m i l l i o n megabytes), and t h e n say t h a t we "saw" the shape o f a n e u t r o n , or " h e a r d " the delta resonance, or " f e l t " the vibra­ tions o f the dancing gluons? T h e evidence is i n d i r e c t , and so we need t o proceed deliber­ ately, one step at a t i m e . B u t the idea that w i t h i n m y lifetime a whole new w o r l d has been glimpsed inside o f the p r o t o n and the n e u t r o n is i n t r i g u i n g and exciting, an intellectual adventure o f the highest degree. "Three quarks for M u s t e r M a r k ! " w r o t e James Joyce i n Finnegan^s Wake^ w i t h seeming prescience. By the t i m e he w r o t e t h a t last novel, Joyce was nearly b l i n d . N o w we t o o w i l l find o u t h o w m u c h we can get t o k n o w w i t h o u t really being able t o see.

The Rise and Fall (for the right reasons) and Rise Again of the Quark Hypothesis

I

N P H Y S I C S T E X T B O O K S we are t o l d that quarks were p r o ­ posed i n 1964 by M u r r a y G e l l - M a n n o f Caltech and George Z w e i g o f C E R N , and experimentally verified by a team o f physicists led by H e n r y Kendall and Jerome F r i e d m a n o f M I T and Richard Taylor o f Stanford i n 1972. I t is an account that mimics the way physicists t h i n k their field should progress: a the­ oretical p r e d i c t i o n followed by experimental verification. W h a t is glossed over i n this tale is that the quark hypothesis should have been rejected i n the late 1960s based o n some very g o o d experimental evidence.

N o matter h o w c o m p e l l i n g a hypothesis or t h e o r y is, i n the w o r l d o f science i t really must stand up t o the scrutiny o f experi­ mental evidence. I n the case o f quarks, the whole n o t i o n o f w h a t is g o o d evidence is very tricky. N o one has ever seen an isolated quark, and there is g o o d reason t o t h i n k that n o one ever w i l l . S t i l l , after m o r e t h e n three decades o f experimental and theoreti­ cal investigation, we k n o w a great deal about t h e m . We k n o w that they have " s p i n " and come i n different q u a n t u m "colors" and "flavors," and possess other characteristics that w i l l be dis­ cussed i n this book. The fact that we k n o w a n y t h i n g about s o m e t h i n g so small is i n itself amazing. A n e u t r o n or a p r o t o n is 10" meter across, and 15

16

CHAPTER 2

quarks are even smaller. T o give an idea o f w h a t that scale means, there is r o u g h l y a factor o f 1 0 0 , 0 0 0 between the size o f a person and the size o f a cell. There is another factor o f 100,000 between the size o f a cell and the size o f an a t o m , and one final factor o f 100,000 between the size o f an a t o m and the size o f a n e u t r o n . A n d all experimental evidence tells us t h a t quarks are signifi­ cantly smaller. H o w e v e r , one m i g h t argue t h a t the discussion o f quarks by themselves is a b i t academic. Quarks are always i n b o u n d states— i n confinement—and i t is this confinement t h a t lies at the heart o f the story o f the near rejection o f the quark hypothesis. I n one sense confinement is just the statement o f the observational fact t h a t we have never seen a free quark, t h a t they are always confined inside protons o r neutrons, or some other particle. O n the other h a n d , confinement is perhaps the most u n i q u e feature o f the quark w o r l d . T h e n e u t r o n and the p r o t o n cannot be described simply as scaled-down atoms o r t i n y solar systems. Rather, they are s o m e t h i n g whose elements we can never really hope t o p u l l apart. T h a t is n o t t o say we cannot unravel some o f t h e i r myster­ ies. I t is just that the techniques w i l l be a g o o d deal different f r o m simple dissection. S t i l l , confined w i t h i n the p r o t o n and the n e u t r o n , and many m o r e exotic particles, is a w o r l d w i t h its o w n etiquette and landscapes. We k n o w s o m e t h i n g about the rules by w h i c h quarks order and g r o u p themselves, and we are just learning t o decipher and chart the choreography o f quarks as they o r b i t and dance a r o u n d each other. I t was a l o n g road f r o m the days o f s p l i t t i n g the nucleus i n t o protons and neutrons t o the p o i n t where physicists were ready t o accept the fact t h a t the constituents o f the next smaller scale were, i n the o l d sense, beyond t h e i r reach. I t t o o k t i m e t o accept the concept o f confinement, b u t this abstract concept was always d r i v e n by g o o d , solid experimental evidence. I n reality, the evi­ dence t h a t eventually persuaded the particle physics c o m m u n i t y t o accept the quark hypothesis d i d n o t arise f r o m a single line o f investigation. Rather, i t was the result o f t w o different lines o f study, t w o separate threads. T h e first thread f o l l o w e d the discov-

RISE OF THE QUARK HYPOTHESIS

17

ery o f a m u l t i t u d e o f elementary particles i n the 1950s. T h e second thread followed the p r o b i n g o f the structure o f the p r o t o n . I n the late 1950s, the field o f subatomic physics had become a crowded place, w i t h new particles being discovered almost weekly. T h e w o r l d was n o t populated only by the familiar elect r o n ( r ) , p r o t o n (p) and n e u t r o n (w), b u t n o w also muons pions (ft), T , neutrinos ( v ) , Cû, A , p, T|, and S. The Greek alphabet was nearly exhausted! One o f the chief features o f many o f these new particles was a new property called "strangeness." I n the 1950s, this property was exactly w h a t its name i m p l i e d — strange. Experimenters l o o k e d at photographic plates, w h i c h had been exposed t o cosmic rays o n m o u n t a i n t o p s or d u r i n g h i g h elevation b a l l o o n or airplane flights. The photographic plates were sensitive t o charged particles passing t h r o u g h t h e m , and o n t h e m physicists w o u l d see the apparently spontaneous product i o n o f a p r o t o n and a p i o n (rr) (see figure 2 . 1 ) . They inferred that some type o f neutral particle, w h i c h left n o track, must have decayed. I t c o u l d n o t be a n e u t r o n (the n e u t r o n d i d n o t have enough mass, or energy, t o create a p r o t o n and a pion)—so they named the particle the A , because o f the inverted F that they saw on their photographic plates. y

This same particle was soon observed at accelerators that slammed high-energy protons i n t o targets and produced a shower o f particles. B u t i t was n o t the only "strange" particle. T h e S, the kaon ( JQ, and the S were also observed, each w i t h its own unique mass and other characteristics, and eventually each o f t h e m decayed i n a pattern similar t o the "strange decay" first observed w i t h the A . W h e n the w o r l d was just made o f the electron, p r o t o n , and n e u t r o n , life seemed simple and complete. B u t w i t h the arrival o f so many newcomers the question became a p r o b l e m o f taxonomy: h o w d o we organize all o f them? Experimenters had been able t o measure lifetime, masses, charge, and decay patterns, and now they dearly desired t o create some type o f classification scheme, reminiscent o f D m i t r i Mendeleev's Periodic Table of the Elements^ the most basic table for chemistry.

18 - CHAPTER 2

Figure 2.1 Much o f the early data for strange decay came from photo­ graphic plates like this one. Charged particles leave tracks i n the plate, and the curvature indicates the charge and momentum o f the particle. (Courtesy o f Brookhaven National Laboratory) T h e periodic table has embedded i n i t the structure o f the a t o m and even the t w o spin states o f an electron. T h i s is remark­ able since Mendeleev first c o m p i l e d the table i n 1 8 7 1 , decades before the discovery o f the a t o m and h a l f a century before the q u a n t u m description o f its structure. H e ordered the elements first by increasing w e i g h t and also started a new r o w w h e n physi­ cal characteristics o f elements repeated. T h u s , h y d r o g e n , l i t h i u m , s o d i u m , potassium, and so o n are all i n the first c o l u m n or " g r o u p " because chemically they are very similar. T h e last col­ u m n contains the noble or i n e r t gases: h e l i u m , n e o n , argon, k r y p t o n , and so o n . T h e structure o f the a t o m t h a t creates these chemical characteristics is the electron o r b i t . T h e first r o w i n the table contains t w o elements, because the first o r b i t can c o n t a i n

RISE OF THE QUARK HYPOTHESIS

19

up t o t w o electrons. The second r o w has eight elements, and the second o r b i t o f the a t o m can have up t o eight electrons. T h e elements i n the last c o l u m n , the noble gases, all have t h e i r orbits completely filled and so are chemically i n e r t . T h e elements i n the first c o l u m n , the alkali metals, have only one electron, w h i c h is easily oxidized or i o n i z e d , i n t h e i r outermost o r b i t . T h u s by ar­ r a n g i n g the elements according t o the r i g h t characteristics, the periodic table h i n t e d at t h e i r u n d e r l y i n g structures. I n 1 9 6 1 , M u r r a y G e l l - M a n n organized the recently discovered particles i n a paper e n t i t l e d " T h e E i g h t F o l d Way: A T h e o r y o f S t r o n g I n t e r a c t i o n Symmetry." Strongly i n t e r a c t i n g particles are those t h a t b i n d i n the nucleus, and include all the particles listed above except the electron and its exotic heavier siblings, the | l and the x, and the neutrinos ( v ) . " T h e E i g h t F o l d W a y " sounds Taoist—as i t was meant t o sound—but the t i t l e t r u l y reported the results o f identifying the r i g h t symmetries o r characteristics that are used t o classify all these new particles, and w o u l d h i n t at the u n d e r l y i n g quark structure. G e l l - M a n n w a n t e d t o p l o t the particles o n some type o f graph. One m i g h t t r y t o list the particles by mass, b u t many particles w o u l d sit almost, b u t n o t quite, o n t o p o f each other; for exam­ ple, the p r o t o n and the n e u t r o n have masses o f 939.5 M e V / c and 9 3 8 . 2 M e V / c

2

(about 1.7 x 1 0

2 7

k g or 3.7 x 1 0

2 7

2

lbs), a

d i s t i n c t i o n o f only 0.1 percent. Perhaps they c o u l d be spread o u t by placing charge along another axis? (See figure 2.2.) B u t n o useful patterns emerged. Perhaps that should n o t be surprising, since w e i g h t by itself had even upset Mendeleev's periodic table originally. A r g o n was more massive t h a n potassium, b u t the char­ acteristics o f argon p u t i t just before potassium. W h e n the peri­ odic table was readjusted by atomic n u m b e r instead o f by atomic w e i g h t , the patterns cleared. F o r G e l l - M a n n , the key w o u l d be t u r n i n g t o the decay patterns. Decay patterns tell us about q u a n t u m number. T h e premise o f this argument is t h a t particles have t w o types o f properties. They have transitory properties, like m o m e n t u m or p o s i t i o n , w h i c h are continuously changing, and more consistent properties, such

20 • CHAPTER 2

Figure 2.2 Particles arranged by charge and mass.

as charge (or even atomic mass), w h i c h resist changing. I f a p a r t i ­ cle decays freely and quickly b y a particular mechanism or p r o ­ cess, t h e n n o q u a n t u m n u m b e r has changed. H o w e v e r , i f a decay takes a l o n g t i m e , and so a particle is relatively stable, t h e n t h a t decay changes a q u a n t u m number. F o r example, the particle N ( 1 4 4 0 ) , the " R o p e r " particle, w i l l decay i n t o a n e u t r o n i n about 10

2 4

second, w h i c h is the a m o u n t o f t i m e i t takes l i g h t t o

cross a p r o t o n ! So we say a Roper is unstable, and i n t h a t decay n o q u a n t u m n u m b e r has changed. A p r o t o n has never been seen decaying i n t o a p o s i t r o n (p —» e* v ) , a l t h o u g h there is e n o u g h energy t o allow this t o happen. We say that a p r o t o n is stable— and q u a n t u m numbers such as " b a r y o n n u m b e r " and "charge" are conserved. N o w i n the case o f a strange particle, such as a E, i t can decay i n t o a A and a K quickly (10~ second o r faster), b u t 20

the decay o f the A is rare—10"

10

second, o r 10 b i l l i o n times

slower. T h e first decay passes strangeness f r o m the Z t o the A , b u t

RISE OF THE QUARK HYPOTHESIS

21

Figure 2.3 Particles arranged by isospin and strangeness. T h e baryon octet, which includes the neutron and the proton, inspired Gell-Mann to call this the eight-fold way. t h e n the second decay is very slow. T h e A is the lightest strange particle, so w h e n i t decays, strangeness w i l l be lost and the strangeness q u a n t u m n u m b e r w i l l go t o zero. I n later years we w o u l d understand this i n terms o f the decay o f a strange quark— but that is g e t t i n g ahead o f ourselves. T w o q u a n t u m numbers that had been determined at the t i m e for all particles were strangeness and isospin. Isospin plus is a measure o f h o w " p r o t o n l i k e " the particle is, whereas isospin m i n u s is a measure o f n o w " n e u t r o n l i k e " i t is. W h e n G e l l - M a n n p l o t t e d all the particles o n his isospin-versus-strangeness graph, several patterns emerged for various types o f particles (see figure 2.3). T h e most i m p o r t a n t pattern, for most o f the matter i n o u r normal w o r l d , is the one that includes the p r o t o n and the neu­ t r o n , the b a r y o n octet or group o f eight particles (hence the t i t l e o f G e l l - M a n n ' s paper). There were t w o i m p o r t a n t consequences

22 • CHAPTER 2

Figure 2.4 The first observation o f the Q particle—1964. (Courtesy o f Brookhaven National Laboratory) o f these patterns. First, i n the b a r y o n decuplet, or ten-particle g r o u p , the Q had n o t been observed. So these patterns predicted a new particle, and by extrapolating the masses o f other particles i n the decuplet, they were able t o make a reasonable p r e d i c t i o n o f the mass o f the O. I n 1 9 6 4 , the O was observed i n g o o d agreement w i t h predictions (see figure 2 . 4 ) . T h e second feature is that the patterns that emerged were rec­ o g n i z e d by mathematicians and theoretical physicists as belong­ i n g t o s o m e t h i n g i n g r o u p t h e o r y called "su(3)"—special u n i t a r y three. T h e 3 refers t o the three bases or b u i l d i n g blocks o f the g r o u p . I n mathematics the bases have a very clear role. There are certain and well-prescribed ways i n w h i c h one can c o m b i n e the bases t o create new objects. S t i l l , i t was n o t clear whether these three bases or b u i l d i n g blocks corresponded t o a n y t h i n g physi­ cal, and i f they d i d , w h a t i t was that they m i g h t be. People t r i e d

RISE OF THE QUARK HYPOTHESIS

23

to develop schemes where the bases m i g h t be the p r o t o n , neu­ t r o n , and A particle. There was even s o m e t h i n g referred t o as the "boot-strap" theory, where the bases are a h y b r i d o f all the particles, and all particles are created f r o m these bases. I t may seem cyclic—similar t o l i f t i n g yourself up by y o u r o w n boot­ straps. Boot-strap theories were also referred t o as "nuclear de­ mocracy," since all particles were equally i m p o r t a n t . Finally, i n the critical year o f 1964, quarks were proposed twice. I n January, f r o m Caltech, G e l l - M a n n s u b m i t t e d a letter to a j o u r n a l for p u b l i c a t i o n , i n w h i c h the t e r m " q u a r k " first de­ buted. T h a t same m o n t h , i n C E R N , near Geneva, George Z w e i g w r o t e a paper o n the possibility o f subconstituents o f protons and neutrons. Z w e i g was a "post-doc" at the t i m e , w h i c h meant that he had just finished his P h . D . and was viewed as being "green" and the most j u n i o r o f scientists. H i s t e r m for a quark was "ace," as i n the phrase "ace i n the hole," w h i c h has nearly been forgotten. B o t h papers, however, made t w o s t a r t l i n g state­ ments. Each paper included, first, the p r e d i c t i o n o f fractional charges for the quarks, and second, the p r e d i c t i o n that one c o u l d observe quarks i n n o r m a l matter: It is fun to speculate about the way quarks would behave i f they were physical particles o f finite mass (instead of purely mathemati­ cal entities as they would be i n the limit of infinite mass). . . . Ordi­ nary matter near the earth's surface would be contaminated by stable quarks as a result of high-energy cosmic ray events through­ out the earth's history, but the contamination is estimated to be so small that it would never have been detected. A search for stable quarks of charge -1 or +1 and/or stable di-quarks of charge -1 or + ~ or + | at the highest energy accelerators would help to reassure us o f the non-existence o f real quarks. Murray Gell-Mann Phys. Letts. 8 p. 214 (Feb. I , 1964) y

y

G e l l - M a n n and Z w e i g b o t h felt that the eight-fold way and su(3) had d r i v e n t h e m t o postulate the quark and the - | or + | fractional charge. They also knew that there were n o candidates for such a particle. So only w i t h great reluctance d i d they publish

24

CHAPTER 2

their hypotheses. I n later years G e l l - M a n n confessed that he chose the j o u r n a l Physics Letters, because he felt that such a radical idea had a better chance o f g e t t i n g accepted and published i n a j o u r n a l that specialized i n quick reviews and p r i n t i n g . Z w e i g published his article i n a C E R N in-house report, and t h e n , as a j u n i o r theorist, was n o t able t o publish i t elsewhere for years. One p o i n t o n w h i c h G e l l - M a n n and Z w e i g differed was the question o f the physical significance o f a quark. F r o m the begin­ n i n g Z w e i g saw t h e m as real particles—the physical b u i l d i n g blocks o f protons, neutrons, As, and so o n . I n his view, most decays c o u l d be explained by the rearrangement o f quarks, and t h e n the reluctance o f the A t o decay is because the strange quark really must decay and t u r n i n t o s o m e t h i n g else—or at least i n t o a different type o f quark. G e l l - M a n n was more guarded, and he usually referred t o quarks as "purely mathematical entities" for nearly a decade. H e often referred t o his hypothesis as a useful mathematical m o d e l that should be discarded once the desired results were obtained. I n fact, his p r o m o t i o n o f the search for fractional charges was i n part p r o m p t e d by a desire t o discredit the physical quarks. So what is a fractional charge and w h y is i t radical? Ever since M i l l i k a n had published his paper i n 1910, the charge o f an elec­ t r o n or a p r o t o n seemed t o be unique and singular. There was n o evidence o f any other charge, except for multiples o f the charge o n an electron (or p r o t o n ) , i n the l o n g list o f k n o w n phys­ ical phenomena. I n the o p e n i n g paragraph o f M i l l i k a n ' s famous paper, he writes: Among all physical constants there are two that w i l l be universally admitted to be o f predominant importance; the one is the velocity of l i g h t . . . and the other is the ultimate, or elementary, electrical charge. R. A. Millikan, Phil Mag. (series IV) vol. 19, p. 209 (Feb. 1910) T h a t prejudice o f " p r e d o m i n a n t i m p o r t a n c e " s t i l l stood h a l f a century later.

RISE OF THE QUARK HYPOTHESIS

25

W h a t M i l l i k a n and his co-worker M r . Begeman d i d i n w h a t every physics student n o w knows as the " o i l d r o p " experiment was t o create a mist o f water droplets i n a chamber. They t h e n irradiated t h e m w i t h a radioactive r a d i u m source, thus k n o c k i n g out electrons and leaving some o f the atoms i n the droplet i o n ­ ized. N e x t they let the droplet fall w h i l e observing i t w i t h a m i ­ croscope, and determined its t e r m i n a l velocity, f r o m w h i c h they c o u l d calculate the mass o f the droplet. T h e n they t u r n e d an electrostatic field o n , w i t h the negative up and the positive d o w n . Since the i o n i z e d droplet carried an excess o f protons, there w o u l d be an electrostatic force u p w a r d , usually raising the particle up against gravity. By balancing the electrostatic force against the gravitational force, M i l l i k a n c o u l d deduce the charge on the droplet. The experiment is extremely tedious. T r y i n g t o generate these clouds o f droplets, or artificial fogs, by v a r y i n g the pressure w i t h i n the experimental chamber. T h e n t r y i n g t o ionize a drop­ let before the whole c l o u d fell under gravity. T h e n w a t c h i n g a droplet fall, t i m i n g i t , raising i t , and balancing the forces o n i t , all w h i l e w a t c h i n g i t t h r o u g h a microscope, and all before i t evap­ orated. I t is n o w o n d e r that i n M i l l i k a n ' s paper he only reports measuring t h i r t y - t h r e e drops total. H e also w r o t e about one pe­ culiar droplet: I have discarded one uncertain and unduplicated observation ap­ parently upon a single charged drop, which gave a value o f the charge on the drop some 30 per cent lower that the final value o f e. R. A. Millikan, Phil May. (series IV) vol 19, p. 209 (Feb. 1910) Some searchers for free quarks have cited this, i n jest, as the first observation o f fractional charge. M o r e i m p o r t a n t t o the serious search for fractional charged particles, the M i l l i k a n technique inspired a series o f experiments f r o m the m i d 1960s t h r o u g h the 1970s. These updated experiments used for their droplets a variety o f materials: water, mineral o i l , graphite, i r o n , and n i o ­ b i u m . By using the o i l or solid droplets, these experiments solved one o f M i l l i k a n ' s persistent problems: his water drops w o u l d

26

CHAPTER 2

evaporate and change t h e i r mass t h r o u g h the course o f a mea­ surement. Experimenters also used materials w i t h t a i l o r e d mag­ netic and dielectric properties, together w i t h complex feedback loops t h a t automatically balanced the fields. A t one p o i n t , George LaRue, W i l l i a m Fairbank, and A r t h u r H e b a r d at Stan­ f o r d had one n i o b i u m sphere t h a t consistently yielded a | charge measurement. T h i s created a great deal o f excitement at the t i m e , p a c k i n g a u d i t o r i u m s at conferences. B u t i t stood alone as an iso­ lated data p o i n t . After an extensive study o f the systemic error o f t h e i r experiment, they finally retracted t h e i r claim t o the obser­ v a t i o n o f a fractional charge. As sophisticated as the m o d e r n ex­ periments were, w i t h specially designed electrostatics and stabi­ l i z i n g magnetic fields, s t i l l n o fractional charges were observed. People were also l o o k i n g for fractionally charged particles— quarks—in cosmic ray studies and at accelerator laboratories. M o u n t i n g an experiment is a very involved process, often t a k i n g years o f preparation. Yet w i t h i n a few m o n t h s o f the p u b l i s h i n g o f the o r i g i n a l theoretical papers about quarks, physicists were " m i n i n g the data." F o r example, i n an experiment designed t o search for s o m e t h i n g like the Q particle, o r t o measure the d i s t r i ­ b u t i o n s o f protons i n carbon, o r a h u n d r e d other t h i n g s , there are hundreds o f thousands o f events (like the photographic plate shown above) that d o n o t c o n t a i n an Q, b u t are s t i l l recorded. " M i n i n g the data" means g o i n g back t o the archives o f data al­ ready taken and l o o k i n g for the fractionally charged particle. I t was a l o n g and tedious task t o pore over thousands and thousands o f photographic plates, and s t i l l n o evidence was f o u n d for a free quark i n the archives o f any laboratory. T h e accelerator and cosmic ray experimentalists t h e n m o u n t e d n e w experiments specifically designed t o l o o k for fractional charges. One o f the t r a d i t i o n a l methods o f measuring the charge o f a particle is t o see h o w its trajectory curves i n a magnetic or electric field. T h e p r o b l e m w i t h this technique is t h a t i t really measures the ratio o f charge t o m o m e n t u m o r mass ( d e p e n d i n g o n whether an electric or a magnetic field is used). I f the particle is a k n o w n particle—such as a p r o t o n o r electron—then we k n o w

RISE OF THE QUARK HYPOTHESIS its charge and even its mass, so we can calculate

27

momentum,

velocity, and even energy, given a map o f its trajectory. B u t for an u n k n o w n particle type, knowledge o f the charge t o mass ratio alone tells us l i t t l e . Either some assumptions are needed—such as the mass o f the quark is o n e - t h i r d o f the mass o f a p r o t o n — or we need t o make more measurements. Two

additional measurements that can be made o n an u n ­

k n o w n particle are the energy o f the particle and the rate at w h i c h i t loses its energy. T h e energy can be measured by l e t t i n g the particle collide w i t h a special material called "scintillator." Scintillator converts the energy o f the particle i n t o l i g h t . T h e a m o u n t o f l i g h t is p r o p o r t i o n a l t o the energy o f the particle, and is easily detected and measured. Scintillators actually are a whole range o f materials that include b o t h liquids and solids. A n experi­ menter w i l l choose the material depending u p o n its detection efficiency, response t i m e , and, o f course, cost, together w i t h the expected energy o f the particle. Typical plastic scintillator looks like Plexiglas u n t i l i t is exposed t o particles or radiation. I n the presence o f an ultraviolet l i g h t (such as a "dark l i g h t " ) the Plexi­ glas is unchanged, b u t the plastic scintillator w i l l g l o w blue. T o measure the rate at w h i c h energy is lost one can construct the detector w i t h several layers o f scintillator. T h e total l i g h t f r o m all layers is a measurement o f the total energy o f the particle, b u t by l o o k i n g at h o w m u c h l i g h t is i n each layer, one measures h o w fast the particle loses energy—or h o w m u c h i t interacts w i t h the material. T h e interaction w i t h material is p r i m a r i l y an electric charge interaction. So a particle that loses its energy quickly has a greater charge t h a n a particle w i t h a small charge. Actually, even particles w i t h n o charge (such as neutrons and the ultraviolet l i g h t m e n t i o n e d above) lose energy, b u t at a very different rate. I n fact, by just k n o w i n g the energy and the rate o f energy lost, experimentalists c o u l d measure charge. Therefore, they b u i l t de­ tectors w i t h layers and layers o f scintillator. Some o f t h e m p o i n t e d skyward t o detect cosmic rays. Some o f t h e m p o i n t e d t o w a r d the targets o f an accelerator laboratory. The advantage o f

28

CHAPTER 2

cosmic rays is that they are everywhere, and they are free ( o f cost)—but the c o u n t rate is low. Accelerators have the advantage o f very h i g h rates, and they can be t u n e d t o produce particles at a c o n t r o l l e d h i g h energy—but they are n o t free. H o w e v e r , despite considerable expense, a rush o f activity, and a flood o f new data, there were s t i l l n o free quarks. W i t h hundreds o f experiments performed w o r l d w i d e t h r o u g h ­ o u t the late 1960s and early 1970s, and absolutely n o evidence o f a free quark, i t w o u l d have been a reasonable and rational conclusion t o reject the physical quark hypothesis completely. Yet i t persisted for t w o reasons. First, i t was s t i l l a s o l u t i o n t o the p r o b l e m that had originally p r o m p t e d i t : i t explained all the par­ ticles seen i n the 1950s, and i t explained the symmetries de­ scribed i n the eight-fold way. Second, there was m o u n t i n g , al­ t h o u g h unclear, evidence that there was s o m e t h i n g g o i n g o n inside the p r o t o n . T h e second thread or line o f investigation that c o n t r i b u t e d t o the evidence that persuaded physicists o f the existence o f quarks dates back t o the early 1950s, w h e n physicists at Stanford had been s t u d y i n g the p r o t o n w i t h electron scattering. I n 1 9 5 5 , a g r o u p led by Robert Hofstadter had scattered electrons o f f a hy­ d r o g e n ( p r o t o n ) target, and measured the size o f the p r o t o n t o be r o u g h l y 10"

15

meter, or 1 0 0 , 0 0 0 times smaller t h a n the size

o f an a t o m . T h a t is small, b u t i t is n o t zero. T h e natural question t h a t physicists were asking themselves was, W h a t is the structure o f the proton? The experiments t h a t searched for fractional charge or exotic particles were i n some sense searches for anomalies. W h a t was measured were the properties o f i n d i v i d u a l particles. I f any one particle was observed t o have the sought-for properties, t h e n the search was considered a success, be i t fractional charge or strangeness = - 3 (the Q particle). Scattering experiments, i n contrast, are statistical and i n d i r e c t . I n Hofstadter's, and subse­ quently all Stanford (later S L A C ) experiments, they scattered an electron o f f a target and measured at w h a t energy and i n w h i c h d i r e c t i o n the electron emerged. N o t e that here, the particles t o

RISE OF THE QUARK HYPOTHESIS

29

be studied reside i n the target, and the electron is merely a probe. T h e i n i t i a l energy o f the electron w i l l determine the resolution o f the probe—the smallest t h i n g i t can observe. A low-energy electron w i l l have a l o n g wavelength and l o w resolution, whereas a high-energy electron w i l l have a short wavelength and a higher resolution. I f an experiment uses a beam o f electrons w i t h 1 M e V o f en­ ergy, the electrons have a wavelength o f 2 x 10" meter, or one 13

five-hundredth

o f the diameter o f an a t o m . Therefore, atoms are

distinguishable, b u t the nucleus ( w h i c h is 2 0 0 times smaller) is s t i l l seen as a p o i n t that carries the sum o f the charges o f all the protons i n t h a t nucleus. T h e electron w i l l scatter exactly as i f i t was scattering f r o m a p o i n t l i k e object w i t h charge +Z. A t a higher energy—for example, 100 M e V — t h e wavelength is 2 x 10"

15

meter, or 2 fermis. T h i s is the scale o f the nucleus, and we begin to resolve the structure o f the nucleus. I f o u r target is a heavy element w i t h a l o t o f protons and neutrons, the electron w i l l scatter n o t f r o m a single p o i n t , b u t f r o m a spatially d i s t r i b u t e d charge. (We w i l l discuss electron scattering i n m o r e detail i n later chapters, since i t plays a critical role i n the g a t h e r i n g o f evidence about quarks.) I n 1 9 5 5 , Hofstadter's g r o u p was using Stanford's new acceler­ ator w i t h a 550 M e V electron beam o n a hydrogen target. A t these energies a hydrogen target is essentially a p r o t o n target. W h a t they saw was that the charge o f the p r o t o n was spatially extended over a region o f space 0.8 fermi across. A t the t i m e many people were satisfied w i t h the n o t i o n that the p r o t o n had a finite size, b u t there was always that question i n the back o f physicists' heads: "We l o o k e d inside the a t o m and saw the n u ­ cleus, we l o o k e d inside the nucleus and f o u n d the p r o t o n and the n e u t r o n . I f we l o o k inside the p r o t o n or the n e u t r o n , w h a t w i l l we see there?" P r o m p t e d by the measurement o f the finite size o f the p r o t o n , and driven by the desire t o see inside i t , i n 1957 Stanford Univer­ sity proposed the b u i l d i n g o f a newer and m u c h larger accelera­ tor. T h i s accelerator and laboratory became k n o w n i n the 1960s

30

CHAPTER 2

as S L A C , and delivered a 2 0 G e V beam t o a target. T h e accelera­ tor itself is over 2 miles l o n g . A t the t i m e i t was the largest and most expensive physics t o o l i n the w o r l d , designed t o l o o k at the smallest scales—a resolution o f about one o n e - h u n d r e d t h o f the diameter o f a p r o t o n . W h a t experimentalists saw defied any expectations based o n o l d models o f the p r o t o n . T h e p r o t o n is n o t just a ball o f charge—there

is s o m e t h i n g , rather, there are many t h i n g s ,

w i t h i n i t . However, i t was s t i l l a l o n g road t o understanding w h a t t h a t structure is. Remember t h a t the data can only be o f the f o r m "we saw n electrons scattered i n this d i r e c t i o n and energy, and m electrons scattered i n t h a t d i r e c t i o n and at t h a t energy." T h e experiments cannot see directly anymore w i t h o u t extensive the­ oretical interpretation. Separately f r o m the experiments, theo­ rists start f r o m a m o d e l and calculate w h a t the experiments w o u l d see i f that m o d e l were t r u e . J. D . Bjorken was a y o u n g theorist at S L A C w h o t u r n e d his a t t e n t i o n t o these bizarre results. H e believed that i t was unlikely t h a t protons had subconstituents, and he had at his

fingertips

the skills t o test this hypothesis. Based u p o n an abstract branch o f theoretical particle physics called "current algebra," Bjorken calculated certain features o f the scattering t h a t w o u l d be ex­ pected i f protons and neutrons had subconstituents. O n e o f the first predictions was the idea t h a t the n u m b e r o f electrons scat­ tered w o u l d n o t depend m u c h o n t h e i r energy. T h i s is very differ­ ent f r o m the scattering o f electrons f r o m atoms or f r o m a w h o l e nucleus, where the "cross section" is strongly energy-dependent. T h e second p r e d i c t i o n is s o m e t h i n g called "scaling." Bjorken convinced the experimentalists t o p l o t t h e i r data n o t as counts versus energy, b u t rather as counts versus a new q u a n t i t y called "the B j 0 r k e n - x variable," or just " x . " " x " is a c o m b i n a t i o n o f energy-lost and m o m e n t u m - l o s t i n f o r m a t i o n . W h e n p l o t t e d , the data asymptotically reached the " B j o r k e n l i m i t , " w h i c h meant t h a t the p r o t o n had subconstituents, c o n t r a r y t o B J 0 r k e n ' s o w n expectations!

RISE OF THE QUARK HYPOTHESIS

31

Experimentalists were wary o f such an abstract theory. A n ex­ planation i n terms o f current algebra may be a v i n d i c a t i o n for the current algebra m o d e l , b u t i t left s o m e t h i n g missing i n terms o f physically understanding what was g o i n g o n . W h e n Richard Feynman o f Caltech visited the newly c o m ­ pleted S L A C experimental facilities, he was i n t r i g u e d by the data and by Bjorken's current algebra treatment and results. H e is reported t o have developed—in just a few days—something he called the p a r t o n m o d e l . The " p a r t o n m o d e l " is based o n a field theory approach. (Field t h e o r y is what particle theorists w h o d i d n ' t d o current algebra d i d . ) T h e p a r t o n m o d e l essentially states that the p r o t o n is made up o f p o i n t l i k e subconstituents. These partons interact w i t h themselves t o a create a c l o u d o f vir­ tual partons a r o u n d itself. T h i s c l o u d is what gives rise t o the spatial extent o f the p r o t o n . Feynman was less t h a n specific about the details o f his partons. H e d i d n o t define h o w many there are, or what charges they have, or i f there is only one type or many types o f partons. S t i l l , he c o u l d accomplish t w o objectives w i t h his m o d e l . H e c o u l d reproduce the " B j o r k e n l i m i t , " and he c o u l d explain the new S L A C data i n terms o f the scattering o f pointlike partons w i t h i n the protons. One o f the refinements o f the p a r t o n model was t o realize that i f the p r o t o n at h i g h energies was seen as a collection o f partons, t h e n the scattering w o u l d be p r o p o r t i o n a l t o the sum o f the square o f the charges o f all the partons i n the p r o t o n . I n fact, there are t o o many things g o i n g o n i n the p r o t o n for this t o be a unique or d i s t i n g u i s h i n g test. However, since most, b u t n o t all, o f the same things are happening i n the n e u t r o n , subtracting the scattering spectrum o f the p r o t o n f r o m that o f the n e u t r o n provides a signature o f the difference i n their subconstituents. A n d that difference was the fractional charge on one quark, as predicted i n the quark models o f Z w e i g and G e l l - M a n n . This remarkable result is considered the most persuasive evidence sup­ p o r t i n g the quark theory, perhaps even raising its status f r o m theory t o recognized fact o f nature.

32

CHAPTER 2 So S L A C saw subconstituents o f the p r o t o n and the n e u t r o n ,

w h i c h were called partons, the Bjorken l i m i t was satisfied, and the particular p a r t o n m o d e l t h a t rose t o the t o p was the quark m o d e l . I t had the r i g h t charges, the r i g h t spins, and the r i g h t isospins. B u t s t i l l n o fractional charged particle was observed by itself, only the i n d i r e c t signature! N o free quarks. As the n u m b e r o f searches for free quarks waned i n the late 1970s, a new t e r m appeared i n the c o m m u n i t y — " c o n f i n e m e n t . " T h i s was at first merely a statement o f the experimental observa­ t i o n . A l l quarks seemed t o be confined i n particles like neutrons and protons. Later, confinement came t o mean that n o t only d o quarks seem t o prefer t o be confined b u t , observationally speak­ i n g , they must be confined. Finally, confinement became l i n k e d t o the d o m i n a n t t h e o r y o f s t r o n g interactions, that o f the inter­ actions

between

quarks,

called

quantum

chromodynamics

( Q C D ) . Confinement is presently expected t o be the most natu­ ral consequence o f Q C D — s o m e t h i n g that we w i l l develop later. Q u a n t u m chromodynamics is presently the g u i d i n g t h e o r y for physicists studying the w o r l d at a scale o f 1 fermi (10~

15

meter)

or smaller. I t is a complex t h e o r y whose complete s o l u t i o n has defied even the best mathematical physicists and the biggest computers. Yet i t has i n a few l i m i t e d cases p r o v i d e d quantitative predictions confirmed by experiments. I t has also p r o m p t e d a class o f models for a w i d e range o f cases that are an approxima­ t i o n t o this theory. T h r o u g h o u t this b o o k a great deal o f what I talk about w i l l be described i n terms o f one o f these approximate models, the "constituent quark m o d e l . " A t this scale we always need some type o f m o d e l . F r o m a m o d e l we can make predictions that can be confirmed i n the l a b o r a t o r y — b u t g o i n g the other way is nearly impossible. A m o d e l also gives us pictures and de­ scriptions o f w h a t is g o i n g o n inside neutrons and protons, t h i n g s we can talk about, discuss, and debate. So, w h a t is inside a p r o t o n or neutron?

The Players and the Stage

A

F T E R T H E concertgoers settle d o w n i n t o their seats at Symphony H a l l , w h i l e others are f i n d i n g their places and I s t o w i n g their coats and umbrellas, they are advised t o glance at the p r o g r a m notes that the usher has left w i t h t h e m . N o w , before the house lights d i m , is the t i m e t o transform t h e m ­ selves f r o m harried navigators o f street traffic i n t o placid recep­ tors o f the Muses. A n d the p r o g r a m notes can assist i n this meta­ morphosis, by preparing the m i n d for that w h i c h is soon t o come. The notes n o t only list compositions about t o be heard, but also tell s o m e t h i n g about the w o r l d i n w h i c h the piece was w r i t t e n . A n attempt is made n o t only t o draw us i n t o the t i m e and place o f M o z a r t or Telemann, b u t also t o draw us inside the composers' thoughts and methods—the orderly systems o f Bach, the drama o f Beethoven. I n the theater, we m i g h t feel that W i l l y L o m a n f r o m Death of a Salesman is essentially n o different from o u r neighbor. Perhaps his character is as familiar as an uncle we see every holiday. So we d o n ' t need a l o t o f preparation t o meet h i m . Even H a m l e t , that fated prince o f D e n m a r k , can step across nearly four centuries w i t h o u t i n t r o d u c t i o n . We have often met his type i n contempo­ rary drama, and H a m l e t himself is an o l d acquaintance w h o m we have k n o w n since h i g h school English class. B u t , w h e n we t u r n to the less familiar Shakespearean faces, the p r o g r a m notes are

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there t o help us. A t the very least they place Laertes as "Son o f P o l o n a i s " and Polonius as " L o r d C h a m b e r l a i n . " T h e n we are prepared, because w h e n Polonius makes his entrance i n f r o n t o f H a m l e t ' s uncle, i t is n o t apparent t o the u n i n f o r m e d w h a t exactly his role i n the c o u r t is. Shakespeare doesn't stop t o i n t r o d u c e h i m — t h e t o r r e n t o f verse nearly drowns the event, and there is n o t i m e t o go back. I n the case o f a Greek drama, we k n o w t h a t b u r i e d at its very heart are some universal themes, b u t s t i l l the need for g o o d p r o ­ gram notes is never stronger. N o t only are all the names Greek t o us, b u t the culture o f the s e t t i n g , the H e l l e n i c references, and the style and structure o f the play c o m b i n e t o make the drama a potentially challenging event. A l t h o u g h the c u l t u r e o f A n t i g o n e is at the most basic level t r u l y universal, superficially i t is a very different w o r l d f r o m M a n h a t t a n o r M o n t a n a . Even the c h o i r t h a t repeatedly parades o u t o n t o the stage t o spout ballads o f w i s d o m is sometimes more confusing t h a n clarifying. M y p o i n t is simply this: w h e n v e n t u r i n g i n t o a new arena, p r o ­ gram notes can sketch o u t the players o r musical pieces for us, and explain what the stage i n front o f us is meant t o be. K n o w i n g s o m e t h i n g about the players and the stage doesn't detract f r o m the performance—instead, i t prepares us and allows us t o be ready for the i m p o r t a n t events w h e n they are played o u t i n f r o n t o f us. T h e p r o g r a m notes give us the static features. I t is the performance itself that is dynamic and makes us w a n t t o care about the characters. So before we move i n t o the dynamics o f quarks inside the p r o t o n or the n e u t r o n , let's set the stage and i n t r o d u c e the players. T h e stage is the nucleus o f the a t o m . There is a factor o f 100,000 between the height o f a person and the diameter o f a red b l o o d cell (usually considered an "average c e l l " ) . There is another factor o f 100,000 between the size o f a cell and the size o f an a t o m . Finally, there is another factor o f 100,000 between the size o f an a t o m and the w o r l d o f the nucleus. Spanning fifteen orders o f m a g n i t u d e is n o t a conceptually easy task. I m i g h t make the analogy that i t is the same as one penny o u t o f t e n t r i l l i o n

THE PLAYERS AND THE STAGE - 35 dollars, b u t I w a n t t o concentrate o n an analogy whose d i m e n ­ sions are l e n g t h . Let us magnify an a t o m so i t has t r u l y macroscopic m a g n i t u d e , increasing a carbon a t o m t o the size o f the E a r t h and a hydrogen a t o m t o the size o f the m o o n . T h e n a red b l o o d cell w o u l d oc­ cupy the w h o l e area inside the Earth's o r b i t a r o u n d the Sun. I f the cell was i n the foot o f an adult, the head o f this magnified creature w o u l d reach t o the next star. W h a t about the nucleus? W i t h an a t o m magnified t o the size o f the E a r t h , the nucleus ranges f r o m 50 t o 2 5 0 meters across, r o u g h l y the size o f a Broadway stage for hydrogen o r d e u t e r i u m to the size o f a stadium for the heaviest elements. I t is o n this stage (or playing field) that the drama o f quark dynamics w i l l be played o u t . B u t before we peer i n t o the n e u t r o n o r the p r o t o n to see the quarks themselves, let us stop and l o o k a r o u n d . There is a l o t t o be seen o n this scale, phenomena that c o n t a i n a h i n t o f the u n d e r l y i n g quarks. L o n g before the advent o f the quark m o d e l we k n e w that the nucleus was made up o f neutrons and protons. O n e other t h i n g that we k n o w about the nucleus is t h a t the force between a neu­ t r o n and a n e u t r o n is essentially the same as between a n e u t r o n and a p r o t o n , or between a p r o t o n and a p r o t o n . O n e o f the first consequences o f this symmetry is the use o f the t e r m "nucléon" to mean either a n e u t r o n or a p r o t o n . N o t only is the force be­ tween neutrons and protons, or nucléons, symmetric, b u t they also have nearly identical masses. Therefore, a useful description o f these t w o particles is that they are the same particle, w i t h " s o m e t h i n g " changed. I n nuclear physics that s o m e t h i n g is called isospin. As we w i l l see later, i t is the quark structure that lies at the heart o f that similarity, as w e l l as the differences. T w o other features o f the nucleon-nucleon force, o r "nuclear force," is that i t is repulsive (positive) at about h a l f a f e r m i , i t is attractive (negative) at a f e r m i , and i t essentially vanishes at about 3 t o 4 fermis (see figure 3.1). T h a t means t h a t t w o nucléons w i l l be oblivious t o each other i f they are separated by more t h a n 4 fermis, and they cannot get closer t o each other t h a n h a l f a fermi.

36

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Figure 3.1 T h e nucleon-nucleon force is repulsive (positive) at about half a fermi, attractive (negative) at a fer mi, and essentially vanishes at about 3 to 4 fermis.

T h e whole regime o f nuclear physics is essentially defined by this distance scale. Since the scale o f the quarks and the scale o f the nucléon and nucleus are so similar, we should be able t o explain nuclear physics, and i n particular the nuclear force, i n terms o f the quarks. T h i s w i l l also serve as a g o o d bridge between the physics o f directly observed particles like protons and the physics o f confined particles—quarks. N o w , before we submerge ourselves i n the u n i q u e and p a r t i c u ­ lar causes o f the nuclear force, let us revisit the more familiar gravitational force. W i t h gravity, as w e l l as w i t h C o u l o m b or elec­ trostatic forces, the force diminishes w i t h distance, b u t never vanishes. A speck o f dust does n o t have a great deal o f gravity associated w i t h i t , and that force decreases as the square o f the separation distance—according t o N e w t o n ' s law o f g r a v i t y — b u t even at the other side o f the universe that force is n o t zero. I t

THE PLAYERS AND THE STAGE

37

may be "essentially," or "effectively," or "for all practical pur­ poses" zero, b u t mathematically, i t is s t i l l finite. T h i s is because o f the mass o f the particle that carries the force—or "conveys the i n t e r a c t i o n . " I n the case o f gravity the particle is called a graviton,

an as-of-yet undetected particle (gravity is very weak). I n

electromagnetism the particle is the p h o t o n , a particle o f l i g h t . B o t h o f these particles are massless, and b o t h o f these forces therefore have an infinite range. T h e nucleon-nucleon force is not so simple. There are several theories that describe the characteristics o f the nuclear forces. T h e first successful t h e o r y was i n t r o d u c e d by H i d e k i Yukawa, a Japanese physicist, w h o i n 1935 postulated the existence o f a new particle—the 7C-meson, or " p i o n " as we usually call i t . H e proposed that a nucléon c o u l d e m i t a particle that w o u l d carry away w i t h i t some m o m e n t u m , and t h e n be ab­ sorbed by a second nucléon. By this means m o m e n t u m c o u l d be transferred between the nucléons, and the b i n d i n g o f the nuclé­ ons c o u l d be affected. T h a t part o f Yukawa's t h e o r y was n o differ­ ent f r o m the exchange o f photons i n electromagnetism. W h a t was unique about Yukawa's meson is that i t had mass. I f a nucléon emits a p i o n that has a mass, b u t the nucléon has lost n o mass, t h e n i t has defied the law o f the conservation o f mass—one o f the staunchest laws i n physics! B u t there is an infinitesimal l o o p ­ hole i n this law. A c c o r d i n g t o Heisenberg's uncertainty relation­ ship, there is always an intrinsic uncertainty i n certain pairs o f measurable quantities, such as p o s i t i o n and m o m e n t u m , t i m e and energy, or distance traveled and mass. I n some sense a system can " b o r r o w " mass, i f it doesn't reach beyond the r e g i o n defined by the Heisenberg uncertainties. Given this constraint, and the average distance that the nuclear force reaches, Yukawa expected that the 7t-meson w o u l d have a mass o f 100 t o 2 0 0 M e V / c , or 2

about a fifth o f the mass o f a nucléon. A t about this t i m e — i n 1937—cosmic ray studies first saw such a particle. O r i g i n a l l y i t was called a |i-meson. B u t , except for mass, i t had all the w r o n g properties. I n particular, i t had spin i , just like an electron. Yukawa k n e w his particle had t o have an

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integer spin i n order t o w o r k as an i n t e r m e d i a r y between nuclé­ ons. S t i l l , i t was very t e m p t i n g . I t had the about the r i g h t mass, and its existence was at the t i m e w e l l d o c u m e n t e d . I n Japan, Yukawa and his colleagues were cut o f f f r o m the rest o f the scientific w o r l d due t o W o r l d W a r I I . So they f o r m e d an association they called the M e s o n C l u b , w h i c h m e t and presented papers and t r i e d t o hash o u t the p r o b l e m o f w h y the particle t h a t Yukawa needed for his t h e o r y l o o k e d so different f r o m the " | l m e s o n " t h a t had been observed. Finally, i n 1 9 4 2 , i t was proposed t h a t there were i n fact t w o different mesons, the "7r-meson" a n d the " | i - m e s o n . " T h i s proposal resolved a great many problems, and later w o u l d be v i n d i c a t e d experimentally. I n fact, by m o r e recent taxonomy, we w o u l d n ' t even call the "u>meson" a meson at all. I t is electronlike, w i t h spin | , and i t usually has a charge o f - 1 . T h a t means t h a t i t is a " l e p t o n " ( i n pseudo-Greek, a " l i g h t p a r t i c l e " ) that we n o w just call a m u o n . T h e 7i-meson, or p i o n , really is a meson ( i n pseudo-Greek a meson is a " m i d d l e p a r t i ­ cle"—a particle t h a t goes i n between o t h e r particles, or mediates a force). I t has spin 0 and i t can have a charge o f + 1 , - 1 , or 0, w h i c h was just fine for Yukawa's needs. After W o r l d War I I , the Japanese t h e o r y was published w o r l d ­ w i d e . I n the U n i t e d States a similar t h e o r y had been developed by R o b e r t Marshak and Hans Rethe—but Yukawa's t h e o r y was deemed t o have been a great deal m o r e developed and m a t u r e . Finally, i n 1 9 4 7 , i t was demonstrated t h a t the |Ll-particle d i d n o t participate i n the nuclear force, m e a n i n g that i t c o u l d n ' t be Yakawa's meson. Also i n the same year cosmic ray experiments f o u n d another particle—Yukawa's p i o n ! I t had the r i g h t spin, charges, and even mass. So o n o u r stage o f the nucleus we can view the nuclear force as a nucléon e m i t t i n g a p i o n . T h e p i o n travels a f e r m i or so, and t h e n is reabsorbed i n t o a second nucléon. Physicists like t o dia­ gram such a process as shown b e l o w (figure 3.2). T h i s is essen­ tially a "Feynman d i a g r a m . " Feynman diagrams always s o u n d mysterious, especially w h e n a theorist pronounces t h a t she w i l l calculate (or occasionally has

THE PLAYERS AND THE STAGE - 39

Figure 3.2 The Feynman diagram for a pion exchange. already calculated) the occurrence p r o b a b i l i t y o f some process using a Feynman diagram. I t is true that these diagrams are used as a type o f shorthand n o t a t i o n t o describe a l o n g and precise calculation, b u t for us t h e i r p r i m a r y role w i l l be as a description o f the sequence o f events i n an interaction. Actually, we can dia­ gram any i n t e r a c t i o n — n o t just subatomic particles. Just bear i n m i n d that each line represents a particle. T h e vertical distance shows separation ( b u t n o t precisely) and the h o r i z o n t a l scale i n ­ dicates t i m e progression. As an example, we can diagram a simple baseball sequence. A pitcher t h r o w s the ball, a batter hits i t and runs. Pictorially we see this i n figure 3.3. I n a Feynman diagram we c o u l d represent this w i t h figure 3.4. T h i s tells us t h a t the first event i n the se­ quence was that the ball was t h r o w n ( A ) . T h e second event was the batter h i t t i n g the ball ( B ) . We are also t o l d t h a t b o t h the batter and the ball changed t h e i r m o t i o n as a result o f the h i t ( C ) — b o t h back t o w a r d the p i t c h e r — w i t h the ball leading. A more exciting baseball play is shown i n figure 3.5. T h e se­ quence t h a t we can read off f r o m the diagram, and o u r interpre­ t a t i o n since we have seen baseball, is s o m e t h i n g like this. (A) T h e ball is pitched. ( B ) T h e batter hits i t . As a result o f the h i t , the batter runs and the ball flies. ( C ) As soon as l i g h t f r o m the ball reaches the pitcher, the short-stop, and the left-fielder, they all start t o r u n . ( D ) T h e pitcher and short-stop collide and stop t h e i r m o t i o n . ( E ) T h e left-fielder fields the ball, the r u n n e r sees t h i s , turns, and heads for the d u g o u t , and the team i n the field heads

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Figure 3.4 A baseball Feynman diagram for the same sequence as the previous figure.

THE PLAYERS AND THE STAGE

41

Figure 3.5 A baseball Feynman diagram for a more complex play. to its starting positions. As we can see by this example, the dia­ gram can keep track o f the interactions, the t i m e o r d e r i n g , and the causes and effects. Back t o o u r "magnified stage," where the nucléons are playing o u t this drama. I f the whole nucleus is the size o f a playing field, t h e n the p r o t o n and the n e u t r o n are balls 50 meters i n diameter a few dozen meters apart, and there is a nearly continuous stream o f pions, about 2 0 t o 30 meters i n diameter, b e i n g e m i t t e d by one nucléon and absorbed by another. As I m e n t i o n e d before, these pions have the peculiar nature o f defying the conservation o f energy i n their role as intermediaries between nucléons. We therefore refer t o t h e m as v i r t u a l particles. Pions d o n o t always have this type o f existence. W h e n we observe t h e m i n the l a b o r a t o r y — i n the macroscopic w o r l d — t h e y must conserve mass and energy, and t h e n we refer t o t h e m as real p a r t i ­ cles. B u t i n their role as the mediators o f the nuclear force, w h a t we observe is n o t a p i o n , b u t rather the fact that some protons and neutrons have b o u n d themselves together t o f o r m the n u ­ cleus o f an a t o m . The larger objects—the nucleus and the a t o m — d o conserve energy and mass the way we t h i n k they should.

42

CHAPTER 3 A p r o b l e m t h a t I always encounter w h e n t r y i n g t o visualize

b i n d i n g due t o the exchange o f v i r t u a l particles is the question: h o w does a p i o n k n o w i n w h i c h d i r e c t i o n t o travel? I n the case o f a ball r o l l i n g d o w n a h i l l , the ball w i l l f o l l o w the trajectory defined by the topography o f the h i l l , f o l l o w i n g the lines o f the gravitational force d o w n . B u t the p i o n cannot p e r f o r m the analo­ gous m o t i o n , since the pions are the creators o f the nuclear force. T h a t is, the nuclear force can be viewed as the sum o f the effects o f innumerable pions b e i n g exchanged. O n e way t o reconcile this p r o b l e m is t o realize that, given e n o u g h t i m e , energy and m o m e n t u m really must s t i l l be conserved. O u r macroscopic per­ spectives are t r u l y fundamental and w e l l justified over larger dis­ tances or longer times. So i f a p i o n were e m i t t e d i n a d i r e c t i o n i n w h i c h i t w o u l d n o t be absorbed, i t w o u l d be lost t o the n u ­ cleus, and mass-energy w o u l d n o t be conserved. Yukawa's nuclear force theory, defining the nucleus i n terms o f neutrons, protons and pions, can describe the w o r l d o n the scale o f a few fermis ( 1 0 0 meters or so i n o u r magnified w o r l d ) . I f we increase o u r r e s o l u t i o n a b i t , these particles dissolve i n t o vaporous clouds—now defined by the s w i r l i n g collection o f t h e i r subconstituents. W i t h a diameter a t h i r d that o f the n e u t r o n or p r o t o n (~ 0.3 f e r m i , or 15 meters o n o u r stage), we see for the first t i m e the "constituent quarks." I n a very real sense the w o r l d o f these quarks is just below the surface o f the w o r l d o f the nuclé­ ons, b e i n g separated by less t h a n a f u l l order o f m a g n i t u d e . There are few places i n physics where the scales o f t w o different classes o f phenomena are so close. T h e difference between the nucleus and the a t o m is five orders o f m a g n i t u d e . T h e difference between the solar system and the galaxy is seven orders. N o t only are n u ­ cléons and pions made up o f quarks, b u t because o f the similarity o f scale we can expect t h a t we s h o u l d be able t o describe the nucleus and the nuclear force i n terms o f quarks. Inside neutrons and protons we see three quarks, each r o u g h l y a t h i r d o f the size o f a nucléon. Likewise, inside a p i o n we see t w o quarks—or more precisely, a quark and an antiquark. N o w let us reconsider the p i o n exchange Feynman diagram for the

THE PLAYERS AND THE STAGE

43

Figure 3.6 Feynman diagram of a pion exchange between two protons, in terms o f quark exchange. Yukawa m o d e l f r o m a few pages ago. I f we draw i t i n terms o f its quark constituents, we w i l l have a m o r e complex diagram (see figure 3.6).

I n i t i a l l y we started w i t h a n e u t r o n and a p r o t o n —

w h i c h is six quarks d i v i d e d i n t o t w o clusters. A p i o n is created, and w i t h i n the p i o n a quark-antiquark pair. W h e n the p i o n is absorbed, we lose this quark-antiquark pair and r e t u r n t o o u r i n i t i a l complement o f six quarks. However, m o m e n t u m has been transferred and the n e u t r o n and the p r o t o n r e m a i n b o u n d e d . T h e most peculiar feature o f this diagram is the antiquark. A n t i q u a r k s are a type o f antimatter—the stuff science fiction is made o f — w h i c h really does have all those characteristics that drive futuristic spaceships. W h e n a quark and an antiquark ( o f identical type) collide, they annihilate, g i v i n g o f f energy. W h a t makes this real, and n o t science fiction, is that i t is a c o m m o n and everyday occurrence. A t the t i m e a p i o n is produced ( t i m e A i n the above Feynman diagram), a quark-antiquark pair was created i n the t o p p r o t o n . By creating a pair, e v e r y t h i n g t h a t should be conserved, is—except energy. T h e charge o n the a n t i ­ quark is the exact opposite t o t h a t o n the quark, so the charge

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created is zero. T h e spins and the "quark-type n u m b e r " also add up t o zero. Mass or energy is n o t conserved at this m o m e n t , b u t t h a t is the same p r o b l e m that we had w i t h the creation o f the p i o n . T h e s o l u t i o n is also the same: the system borrows some mass-energy and t h e n returns i t w i t h i n a very short p e r i o d o f t i m e . A t t i m e U , a quark-antiquark pair is annihilated and the p i o n is absorbed—giving up exactly the a m o u n t o f mass-energy that was b o r r o w e d . A n o t h e r interpretation o f w h a t this diagram means, w h i c h is h i n t e d at i n the n o t a t i o n for the antiquark, is that an antiquark is the same as a real q u a r k — o n l y m o v i n g backward i n t i m e . I n the case o f all regular particles, the line i n the diagram has an arrowhead p o i n t i n g t o the r i g h t t o show t h a t this particle is m o v ­ i n g f o r w a r d i n t i m e . B u t an antiparticle is marked w i t h an arrow­ head p o i n t i n g t o the left, as i f i t was m o v i n g t o w a r d the past. So we c o u l d interpret the Feynman diagram as saying that a quark f r o m the b o t t o m p r o t o n moves f o r w a r d t o t i m e B. I t t h e n turns back i n t i m e t o t i m e A and t h e n continues i n the lower p r o t o n . I n o u r first interpretation o f this diagram, we b o r r o w e d mass i n a small region o f t i m e / e n e r g y . I n this second i n t e r p r e t a t i o n , we have b o r r o w e d t i m e i n the same small region—as allowed by the Heisenberg uncertainty relationship. D o antiparticles really g o backward i n time? Descriptions o f antiparticles as particles w i t h all t h e i r qualities and q u a n t u m numbers backward, or as particles w i t h the r i g h t qualities b u t m o v i n g backward i n t i m e , are absolutely indistinguishable. For example, t h i n k about a quark w i t h a + | charge. I t w o u l d be at­ tracted t o an electron w i t h charge o f - 1 by n o r m a l electrostat­ ics—opposites attract. T h e antiquark o f that same type w o u l d have a charge o f - 1 , and be repulsed by the electron—like charges repel. However, i f we v i e w the antiquark as h a v i n g the same quantities as the quark, b u t time-reversed, t h e n i t has the same +1 charge as the quark. We w o u l d expect i t t o be attracted t o the electron, b u t instead o f m o v i n g t o w a r d the electron we see the video b e i n g played backward, and the antiquark appears t o move away f r o m the electron. So the action o f the antiquark is the same

THE PLAYERS AND THE STAGE - 45

Figure 3.7 Water: Van der Waals force. i f i t were a quark m o v i n g backward i n t i m e , o r a quark w i t h all o f its q u a n t u m numbers backward. T h e concepts related t o descriptions o f the nuclear force i n terms o f quarks—antiparticles, the v i o l a t i o n o f mass-energy con­ servation, and particles m o v i n g back i n time—are all concepts that are consistent w i t h Yukawa's t h e o r y o f p i o n exchange. W h a t really has changed is the scale o f the players and the n u m b e r o f players o n the stage. The last m o d e l o f the nuclear force that I w i l l describe carries this one step further, a d d i n g m o r e details and w o r k i n g at an even smaller scale. Originally, this last m o d e l viewed the nuclear force as a " c o l o r Van der Waals force," b u t m o r e recently this has been m o d i f i e d i n t o s o m e t h i n g called the " q u a r k exchange"

or

"flip-flop"

m o d e l . T h e t e r m " V a n der Waals force" is familiar i n chemistry, where i t is a very short-range force acting between t w o neutral molecules. F o r instance, the force between water molecules that gives rise t o surface tension is V a n der Waals. A water molecule— g o o d o l d H 0 — i s electrically neutral. T h a t is, there are the same 2

n u m b e r o f electrons as protons—ten each. Therefore, the w h o l e molecule is neutral. H o w e v e r , i f you are close e n o u g h t o the m o l ­ ecule, there is some electrostatic force, for the electrons are con­ centrated at one end ( o n the oxygen a t o m ) , leaving the protons i n the h y d r o g e n somewhat bare (see figure 3.7). I t is n o t as i f the hydrogen is completely stripped, just that there is a tendency for the electrons t o spend more t i m e at the oxygen end o f the mole­ cule. T h i s means t h a t the molecule is polarized. I f y o u are far

46

CHAPTER 3

away, the effect o f the t e n electrons and ten protons essentially cancels, b u t at a distance t h a t is similar t o the size o f the water molecule, there is a charge. A second water molecule that is very close w i l l line itself up such t h a t its oxygen—slightly negative— w i l l be near the first molecule's hydrogens—slightly positive. T h i s sort o f short-range force seemed t o be very similar t o the nuclear force. T h e m a i n difference is t h a t i n quark dynamics we have " c o l o r charge" instead o f electric charge. I n electric charge, there is one type o f charge, w h i c h can be positive or negative, -e or +e. I n color charge, there are three types o f charges, w i t h the whimsical names " r e d , " "green," and " b l u e . " Each color charge can also come i n positive and negative. We usually w r i t e t h e m às A p r o t o n or a n e u t r o n has three quarks. Each o f the quarks is a different c o l o r — w h i c h adds u p t o a color-neutral, o r color " w h i t e , " particle. T h e rules t h a t govern color interactions are (1) opposites attract, ( 2 ) the same color repels, and ( 3 ) w h i t e o r neutral systems d o neither. T h i s is just the same as electric charge or even magnets. T h e difference is i n i d e n t i f y i n g the opposite color. A red quark w i l l repel a red quark and be attracted t o an anti-red antiquark, b u t i t w i l l also be attracted t o a green q u a r k blue quark pair, as is necessary t o f o r m the n o r m a l nucléons. T h i s also seems t o allow for the possibility o f a color V a n der Waals force. So i f t w o nucléons are w i d e l y separated, a quark i n the first nucléon w i l l v i e w the three quarks i n the second nucléon n o t as individuals, b u t rather as a collected set o f quarks, whose c o m ­ b i n e d color is w h i t e or neutral, and there is n o attraction o r re­ pulsion. H o w e v e r , i f the nucléons are close t o each other, the story changes. T h e n a quark i n the first nucléon can d i s t i n g u i s h the quarks i n the second nucléon. W i t h o u r experience w i t h water molecules l i n i n g themselves up head t o t a i l , we can expect the nucléons t o line themselves u p i n an o r i e n t a t i o n such t h a t the red quark o f the first nucléon is near the blue and green quarks and far f r o m the red quark o f the second nucléon (see figure 3.8). I n this arrangement the p r i m a r y lines o f interaction

THE PLAYERS AND THE STAGE - 47

Figure 3.8

C o l o r Van der Waals—quark flip-flop.

(indicated by the thicker lines i n the figure) are s t i l l the ones that define the nucléon. B u t there c o u l d easily be secondary lines o f force (indicated by the dashed lines i n the figure) between quarks i n different nucléons. T h e p r o b l e m w i t h the color V a n der Waals description o f the nuclear force is that a l t h o u g h V a n der Waals and nuclear forces are short-range, V a n der Waals is n o t short-range enough. A true Van

der Waals force is very, very small as the separation grows,

but i t doesn't completely vanish. T h e nuclear force does vanish. However, i n the pictures we have d r a w n lies the g e r m o f the flipflop m o d e l . I n the flip-flop m o d e l we recognize that w h a t i n i t i a l l y defines the nucléons are the connections indicated by the dark lines i n this figure. B u t because they are so close t o each other there is a p r o b a b i l i t y that the quarks w i l l re-pair themselves as shown i n the Feynman diagram below (see figure 3.9). I n this case the red quarks have been exchanged, or flipped, between nucléons, and t h a t r e - p a i r i n g is indicated by the lighter c o n n e c t i n g line. T h i s Feynman diagram looks essentially like t w o "pion-as-quark" ex­ change diagrams, one r i g h t after the other. Back o n o u r stage, a nucléon is three quarks racing a r o u n d each other i n a r e g i o n 50 meters across. W h e n these w h i r l i n g objects approach each other, they can overlap. I n the r e s u l t i n g confusion, t w o quarks c o u l d be exchanged, similar t o a square

48 - CHAPTER 3

Figure 3.9 Feynman diagram for quark flip-flop. dance, where everyone swaps partners i n a way that works t o keep the square o f dancers together. So we have three descriptions o f the mechanism t h a t underlies the nuclear force: Yukawa's pions, pions as quark exchange, a n d a quark flip-flop. W h i c h is right? I n one sense the flip-flop is embedded i n the quark exchange, and t h e quark exchange is em­ bedded i n the p i o n exchange, so they all w o u l d seem t o be equally true. B u t there are real distinctions related t o scale a n d complexity. T h e simplest m o d e l is Yukawa's. I n his picture we h a d a neu­ t r o n , a p r o t o n , and a p i o n — o n l y three particles. C o m p u t a t i o n ­ ally this is the simplest. B u t this m o d e l breaks d o w n as we ap­ proach the size o f the nucléons themselves, w h e n w e must i n t r o d u c e the quarks as i n d i v i d u a l particles. I n the quark ex­ change m o d e l we deal w i t h six quarks, and t h e n the creation a n d a n n i h i l a t i o n o f a quark-antiquark pair. T h a t is, w e have u p t o eight particles and we can describe matter d o w n t o t h e distance between quarks. T h e t h i r d model—the flip-flop—has six quarks, b u t also t w o sets o f inter quark b i n d i n g as w e l l as m u l t i p l e p e r m u -

THE PLAYERS AND THE STAGE

49

tations o f rearrangements. T h i s last m o d e l should w o r k d o w n t o a scale defined by the interquark b i n d i n g . So as we go t o models w i t h higher resolution we also gain degrees o f complexity. B u t that is just a c o m p u t a t i o n a l p r o b l e m that we should be able t o overcome w i t h faster and better c o m ­ puters. W h y t h e n d o we s t i l l m a i n t a i n the p i o n exchange model? I t is because nature really does produce a particle called the p i o n . I t has been observed and measured. Physicists can even f o r m t h e m i n t o a beam that can be steered across a laboratory and used as an experimental probe. I n fact, i f we had only l o o k e d at the nuclear force we m i g h t never have guessed at the existence o f the quark. There are small details about the nuclear force that are not

completely accounted for by the p i o n . H o w e v e r , w i t h the

a d d i t i o n o f other, less abundant particles, such as the p-meson or the co-meson, w h i c h also have been observed, a complete de­ scription o f the nuclear force does arise. I f I w a n t t o describe nature d o w n t o a scale o f one f e r m i , I d o n ' t need quarks. A l l the effects o f quarks and antiquarks, and t h e i r interactions, generally sum up t o very discrete and u n i q u e bundles—the nucléons and the mesons. So w h y b r i n g up the quark picture at all? T h e quark m o d e l s t i l l performs the same f u n c t i o n a m o n g these exchange mesons as i t d i d a m o n g the particles that i n the 1950s and 1960s drove G e l l - M a n n t o the eight-fold way. The pions (7i , 7t°, 7 T ) , the p+

mesons ( p , p°, p~),the co-mesons, and so o n are all explained i n +

terms o f t w o types o f quarks. A n d finally, quarks d o come i n t o their o w n at the r i g h t scale, at less t h a n a fermi. One final c o m m e n t w h e n discussing quarks and t h e i r scale. W h a t is the size o f a quark? I said that a constituent quark was r o u g h l y a t h i r d o f a fermi across. B u t a constituent quark is n o t a true or "bare" quark; rather, i t is a bare quark wrapped up i n self-interactions. Theorists believe that a bare quark is p o i n t l i k e , and all experiments c o n f i r m this. I n reality, experiments can only say that the quark is smaller t h a n the resolution o f the largest accelerator. W i t h the accelerator at F e r m i L a b , near Chicago, p r o -

50

CHAPTER 3

d u c i n g a t r i l l i o n e V o f energy, we can say that a quark is less t h a n one-thousandth o f the size o f a nucléon. I n the magnified analogy w i t h w h i c h we started this c h a p t e r — w i t h a h u m a n reaching the stars, atoms the size o f E a r t h , and nucléons

fitting

inside a playing field o r o n a stage—we can say that a bare quark must be smaller t h a n 10 centimeters, or about 2 - inches, across.

The Nature of the Evidence

I

N T H E H I S T O R Y o f the study o f matter, w h a t sets quarks apart f r o m every other level o f matter is that we are n o t able t o isolate and observe t h e m directly. N o t only is this l i m i t a t i o n inconvenient and frustrating for the investigator, b u t i t has also created a psychological barrier. T h e quark hypothesis has been w i t h us for t h i r t y years and we k n o w a great deal about w h a t quarks are like, yet there are s t i l l physicists w h o treat t h e m as merely a "mathematical convenience." T h r o u g h o u t the history o f physics we have been able t o con­ firm our hypotheses about the basic constituents o f matter by d i s t i l l i n g and isolating those constituents. I n the eighteenth and nineteenth centuries, chemists were busy isolating the "ele­ ments" before D m i t r i Mendeleev c o u l d compile his Periodic Table of the Elements. A h u n d r e d years ago J.J. T h o m s o n , at Cav­ endish Laboratories i n Cambridge, E n g l a n d , launched o u r study o f the a t o m and the fundamental particles by observing the elec­ t r o n i n detail. This was soon followed by Ernest Rutherford's discovery o f the nucleus and later the p r o t o n , and shortly thereaf­ ter James Chadwick's observation o f the n e u t r o n . B u t we have never isolated a quark. I n accelerators we have created fireballs w i t h 1,000 times more energy t h a n is necessary t o create a p r o ­ ton. Yet we s t i l l have never seen a quark, and there are many reasons t o t h i n k that we never w i l l .

52

CHAPTER 4 Yet this synopsis o f the study o f matter is n o t o n l y simplistic

b u t deceptive. A truer h i s t o r y should h i g h l i g h t the linkage be­ t w e e n size and w h a t counts as direct evidence. T h e t r e n d i n the study o f matter is n o t just a m i g r a t i o n t o smaller and smaller scales, i t is also a m i g r a t i o n t o m o r e abstract evidence, evidence t h a t can only be understood t h r o u g h a g r o w i n g reliance u p o n theoretical interpretation. I n this l i g h t we can l o o k at the w o r k o f A n t o i n e L a u r e n t Lavoisier, w h o isolated oxygen d u r i n g the late p a r t o f the eigh­ t e e n t h century. A l t h o u g h he was dealing w i t h a substance t h a t he c o u l d n o t directly "see," he k n e w t h a t i t occupied space and displaced water. M o r e i m p o r t a n t , flames b u r n e d i n its presence and were extinguished i n its absence. I n some sense, he k n e w t h a t he had oxygen because he c o u l d p u t i t i n a b o t t l e , label i t , and p u t i t o n a shelf. Likewise, J. J. T h o m s o n , i n 1 8 9 7 , is generally credited w i t h the "discovery" o f the electron—or at least the d e t e r m i n a t i o n o f many o f its characteristics. I n i t i a l l y he was t r y i n g t o understand w h a t cathode rays are. A t the t i m e there were t w o c o m p e t i n g hypotheses about t h e i r nature. O n e school o f t h o u g h t m a i n ­ tained that cathode rays were "aether" o r "wavelike," w h i l e the other school proposed a "particlelike" explanation. T h o m s o n ' s apparatus, the cathode ray tube ( C R T ) , is essentially the same as a television t u b e . W h a t he and his contemporaries called a cath­ ode ray we w o u l d n o w identify as a beam o f electrons. H e sub­ jected this beam t o electric and magnetic fields and measured h o w the beam was deflected. H e also focused the beam o n a metal receptacle, an anode, and measured h o w m u c h charge, heat, and energy were deposited. F r o m these measurements he c o u l d infer the velocity o f the electron, its charge t o mass ratio, and the sign o f its charge. F u r t h e r m o r e , he k n e w t h a t the mass o f the electron was 2 , 0 0 0 times smaller t h a n the mass o f a h y d r o g e n a t o m , and finally t h a t cathode rays were made o u t o f particles— the electrons. A t this stage, w i t h the i n t r o d u c t i o n o f the first elementary par­ ticle, already the nature o f the evidence has changed. F o r L a v o i -

THE NATURE OF THE EVIDENCE

53

sier, matter had classical traits: i t occupied space and i t had mass—it c o u l d be weighed o n a scale. J. J. Thomson's electron was a very different creature. H e never attempted t o measure its size—it w o u l d have been a futile endeavor, as i t s t i l l looks p o i n t ­ like even t o the most sensitive experiments we have today. H i s measurement o f mass, unlike Lavoisier's, was n o t gravitational, but

related t o energy; the k i n e t i c energy converted t o heat, or

the inertial resistance t o being deflected. Finally, he never "saw" the electron. Rather, he saw a b r i g h t spot o n the phosphorescent coating o f his cathode ray tube. I n a series o f experiments at the University o f Manchester i n E n g l a n d , f r o m 1910 t o 1 9 1 8 , Ernest R u t h e r f o r d and his assistant Hans Geiger established the existence o f the nucleus, and later the p r o t o n . They scattered alpha particles, e m i t t e d f r o m a radio­ active r a d i u m source, off a t h i n g o l d leaf. R u t h e r f o r d knew f r o m his earlier w o r k at M c G i l l University i n M o n t r e a l that alphas are relatively massive and so he expected the alphas t o pass t h r o u g h the g o l d w i t h l i t t l e o r n o deflection. H e set up detectors—both a Geiger counter and a zinc sulfide screen ( m o r e sensitive t h a n Thomson's phosphorus screen)—on the far side o f the g o l d f r o m the alpha source. H e observed a large n u m b e r o f alpha particles p u n c h i n g t h r o u g h the g o l d leaf. T h i s is w h a t he expected, for the prevailing m o d e l held that the g o l d a t o m , or any a t o m , was a t h i n , homogeneous c l o u d o f positive charge m i x e d w i t h elec­ trons, w h i c h c o u l d n ' t stop an energetic alpha. R u t h e r f o r d t h e n m o v e d his detectors t o the other side o f the g o l d f o i l and s t i l l observed alphas. These were alphas that had scattered

backward

off some dense and massive core—the nucleus o f the a t o m . These experiments are precursors t o o u r m o d e r n study o f quarks. N o t only d o they i n t r o d u c e us t o the nucléons, b u t they also p o i n t t o the value o f scattering experiments and the develop­ m e n t o f new detector technologies. Scattering experiments and the direct descendants o f the Geiger counter are at the core o f any m o d e r n high-energy o r nuclear physics laboratory. Later, i n 1919, R u t h e r f o r d was able t o scatter alphas off n i t r o ­ gen gas and liberate a hydrogen nucleus—a p r o t o n . H e t h e n

54 - CHAPTER 4 demonstrated t h a t the nuclei o f all atoms c o n t a i n protons and even w e n t o n t o estimate the size o f the p r o t o n . H e measured i t t o be 7 x 10"

13

centimeter, similar t o a m o d e r n measurement

and by far the smallest t h i n g measured at the t i m e . H o w e v e r , R u t h e r f o r d realized t h a t s o m e t h i n g was missing—and he p o s t u ­ lated the existence o f the n e u t r o n t o explain unaccounted mass and even isotopes. I t fell t o James C h a d w i c k , at Cavendish, t o detect the n e u t r o n , the elusive uncharged counterpart o f the p r o t o n . Chadwick's n e u t r o n detector consisted o f essentially a flask o f h y d r o g e n i n f r o n t o f a p r o t o n detector. N e u t r o n s are neutral and w i l l n o t flash o n a zinc sulfide screen o r produce i o n i z a t i o n i n a Geiger counter. H o w e v e r they can collide w i t h and k n o c k o u t protons f r o m hydrogen. These newly liberated protons can t h e n be de­ tected i n a standard p r o t o n detector. I n one sense Chadwick's detector was b u t a slight i m p r o v e m e n t o n the methods o f R u t h ­ erford and Geiger, b u t i n another sense his detector had a w h o l e new element, the hydrogen i n front o f the p r o t o n detector. So C h a d w i c k had detected the particle t h a t c o u l d n ' t be de­ tected. H e had done this by completely understanding the way i n w h i c h protons w o u l d interact i n his device. H e k n e w t h a t the protons t h a t his Geiger counter saw had o r i g i n a t e d i n the h y d r o ­ gen, and that they had n o t been liberated by alphas or protons. W h a t he was left w i t h was a neutral particle w i t h about the same mass as a p r o t o n . T h i s is the d o m i n a n t t r e n d i n the detection o f m o r e and m o r e elusive particles: m o r e sensitive detectors depend o n a m o r e de­ tailed understanding o f h o w the detector works and w h a t other phenomena i t m i g h t see. T h e physicists' observation o f a reaction o r event under study becomes a longer and longer chain:

T h e w o r k i n g physicist realizes t h a t observations are n o t just l i m ­ i t e d by the weakest l i n k o f the chain, b u t are even weaker. T h e uncertainty i n the final results is an accumulation o f the uncer-

THE NATURE OF THE EVIDENCE - 55 tainties f r o m each l i n k . T h e uncertainty also depends o n h o w m u c h data are available, h o w large the statistical sample is. So i n a m o d e r n experiment we have replaced the observation o f a flash or the clicks o f a Geiger counter w i t h an array o f detector sys­ tems, many o f w h i c h have w e l l over 9 9 percent efficiency, as w e l l as specialized data collection computers that can gather data f r o m thousands o f detectors, m i l l i o n s o f times each second. Yet, before we discuss the m o d e r n descendants o f the fluorescing screen and Geiger counter, we need t o understand w h a t we m i g h t detect, i f it is n o t the quarks themselves. T h e identification and measurement o f a particle, be i t an a t o m , a p r o t o n , or a quark, is only one k i n d o f measurement— and only the first stage o f an investigation. T h e goal o f physics is to understand h o w the w o r l d is p u t together, and n o t just w h a t it is made of. I t is the dynamics o f a system that gives rise t o many o f its characteristics, characteristics that can be experimentally measured. I f quarks were b u i l d i n g blocks that were simply added together t o f o r m protons and neutrons, atoms and molecules, i n the way Legos snap together, i t w o u l d n ' t matter i f they were ultramicroscopic b i l l i a r d balls, or just "convenient mathematical concepts." B u t they d o n ' t just add up. A p r o t o n is n o t just the sum

o f three quarks—it is also a p r o d u c t o f the way the quarks

dynamically interact w i t h each other. W h e n John D a l t o n was proposing his atomic t h e o r y o f chem­ istry ( 1 8 0 3 ) , far more i m p o r t a n t t o h i m t h a n a complete list o f the elements were observations, such as the "law o f constant p r o ­ p o r t i o n s . " T h i s law stated that molecules are made up o f definite ratios o f the elements. For example, the ratio o f hydrogen t o oxygen i n water is always t w o t o one by v o l u m e , and eight t o one by weight. D a l t o n recognized that this w o u l d f o l l o w i f t h e w o r l d was atomic i n nature. I f an elementary substance is made o u t o f discrete atoms, i t can only combine i n discrete units. H e p r o ­ posed that the unique and defining feature o f the atoms o f an element was the n u m b e r o f " h o o k s " and "eyes" that i t had o n its surface. H i s sketches o f atoms l o o k a l o t like b u r d o c k , those seed pods that attach themselves t o y o u r socks w h e n y o u w a l k t h r o u g h

56

CHAPTER 4

an a u t u m n meadow. These pictorial " h o o k s " o f D a l t o n end u p corresponding t o the valence electrons o f m o d e r n chemistry. I n atomic physics, i f we had only observed the electrons and the nucléons, we w o u l d have missed one o f the most s t a r t l i n g revelations o f the century. I t was the measurement and explana­ tions o f the atomic spectrum t h a t really opened up the w o r l d o f q u a n t u m mechanics. I t was those b r i g h t lines i n the familiar spectrum that inspired Niels B o h r t o postulate the p r i v i l e g e d orbits o f the electron about the nucleus. " P r i v i l e g e d , " because the electrons d i d n o t radiate away t h e i r energy and spiral i n t o the nucleus i n the way a "classical," n o n - q u a n t u m mechanical system w o u l d . E r w i n Schrôdinger explained w h y these orbits were p r i v ­ ileged by " q u a n t i z i n g " the a t o m , and explained the relative i n ­ tensity o f each line as a result o f the p r o b a b i l i t y and the overlap o f "wavefunctions." Werner Heisenberg explained the w i d t h o f the spectral lines as an i n t r i n s i c p r o p e r t y o f wave mechanics— usually expressed as an uncertainty relationship. Finally, P . A . M . Dirac gave us a detailed relativistic q u a n t u m mechanics, w h i c h i n c l u d e d the spin o f the electrons and the microscopic s p l i t t i n g o f the spectral lines. A l l o f these new developments i n theoretical physics—the Schrôdinger equation, the Heisenberg uncertainty relationship, the Dirac equation—were first postulated t o explain the atomic spectrum. T h e spectrum, i n its m o r e abstract and generalized f o r m , continues t o be the single most i m p o r t a n t way o f pre­ senting b o t h theoretical and experimental results today. F o r a m o d e l o r a t h e o r y t o be tested, i t must be able t o calculate some type o f spectrum. For an experiment t o be a useful test, one must be able t o take the raw data and present i t as a spectrum. I t is the m e e t i n g p o i n t o f these t w o branches o f investigation, t h e o r y and experiment. T h e quintessential atomic spectrum is that o f h y d r o g e n , w i t h a b r i g h t red line at one end and a blue line at the other, w i t h a faint blue and even fainter v i o l e t lines beyond that. T h e intensity o f the red l i n e , for example, means t h a t a large n u m b e r o f " r e d " photons, w i t h wavelength o f 6,562 A , or 1.89 e V o f energy

THE NATURE OF THE EVIDENCE - 57

Figure 4.1 Spectrum as histograms o f counts. (color, wavelength, and energy are all equivalent) were de­ tected—either by a photographic plate or perhaps by eye. So we c o u l d present a spectrum as a histogram o f the n u m b e r o f pho­ tons detected versus the energy o f each p h o t o n (see figure 4 . 1 ) . W i t h a m o d e l such as the B o h r m o d e l o f the a t o m , or better yet the q u a n t u m mechanical atomic theory, these lines are ex­ plained as energy released w h e n an electron drops f r o m a higher energy level (or o r b i t ) t o a lower level. W h a t we measure is the n u m b e r o f photons as a f u n c t i o n o f energy. W h a t we infer t h r o u g h o u r models and theories is the energy b o u n d up i n each orbit. I n nuclear and high-energy physics we measure s o m e t h i n g called a cross section, w h i c h is essentially a generalized o r abstract spectrum. As an example, a simple cross-section

measurement

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w o u l d be t o direct a beam o f electrons at a t h i n f i l m o f carbon, and c o u n t h o w many electrons pass t h r o u g h w i t h o u t being scat­ tered. F r o m this one can infer the size o f the nucleus. B u t the probe and the target need n o t be microscopic t o measure a cross section. Imagine that y o u are t r y i n g t o measure an obstacle i n a dark hallway. Simply station y o u r friend at the far end o f the hallway, t h e n take 100 baseballs and t h r o w t h e m d o w n this dark and mysterious corridor. W h e n y o u r partner reports that seventy balls arrived at his end y o u k n o w t h a t the obstacle blocks r o u g h l y 30 percent o f the hallway. By measuring the cross-sectional area o f the hallway (5 feet by 10 feet, or 50 square feet), y o u can calculate the cross-sectional area o f the obstacle: 30 percent o f 50 square feet—or 15 square feet. So a cross section is a measurement o f an effective area. I f a p r o t o n were a solid l i t t l e ball, and we c o u l d fire infinitesimally small BBs at i t , we c o u l d simply measure its area and size. B u t we d o n ' t have infinitely small BBs. T h e best we can d o is an electron. A l t h o u g h an electron is as small as one c o u l d hope for, i t doesn't collide w i t h the p r o t o n i n the same way a B B w o u l d . I t interacts via the electrostatic, or C o u l o m b , force. T h a t means i t can scatter o f f the p r o t o n even i f i t just passes near i t and doesn't actually t o u c h i t . I n fact, this is n o t so bad, since we understand C o u l o m b scattering so w e l l . Also, f r o m the v i e w p o i n t o f the electron, w h a t defines the boundaries o f the p r o t o n is where the p r o t o n ' s charge is located. So a p r o t o n can be "spongy," and n o t just a h a r d sphere. I f the electron has a l o t o f energy, i f i t is t h r o w n h a r d , i t can penetrate deeply. I f i t loses n o energy, i t misses. I f i t loses a l o t o f energy, i t hits near dead center. We can pick u p one more piece o f i n f o r m a t i o n : at w h a t angle d i d the electron scatter? I n billiards the energy lost and the angle scattered are p r o p o r t i o n a l , a result o f b o t h the conservation of energy and the conservation of momentum. B u t that is n o t purely true for o u r experiments. Energy can be conserved by using some o f i t t o deform the nucleus or the nucléon. They act m o r e like rubber balls t h a n b i l l i a r d balls, as they can h o l d energy by b e i n g "stretched" or "compressed" or "excited," and t h e n release t h a t

THE NATURE OF THE EVIDENCE

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energy a m o m e n t later. A n d this t o o is a useful t o o l for under­ standing the i n t e r i o r o f a nucléon. So w i t h these t w o pieces o f i n f o r m a t i o n , energy lost and angle scattered, we can infer h o w deep the electron penetrated and how m u c h charge the electron encountered. Also, we can probe the various modes or deformations by w h i c h the nucléon can temporarily store energy. T h i s is the type o f i n f o r m a t i o n the S t a n f o r d / S L A C g r o u p (Hofstadter and later Kendall, Taylor, and Friedman) had w h e n they measured the size o f the p r o t o n and "discovered" the quark. T h e interpretation o f this cross sec­ t i o n — o r spectrum—as a f u n c t i o n o f angle can be quite complex. T h e general shape is d o m i n a t e d by the w e l l - u n d e r s t o o d C o u ­ l o m b scattering, b u t there are numerous bumps and valleys, places where the energy is r i g h t for the nucleus o r nucléons t o absorb some energy i n t o a d i s t o r t i o n or t o resonate. B u t there is p o t e n t i a l l y m o r e i n f o r m a t i o n available t h a n just the energy and scattering angle o f the electron. Q u i t e often some other particle or p h o t o n is knocked o u t o f the target as w e l l . We can measure its energy and angle t o o . F o r example, i f we are scattering an electron o f f a carbon target, we w i l l sometimes also k n o c k o u t a p r o t o n . I n this case we can construct a "fivedimensional cross section," a spectrum w h i c h is a f u n c t i o n o f five different measurements: t w o energies and the three angles involv­ i n g the electron beam, scattered electron, and ejected p r o t o n . T h e n u m b e r o f possible measurements increases as we add complications. W h a t i f we knocked o u t t w o protons? O r perhaps a p i o n and a neutron? B u t the problems w i t h "coincidence" mea­ surements are t w o f o l d . First, they are h a r d t o p e r f o r m . I f the detection o f one particle is difficult, t h e n the detection o f t w o coincident particles is difficulty squared. Second,

sometimes

there is no new i n f o r m a t i o n . T h e second or t h i r d particle may only c o n f i r m the conservation o f energy and m o m e n t u m . T h e measurement o f a cross section w i l l always be a difficult experiment. So before an experimenter is ready t o invest years o f labor f r o m dozens o f physicists, and thousands o f dollars' w o r t h o f beam t i m e , he w i l l l o o k for guidance f r o m a theorist. T h e

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theoretical physicist, or theorist, may be able t o p o i n t o u t a p a r t o f a cross section that is u n i q u e l y interesting, sensitive t o i n t r i g ­ u i n g physics. W h e n experimentalists resolve t o "measure the most i m ­ p o r t a n t cross section i n the nucleon-quark t r a n s i t i o n r e g i o n , " that is, study matter at the scale where quark descriptions and nucléon descriptions are b o t h approximately valid, i t is a daunt­ i n g task. S t i l l , plans are drafted, laboratories are b u i l t , and tasks are organized. T h e w o r k load is subdivided i n a cooperative m a n ­ ner such that a single measurement m i g h t draw u p o n the equip­ m e n t and experience o f a collaboration o f 100 physicists and an army o f technical support. T h e task o f the theorist is equally d a u n t i n g : start f r o m the best-established t h e o r y — Q C D — a n d t r y t o predict the w o r l d we see, or at least the cross sections measured by the experimental­ ists. B u t this is a task t h a t defies o r g a n i z a t i o n and subdivision. T h e best techniques are n o t obvious, and Q C D is a difficult the­ ory t o w o r k w i t h . M u c h as quarks insist o n s t i c k i n g together, the equations o f Q C D d o n o t readily unravel. A n d a l t h o u g h i t is o u r favorite t h e o r y and has passed every challenge t o date, Q C D is n o t the only t h e o r y o u t there. Theorists have approached this p r o b l e m f r o m a n u m b e r o f sides. Some concentrate o n o n l y one particular aspect o f the the­ ory, reducing i t t o a simplicity that they can tackle. B u t these simplifications may n o longer describe the real w o r l d . These pa­ pers w i l l be peppered w i t h phrases such as "consider the case o f a one-dimensional universe," or " i n the case o f a massive quark . . . , " or " w i t h only an isotropic d i s t r i b u t i o n . . . . " O n e hopes t h a t techniques m i g h t be developed, o r perhaps a t r e n d w i l l be recognized, w h i c h w i l l p o i n t i n the r i g h t d i r e c t i o n for a f u t u r e , m o r e realistic calculation t o pursue. A n alternative approach is to create a phenomenological m o d e l . H e r e , many o f the subtle­ ties o f a f u l l t h e o r y m i g h t be averaged over, o r replaced w i t h experimental results. T h e constituent quark m o d e l , w h i c h guides a great deal o f the description i n this b o o k , falls i n t o this cate­ gory. We have been speaking o f this m o d e l as i f i t were w e l l estab-

THE NATURE OF THE EVIDENCE - 61 lished and universally recognized. Yet t w o theorists starting f r o m the same m o d e l w i l l differ i n their results. One m i g h t indulge i n an a p p r o x i m a t i o n early i n her calculation that allows a simple interpretation o f results later. The second theorist may choose less a p p r o x i m a t i o n , perhaps s u b s t i t u t i n g computer simulations or numerical solutions, and end up w i t h a result that is more accurate, b u t harder t o understand and less instructive. This whole noncentralized approach may seem a b i t chaotic, but i t lends itself w e l l t o the mind-set o f the theorist. Theorists tend t o w o r k i n groups o f rarely more t h a n three. T o o many theorists, and there w o u l d never be an agreement o n techniques and appropriate approximations. To demonstrate some o f the considerations that go i n t o a cal­ culation, let us consider describing a A particle f r o m a theorist's p o i n t o f view. We w i l l see some o f the r u d i m e n t a r y parts o f the constituent quark m o d e l , the role o f conservation laws, q u a n t u m mechanics, relativity, and spin. We can finally p u l l all these t o ­ gether t o create a cross section. I n Q C D , and all quark models, the p r o t o n and the n e u t r o n are made up o f three quarks. T w o o f t h e m have their spins aligned (parallel) and one is antiparallel. The A particle is essentially the same as these nucléons, except that i t has the spins o f all three o f its quarks aligned. B u t what is spin? Spin is a property that all particles have w h i c h is meant t o con­ jure up visions o f a child's t o p , or the spin a pitcher can p u t o n a curve ball, or the " E n g l i s h " that can be p u t o n a cue ball as i t skids across the green felt o f a b i l l i a r d table. I n all these cases, i n c l u d i n g quarks, the best way t o "see" the spin is t o observe what happens w h e n the ball or particle collides w i t h something. W h e n a b i l l i a r d ball hits the cushion at the edge o f the table, i t is n o t simply reflected; h o w i t reacts depends u p o n the d i r e c t i o n o f its spin. I f the spin rolls i t along the cushion, i t bites i n t o the cushion and bounces away at a shallower angle, closer t o the cushion t h a n before the collision. I f a backspin skids i t along the cushion, i t bounces o f f at a larger angle. Sometimes we can use this type o f scattering t o measure the spin o f a particle.

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Figure 4.3 Combination o f two spin | quarks can be + 1 , 0 , 0, o r - 1 . B u t quarks and electrons cannot have spin i n exactly the same way as a b i l l i a r d ball. They are p o i n t l i k e objects, spheres w i t h zero radius. H o w can the equator spin a r o u n d the axis w h e n the equator doesn't go a r o u n d the axis, b u t is i n the same place as the axis? W h a t they share w i t h a b i l l i a r d ball is the way t h e i r " b o u n c e " depends o n their spin. M o r e i m p o r t a n t , they share the same type o f mathematics. F o r instance, a b i l l i a r d ball s p i n n i n g has essentially t w o orientations; " u p , " w h i c h is counterclockwise w h e n viewed f r o m the t o p , or " d o w n , " w h i c h is clockwise. T h e spin axis may be t i p p e d , b u t w h e n i t comes t o b o u n c i n g o f f the cushion i t is only the p a r t o f the spin that is up or d o w n w h i c h counts (see figure 4 . 2 ) . W h a t makes spin interesting is w h e n t w o particles collide or combine. There are n o "cushions" i n a nucléon—only other par­ ticles w i t h spin. W h e n t w o b i l l i a r d balls are o n a table—or t w o particles are b o u n d together—there are four possible spin c o m ­ binations (see figure 4 . 3 ) . I n the first c o m b i n a t i o n , the sum is t w i c e the spin o f one ball. I n the second and t h i r d cases, the spins cancel and sum t o zero. I n the last case, the

magnitude

o f the sum is the same as the first case—but w i t h the opposite orientation.

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I n the case o f quarks and electrons, the magnitude o f their " i n t r i n s i c " spin is quantized i n t o steps o f | . T h i s is where the analogy t o b i l l i a r d balls breaks d o w n . I n t r i n s i c spin is s o m e t h i n g unique t o the q u a n t u m w o r l d . I f a ball rotates faster, its " s p i n , " or angular m o m e n t u m , increases. I f i t stops r o t a t i n g , its angular m o m e n t u m vanishes. B u t a quark always has spin | . I f we have a pair o f quarks, we can add up their spins, like the b i l l i a r d balls p i c t u r e d above, t o + 1 , 0, or - 1 . I f we have a t r i p l e t o f quarks, like i n neutrons and protons, the spins can add up t o ± | or + | . O u r familiar nucléons, the protons and the neutrons, are the spin | c o m b i n a t i o n . Protons and neutrons have t w o quarks w i t h their spins aligned, and one quark w i t h its spin i n the opposite d i r e c t i o n . The difference between the + | and the - | is just a matter o f o r i e n t a t i o n . W h a t about the c o m b i n a t i o n o f quarks i n w h i c h all three are aligned w i t h each other? This c o m b i n a t i o n — w i t h spin | —is called the A particle. The A was first observed at the Chicago C y c l o t r o n i n 1952, a dozen years before the quark hypothesis was p u t f o r t h . A l t h o u g h its discovery predates G e l l - M a n n and Z w e i g , i t is s t i l l instructive t o start w i t h a quark m o d e l o f the A, t o t r y t o calculate what an experimenter w o u l d observe. I n most ways a A is the same as a nucléon. I t has three quarks that o r b i t each other i n a r o u g h l y symmetric manner. The notable exception t o their similarities is the fact that the quarks i n the A are all aligned. T h e other major difference is their mass. T h e A has a mass o f 1,232 M e V / c whereas the nucléon has a mass o f 9 3 7 M e V / c — a 30 percent difference. I n fact, this mass differ­ ence, and all the other unique characteristics we w i l l encounter, are a result o f the spin flip o f a single quark. 2

2

B u r i e d i n the equations that govern the dynamics o f any quan­ t u m mechanical particle w i t h spin is a t e r m referred t o as a spinspin interaction t e r m . I n the case o f atomic physics, the interac­ t i o n between the spin o f an electron and the spin o f a nucléon w i l l alter the b i n d i n g o f the electron by one part i n a m i l l i o n . T i n y as that is, i t can be seen i n careful spectroscopic studies as the " h y p e r f i n e - s p l i t t i n g " o f a line i n the spectrum i n t o sublines.

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W h e n we move o n t o the scale o f the constituent quarks, this hyperfine-splitting has g r o w n t o a d o m i n a n t role, a mass s p l i t t i n g o f 30 percent! I t may seem peculiar that "interactions" can m o d i f y masses so dramatically. I n o u r everyday experience the mass o f a composite object w i l l have the same mass as the sum o f its parts, i f we i n ­ clude the glue as a part. A n d i t most certainly cannot have less mass t h a n the sum o f the parts. B u t i n the subatomic w o r l d , masses d o w o r k i n this peculiar way. T h e deuteron does w e i g h less t h a n the sum o f the masses o f its constituents: the n e u t r o n and the p r o t o n . T h i s is because w h e n we talk about masses i n the subatomic w o r l d , we mean the energy required t o create such a particle or c o m b i n a t i o n o f particles. W h e n a p r o t o n and a neu­ t r o n are c o m b i n e d t o make a deuteron, excess energy is released. T h u s , i t takes less energy t o create a deuteron t h a n a free p r o t o n and n e u t r o n , and therefore the deuteron has less mass. Always w h e n we measure mass we d o i t t h r o u g h some dynamic property. We collide the particle i n t o a detector and measure its k i n e t i c energy. We bend its trajectory i n a field and measure its m o m e n ­ t u m . I t is always f r o m these types o f measurements that we infer mass. T h e p o i n t is, mass is energy (remember Einstein). I n a d d i t i o n t o mass, o u r quark m o d e l can tell us about the dynamics o f the A. W h e n a c o l l e c t i o n o f three quarks, a nucléon, is excited, the spin o f one o f the quarks can flip t o f o r m a A. I t takes a l o t o f energy t o f o r m that A (the 300 M e V / c mass i n ­ crease) and there is l i t t l e t o stop i t f r o m d u m p i n g t h a t energy and decaying back i n t o a nucléon. Therefore, the lifetime o f the 2

A is very short: 5 x 10~ second—or 5 yactoseconds. T h i s is r o u g h l y the a m o u n t o f t i m e i t takes l i g h t t o travel 1 t o 2 fermis, or t o cross t w o protons. Even i f y o u h i t a A o u t w i t h an electron f r o m the w o r l d ' s largest accelerator and effectively increased its lifetime due t o relativistic t i m e d i l a t i o n , i t s t i l l w o u l d n o t be able 24

to travel all the way t o y o u r detector! W h a t we can detect are the decay products. W h e n the A decays back t o a nucléon, i t w i l l release that 300 M e V o f energy. T h i s c o u l d be a p o w e r f u l p h o t o n , a gamma ray. ( I n contrast, an X ray

THE NATURE OF THE EVIDENCE - 65

Figure 4.4 The decay o f the A particle, as seen on the quark level. w i l l have dozens o f K i l o electron Volts [ K e V ] o f energy, four orders o f magnitude weaker.) Alternatively, the energy may be used t o create a p i o n , w h i c h needs only 140 M e V , and some motion. I n fact, a quark m o d e l based calculation w i l l predict that the latter is by far the d o m i n a n t decay m o d e . Experimentally i t is verified that i t is the decay o f choice 9 9 . 4 percent o f the t i m e . The Feynman diagram for this decay is shown i n figure 4 . 4 . This creation o f a p i o n is m u c h like the process that created the nuclear force p i o n o f chapter 3, except that i t is n o t made o u t o f b o r r o w e d energy for a fleeting m o m e n t o f t i m e . T h i s is a real p i o n , w h i c h w i l l last 2.6 x 10" second—26 nanoseconds. I n the w o r l d o f the nucléon, t h a t is practically forever. A p i o n w i l l travel a few meters before decaying. T h a t is galactic distances compared to quarks and nucléons, and these pions w i l l even show up i n o u r detectors. A peak i n the energy spectrum o f the p i o n is a g o o d 8

signature o f a A. O n e last characteristic o f the A is that i t doesn't have an absolute and unique mass. Rather, i t has a mass o f 1,232 ± 120 M e V / c . T h i s is a consequence o f its short lifetime and the Heisenberg uncertainty relationship. I f the lifetime o f a particle 2

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Figure 4.5 T h e cross section, or counts, as a function o f the kinetic energy of the proton and pion, or the mass o f the À (kinetic energy + masSp + mass = mass ). rt

A

is short, t h e n the uncertainty o f its energy o r mass is large. A short lifetime and fast decay lead t o a broad spectral line. C o n ­ ceptually the A has n o t had e n o u g h t i m e t o settle i n t o a w e l l defined and stable c o n f i g u r a t i o n before i t decays. Therefore, the mass o f the A can vary by as m u c h as 10 percent. So w h e n a theoretician t h i n k s about w h a t a A looks like, she builds her ideas o u t o f a w h o l e range o f theories and models and principles. She has a constituent quark m o d e l : three quarks each w i t h spin | . T h e A is the spin | c o m b i n a t i o n o f the quarks. T h e spin-spin interaction causes a hyperfine-splitting, a difference i n the mass between the nucléon and the A o f 30 percent. T h e decay releases e n o u g h energy t o create a p i o n . A n d finally, due t o the short l i f e t i m e , there is a spread o f range o f masses.

T H I NATURE OF THE EVIDENCE

67

These are the aspects o f the À w h i c h a theorist considers i n her t h o u g h t s and calculations, b u t w h e n i t comes t i m e for the experimentalist t o l o o k for a A , we must present the models i n terms o f a spectrum o r cross section. I n figure 4.5 is shown the theorist's curve, cross section-versus-A mass, and the experimen­ talist's raw data, counts-versus-pion energy. T h e experimentalist w i l l c o u n t pions and measure t h e i r energy. B u t he knows that the pion's energy is exactly dependent o n the mass o f the particle that decayed. H e also knows h o w t o convert his raw counts t o cross section by c o r r e c t i n g for detection efficiency and the n u m ­ ber o f electrons, i n analogy t o c o r r e c t i n g for the n u m b e r o f base­ balls that were t h r o w n at the target. So, finally, i t is here i n the cross section that t h e o r y and experi­ ment meet, and the fleeting A rises up i n the data t o be counted.

Measuring a Rainbow

O

N E O F T H E great ironies o f the study o f nature is that t o see a n y t h i n g very large (the edge o f the universe) or very small (quarks) we need t r u l y enormous instruments. O n a clear n i g h t under the stars, y o u can see forever, all the way t o the shoreline o f the cosmos—to the first instant o f t i m e — b u t y o u w o u l d n ' t recognize i t . Likewise, this b o o k contains about 2 0 oc­ t i l l i o n ( 2 0 x 1 0 ) quarks, r i g h t n o w i n y o u r hand, b u t by staring at paper y o u really d o n ' t "see" quarks. T h e l i g h t w i t h w h i c h y o u see this b o o k — a n d e v e r y t h i n g else—has a wavelength o f 5,000 to 7,000 A ( 1 A = 10~ m ~ 1 a t o m ) , w h i c h means that each l i g h t wave w i l l span a few thousand atoms. T h a t sets the l i m i t o f w h a t we can see w i t h o u r eyes, even w i t h the aid o f lenses and optical microscopes. T o see s o m e t h i n g the size o f an a t o m we need a source that w i l l produce l i g h t w i t h a 1A wavelength. T h a t corres­ ponds t o a p h o t o n w i t h about 2,000 e V o f energy, w h i c h is an X ray. A p h o t o n w i t h 2 0 0 , 0 0 0 , 0 0 0 e V or 2 0 0 M e V o f energy w i l l just barely resolve a nucléon. A n additional order o f magnitude w i l l allow us t o start t o probe the inside o f a nucléon o n the scale o f the constituent quarks, and this is where the massive accelerators come i n t o play. T h e accelerators are effectively l i g h t sources, and the detectors are the eyes t h a t see the nucléons and the effects o f the quarks. 27

10

MEASURING A RAINBOW

69

Perhaps an exhaustive description o f the experimental mea­ surement o f a cross section is a major digression f r o m the central emphasis o f this book—a description o f quarks i n nucléons. B u t by presenting the efforts o f experimentalists i n detail, I hope t o i n s t i l l some sense o f where these images come f r o m and t o w h a t degree we have confidence i n t h e m . We are i n the early days o f subnucleon structure studies, and u n d o u b t e d l y some o f the i m ­ ages i n this b o o k w i l l be m o d i f i e d w i t h t i m e . B u t most o f t h e m are b u i l t u p o n the meticulous labors o f thousands o f experimen­ talists w o r l d w i d e . T h r o u g h t h e i r works we have an impressive array o f data t o support the theories and models w i t h w h i c h we make o u r pictures o f quarks. W h a t d o the "data" l o o k like f r o m an experiment? W h e n the data are collected they are a stream o f digital bits o n a w i r e plugged i n t o a computer. Alternatively, w h e n the data come i n they are electrical pulses related t o a discharging w i r e , or tracks left by a particle as i t coils up i n the magnetic field o f a detector. Or

they are a spectrum, the sum o f a m i l l i o n

events—counts

versus energy and angle. W h e n the data really come i n , they are a curve, a cross section o n a page o f a j o u r n a l . Clearly the "data" are all o f these t h i n g s , m u c h as a b o o k is i n k and paper, letters, w o r d s , sentences, images described, and ideas conveyed. F r o m another p o i n t o f view, a b o o k is the thread that connects authors, editors, artists, publishers, printers, book­ sellers, librarians, and readers. Finally, a b o o k is only a collection o f quarks u n t i l its message is internalized by the readers, m a t u r e d i n their m i n d s , and hopefully added t o their perspective, enlight­ enment, and enjoyment. A n experimental study has at least as many aspects. I n one sense an experiment is b o r n as an idea l o n g before data are actu­ ally taken. I t takes years t o gather a collaboration o f physicists, develop targets, design detectors, and o f course find the f u n d i n g for all this activity. After the data are collected, i t s t i l l takes t i m e and energy t o analyze and synthesize those data. Finally, theses, dissertations, reports, and papers are w r i t t e n and published.

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CHAPTER 5 But that is only one facet o f an experiment. F r o m a laboratory

director's p o i n t o f view, there are accelerators, detectors, and tar­ gets. There are computer networks, data t a k i n g , a r c h i v i n g , and computer analyses. There is even the logistical p r o b l e m o f get­ t i n g a thousand people and a similar n u m b e r o f machines t o w o r k together. There is a c c o u n t i n g , personnel, grounds, u t i l i t i e s , safety, and so o n . F r o m a personal perspective, involvement i n this k i n d o f re­ search is a lifetime c o m m i t m e n t . I t starts w i t h t h a t decision i n h i g h school t o take that extra m a t h course; t h e n m a j o r i n g i n physics i n college; later graduate w o r k , post-doctoral research, a research scientist p o s i t i o n , or perhaps a faculty p o s i t i o n at a university. I t can be a cradle t o grave obsession. I t some respects it may sound like i n d e n t u r e d servitude, b u t research as a lifelong career has a major redeeming quality: the problems t h a t were asked yesterday are different f r o m the problems o f today, and t o m o r r o w promises new and u n i q u e challenges. One final facet o f this k i n d o f experimental study is t h a t ques­ tions about quarks and nucléons span hundreds o f experiments as w e l l as careers. T h e y even cross laboratory boundaries and oceans. The problems discussed i n these pages and the visions o f quarks presented here are derived f r o m dozens o f laboratories w o r l d w i d e , hundreds o f experiments, thousands o f people, and billions o f dollars o f expenditure. T o describe all this is beyond the means o f a single b o o k , per­ haps even beyond the scope o f most libraries. So I w i l l d o w h a t all experimentalists d o — I w i l l d i p m y net i n t o this seething sea o f activity and p u l l o u t a sample w h i c h w i l l , hopefully, be representative. There are laboratories a r o u n d the w o r l d t h a t c o n t r i b u t e t o o u r understanding o f the arrangement o f quarks inside nucléons. Each laboratory is a c u s t o m - b u i l t facility, w i t h its o w n one-ofa - k i n d accelerator, handcrafted detectors, and u n i q u e l y t a i l o r e d experimental p r o g r a m . T h e M I T - B a t e s L a b o r a t o r y near B o s t o n has b u i l t a b e a m - r i n g that w i l l cause the electrons i n the beam to pass t h r o u g h the target repeatedly—billions o f t i m e s — u n t i l

MEASURING A RAINBOW - 71 they finally scatter. T h e "Synchrophasotron" i n D u b n a , near Moscow, can accelerate ions o f almost any k i n d i n w h a t the Guinness Book of World Records has recognized as the w o r l d ' s heaviest machine. I n laboratories f r o m Sweden t o Canada t o Japan there are accelerators w i t h unique traits: f r o m energies t o intensities t o p u r i t y o f beam. They have beams o f every i m a g i n ­ able type: electrons, neutrons, protons, ions, and even h i g h - i n ­ tensity photons. The laboratory I w i l l describe i n great detail i n this chapter is the Thomas Jefferson N a t i o n a l Accelerator Facility, i n N e w p o r t News, V i r g i n i a . U n t i l 1996 i t was called C E B A F ( C o n t i n u o u s Electron Beam Accelerator Facility). Jefferson L a b doesn't have the largest, or the highest-energy, accelerator, b u t i t is the newest laboratory w i t h some o f the highest-quality beam and most so­ phisticated detectors i n the w o r l d . Jefferson Lab was b o r n o u t o f a 1979 r e p o r t by the Nuclear Science Advisory C o m m i t t e e ( N S A C ) , w h i c h identified the need for a new accelerator and complement o f detectors t o study na­ ture at the scale o f the nucleon-quark transition. One o f the most i m p o r t a n t aspects o f the laboratory's organization is that i t is an i n s t i t u t i o n b u i l t o u t o f inclusion and collaboration. T o start w i t h , the grounds, the b u i l d i n g s , and the accelerator are oper­ ated for the D e p a r t m e n t o f Energy ( D o E ) by the Southeastern Universities Research Association ( S U R A ) . As its name implies, S U R A is a group o f universities f r o m the southeast U n i t e d States that w a n t e d the next national laboratory t o be b u i l t i n their part o f the country. They j o i n e d together and proposed Jefferson L a b to f u l f i l l the mission identified by N S A C . B u t that was just the start o f the cooperative and inclusive nature o f the laboratory. W h e n i t came t i m e t o b u i l d the detectors, some o f the construc­ t i o n and most o f the management emanated f r o m Jefferson L a b , but the design and fabrication o f the detector components t o o k place at over a h u n d r e d i n s t i t u t i o n s , f r o m over forty countries w o r l d w i d e . A l t h o u g h i t is a U . S . government laboratory, m i l l i o n s o f dollars have been spent by the Italians, the French, the Rus­ sians, and others.

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CHAPTER 5 T h e laboratory itself occupies a half-square-mile parcel o f l a n d

i n a high-tech park i n N e w p o r t News, V i r g i n i a . T h e nerve center o f the campus is C E B A F Center. A t first i t may appear n o differ­ ent f r o m any other office b u i l d i n g i n the high-tech park, except t h a t i n the l o b b y a m o n i t o r continuously displays the status o f the accelerator: w h a t energy and h o w m u c h beam is b e i n g deliv­ ered t o w h i c h experiment. Also the l o b b y is papered w i t h posters f r o m local schoolchildren w h o have participated i n one o f the many educational outreach programs o f the laboratory. I t s audi­ t o r i u m is as likely t o play host t o a conference o f physicists prepar­ i n g experiments as i t is t o a general public lecture o n a w h o l e range o f science topics, or even the occasional piano concert. O t h e r b u i l d i n g s o n the grounds house technical libraries, fab­ r i c a t i o n and testing laboratories, and the offices o f teams o f phys­ icists and engineers. Nestled next t o a w o o d l o t is the "residence facility," on-site housing for v i s i t i n g scientists. T h i s is a wonder­ f u l resource w h e n y o u r experiment is r u n n i n g a r o u n d the clock: a bed is o n l y a t e n - m i n u t e w a l k away. I n fact, w i t h computer terminals and I n t e r n e t connections i n the rooms, physicists can keep an eye o n their experiments even w h e n they are off-site and t r y i n g t o get some rest. B u t i t is back t h r o u g h the w h i t e oak and p i t c h pine grove f r o m the residence facility, past the w h i t e - t a i l e d deer and the testing laboratory, where the w h o l e reason for the laboratory is focused: the accelerator and the experimental areas. T h e c h a i n - l i n k fence t h a t surrounds the accelerator is p r i m a r ­ i l y for safety rather t h a n security. T h e guards at the gate are n o t interested i n p r o t e c t i n g secrets or "security clearances"; rather, they w i l l insist o n checking y o u r safety clearance—have y o u been t r a i n e d t o w o r k a r o u n d r a d i a t i o n (a m i n i m a l hazard at this labo­ r a t o r y ) , h i g h voltage, or, most hazardous, cryogenics such as super c o l d l i q u i d h e l i u m . Inside the fence the e a r t h seems alive, a l t h o u g h most o f the noises are f r o m v e n t i l a t o r fans. There are a n u m b e r o f buildings: the accelerator c o n t r o l b u i l d i n g , the re­ frigeration plant, the " c o u n t i n g house" where the experiments and the detectors are c o n t r o l l e d , and three great d o m e d m o u n d s . T h e accelerator itself is b u r i e d 10 meters u n d e r g r o u n d i n a race-

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track-shaped t u n n e l 7 / 8 o f a m i l e a r o u n d . T h e three m o u n d s cover and shield the three cavernous experimental halls where the accelerator finally dumps its electron beam i n t o targets and detectors. But experiments d o n o t happen o n t h e i r o w n . Experiments are b o r n o u t o f questions. Questions are raised by previous ex­ periments w i t h unexplained results—or by theories t h a t seem sound b u t predict radically different results t h a n other models. Questions are raised i n j o u r n a l papers, or presentations at colloquia, or conference sessions. Experiments are often conceived i n the back r o w o f an a u d i t o r i u m , t r i g g e r e d by a c o m m e n t o f the speaker. Sometimes they are b o r n i n conversations between col­ leagues d u r i n g a conference, or sharing an experimental shift. Sometimes i t is w h i l e b r o w s i n g the latest journals i n the library, or i n the quiet m o r n i n g or soft evening hours w h e n the w o r l d is s t i l l and t h o u g h t s can mature. Whatever the o r i g i n , i t is often a beautiful m o m e n t . Yet experiments are n o t b o r n f u l l g r o w n and mature. A n ex­ p e r i m e n t must go t h r o u g h a l o n g gestation p e r i o d before the basic ideas are ready t o face the w o r l d . The o r i g i n a t o r w i l l w a n t t o first k n o w i f i t is a practical experiment before g o i n g public w i t h his ideas. Usually, t h r o u g h a series o f calculations or c o m ­ puter simulations, he w i l l address such questions as: Can y o u really see the sought-after effect? Is the experimental apparatus sensitive enough? W h a t other effects m i g h t i m i t a t e o r mask w h a t y o u are l o o k i n g for? There is also a series o f less scientific ques­ tions such as: Is there e n o u g h interest i n the physics c o m m u n i t y to support this measurement? W h e n compared t o other experi­ ments, is i t really unique? A r e there e n o u g h h u m a n and

financial

resources? A well-developed experimental proposal is m o r e t h a n a de­ scription o f w h a t the experiment can achieve. I t is also a detailed study o f the l i m i t a t i o n s o f the experiment. T h e experiment may have a signature, such as the p i o n - n u c l e o n energy peak shown at the end o f the previous chapter. I t w i l l also always have physical and experimental l i m i t a t i o n s .

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CHAPTER 5 Physical l i m i t a t i o n s are concerned w i t h h o w the signature

compares t o " b a c k g r o u n d . " B a c k g r o u n d means an event t h a t c o u l d be mistaken for part o f the signature—like t r y i n g t o recog­ nize the music over the static o n a distant radio station. F o r example, w h e n seeking the A , one m i g h t detect a p r o t o n and a p i o n w h i c h d i d n ' t originate f r o m a A: perhaps they have n o c o m ­ mon

h i s t o r y and only coincidentally have energy similar t o the

peak o f the A resonance. T h e p r o t o n simply may have been knocked o u t o f the target, the p i o n may have resulted f r o m a completely separate interaction o f the beam w i t h the target. Worse yet, the o r i g i n a l beam may have accidentally scraped the edge o f the beam pipe, or the frame h o l d i n g the target, o r some­ t h i n g else. O n e o f the particles c o u l d have come f r o m outside the experiment. Some backgrounds we can m i n i m i z e w i t h a g o o d design, b u t other backgrounds are i n t r i n s i c t o the experiment. Often the signature is small compared t o the w e l l - u n d e r s t o o d background. I n that case we can "subtract" the b a c k g r o u n d t o o b t a i n o u r signature. Occasionally that m i g h t mean a p r e l i m i ­ nary experiment t o first measure the background ( i n a different r e g i o n t h a n the signature), w h i c h c o u l d spur a separate line o f experiments. T h e apparatus w i l l have i n t r i n s i c l i m i t a t i o n s . I f a detector can measure the energy o f a 1 G e V p r o t o n t o a resolution o f 10 M e V , and the signature is only 2 M e V w i d e , i t w o n ' t be seen clearly. T h e experimenter may h u n t a r o u n d for a laboratory t h a t has a detector w i t h higher resolution. I f one doesn't exist, ideally he c o u l d b u i l d a new one, b u t t h a t is expensive (many m i l l i o n s o f dollars), and the experimental p r o g r a m w o u l d have t o be u n i q u e and critical t o justify i t . Finally, there is a "statistical" l i m i t a t i o n w i t h every k i n d o f measurement. I f an experimenter saw 100 events i n one hour, he w o u l d expect t o see 100 i n the next hour, plus or m i n u s ten. T h e size o f the sample tells h o w w e l l t h i n g s are k n o w n , i n this case to 10 percent. T h e m o r e data one collects, the m o r e confidence one has i n the measurement. T o increase the a m o u n t o f data, he c o u l d b u i l d a thicker target, b u t this w i l l d i s t o r t the beam and

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make the i n i t i a l energy o f the event uncertain. T h e experimenter c o u l d also increase the intensity o f the beam, b u t accelerators have upper l i m i t s , and they usually r u n nearly flat o u t . Finally, the experiment c o u l d r u n for a longer t i m e , b u t beam t i m e is expensive and the c o m p e t i t i o n for beam t i m e is intense. Once an experimentalist has convinced h i m s e l f that his experi­ m e n t is feasible, he w i l l b u i l d a collaboration. T h e experiments are far t o o complex and expensive for a single i n s t i t u t i o n t o tackle. Usually he w i l l present his ideas and estimations t o other experimentalists, generally those w h o w o r k i n the laboratory where the experiment w i l l take place, or t o physicists w h o have a unique interest i n that particular k i n d o f experiment. H e will

ask this g r o u p for comments,

criticism, and

perhaps

collaboration. H e l p f r o m collaborators comes i n all forms. M o s t i n s t i t u t i o n s have developed a specialty, perhaps a unique target or an auxiliary detector or a special analysis computer code. Besides resources, collaborators can help fine-tune the experimental proposal w i t h additional i n f o r m a t i o n o n resolutions or backgrounds or new data for comparison f r o m other laboratories. W h e n the collabo­ r a t i o n feels an experiment is feasible and c o m p e l l i n g , i t is t i m e to approach an accelerator laboratory t o request beam t i m e . I f an experiment requires a l o t o f new equipment, i t may also be t i m e t o go t o a f u n d i n g agency t o seek a grant. I n the U n i t e d States the p r i m a r y agency for f u n d i n g this k i n d o f laboratory and research is the D o E . T h e N a t i o n a l Science F o u n d a t i o n ( N S F ) is also an i m p o r t a n t player. Beam t i m e requests are handled slightly differently at every laboratory, b u t i n general, they are reviewed by a Program A d v i ­ sory C o m m i t t e e ( P A C ) , w h o advises the laboratory director as to whether an experiment is b o t h feasible and interesting. T h e collaboration spokesperson, usually the physicist w h o originally conceived the experiment, w i l l present the case for the experi­ m e n t t o the P A C . T h i s is often done i n t w o stages. First, a l o n g and detailed w r i t t e n d o c u m e n t is s u b m i t t e d . Second, a presenta­ t i o n is made, i n w h i c h the spokesperson must present the experi-

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mental m o t i v a t i o n and design. I n this presentation, the spokes­ person must field any conceivable question and defend the experiment against all criticism. T h i s is a very serious endeavor. Beam t i m e is n o t only expensive, b u t i t is also a finite c o m m o d i t y . A laboratory at its best can produce only 365 days o f beam a year, yet every g r o u p o f experimentalists w o u l d like t o have as m u c h beam t i m e as they can. M o r e t i m e means m o r e data, better statis­ tics, and a better measurement. A b o u t half o f all proposals are t u r n e d d o w n . Perhaps the ex­ p e r i m e n t is being done elsewhere, o r i t is technically t o o difficult at the t i m e , or the P A C has doubts whether the measurement can really answer any questions. A n o t h e r fraction o f the propos­ als may be " c o n t i n g e n t l y approved," that is, approved i f a techni­ cal p r o b l e m w i t h a target is resolved, o r i f the expected results f r o m another laboratory c o n f i r m the existence o f the peak t h a t is to be studied, or i f a detector r e s o l u t i o n improves. Finally, some proposals really are approved; beam t i m e is a l l o t t e d and eventu­ ally even scheduled. W i t h an approved experiment i n hand, there can s t i l l be sev­ eral years o f preparation. There is always the reengineering o f targets and detectors for this particular laboratory and experi­ ment. There is the t r a i n i n g o f y o u n g students and the testing o f equipment before i t is m o v e d i n t o the experimental area o f the laboratory. T h e final preparations include an endless list o f interconnect­ i n g systems t o check and m o n i t o r . There are the computers t h a t measure and c o n t r o l the target p o s i t i o n i n g and pressures. There are the thousands o f elements o f the detector. There are the doz­ ens o f physicists w h o w i l l fly i n f r o m a r o u n d the w o r l d t o c o n t r i b ­ ute t h e i r unique skills, and also t o provide the general staffing for around-the-clock data-taking shifts. There are also the new students w h o w i l l spend m o n t h s and years away f r o m t h e i r schools, w o r k i n g at the laboratory. N o t o n l y d o they need t o learn the physics and the laboratory, b u t they need t o find apart­ ments and transportation. There are a thousand other details t h a t require the a t t e n t i o n o f the experimentalist, b u t i t is n o t a

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solo venture—there is an experienced collaboration b e h i n d every experiment. S t i l l , as w i t h any expedition i n t o the u n k n o w n , i t is by dealing w i t h the details that disasters can be averted and the p r o b a b i l i t y o f success enhanced. O n the day w h e n an experiment finally goes o n - l i n e , i t is as complex as any space launch. T h e event should be accompanied w i t h all the drama o f the Kennedy Space Center, b u t i t is n o t quite as heart-stopping. A thousand things can go w r o n g , b u t most o f t h e m can be fixed w i t h o n l y the loss o f precious beam t i m e , precious data. W h a t i f the power supplies t o the magnets t r i p , or short? W h a t i f there is a gas leak? O r w h a t i f the data are o u t o f sequence and scrambled? O r w h a t i f there are n o data at all? A l l o f these things have happened, b u t w i t h adequate plan­ n i n g and experience, generally the experiment, after some false starts, w i l l be ready t o collect its valuable data. " H e l l o — M a c h i n e C o n t r o l Center. T h i s is the crew chief. C o u l d we have beam? 4 G e V at 1 m i c r o a m p please? T h a n k y o u . " O f course, the accelerator operators have k n o w n w h a t w o u l d be requested for months—the experimentalist had sent t h e m a de­ tailed r u n p l a n — b u t n o w i t is t i m e for the accelerator t o deliver. A t one end o f the accelerator (see figure 5.1), far f r o m the experimental area, is the electron source: a h o t , g l o w i n g w i r e — like a l i g h t b u l b . I n fact, the simplest source, the t h e r m i o n i c elec­ t r o n g u n , is essentially the same as the back o f a television tube. I n the g u n , electrons are b o i l e d o f f a w i r e , a cathode (remember J. J. Thomson's cathode ray electron experiment?) by passing a current t h r o u g h the w i r e . T h e electrons are d r a w n electrostati­ cally away f r o m the cathode by the positive charge o n an anode. There is a hole i n the m i d d l e o f the anode t h r o u g h w h i c h some o f the electrons pass t o start the beam. I n between the cathode and the anode is a 100,000 v o l t potential d r o p , so the energy gained by an electron is 100 KeV. Some experiments w i l l require a very different source, i f they need electrons w i t h their spins l i n e d u p i n one particular direc­ t i o n . I n that case, instead o f a h o t w i r e , a laser is focused o n a gallium-arsenide (GaAs) crystal. Because o f the crystal structure

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Figure 5.1 The accelerator at the Thomas Jefferson National Accelerator Facility. and the p o l a r i z a t i o n o f the laser, the electrons t h a t are dislodged t e n d t o have their spins aligned. I t was p o i n t e d o u t t o me b y one o f the physicists w o r k i n g o n the polarized source that the laser t h a t is used is so weak its beam c o u l d be b r o k e n by his h a n d w i t h o u t any i l l effects o n his h a n d — b u t i t w o u l d shut d o w n the entire accelerator. A t the o t h e r end o f the accelerator, the elec­ trons have gained a few b i l l i o n e V o f energy, and y o u d o n o t w a n t t o p u t y o u r h a n d anywhere near i t ! So far the electron has 100 K e V o f energy and has traveled only a few inches o n its t r i p o f about 4.5 miles. N e x t the beam is " c h o p p e d " i n t o micropulses, w h i c h are easier for the accelera­ t o r t o push. T h i s is done by sweeping the beam over a metal plate w i t h 3 holes i n i t . Sweeping an electron beam by rapidly chang­ i n g electric fields is n o t hard. Television sets sweep beams across the screen about 15,000 times a second. A n accelerator w i l l

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sweep its beam about 8 0 0 m i l l i o n times a second, t o produce 2.5 b i l l i o n pulses (remember, there are 3 holes.) T h e rest o f the injector shapes each pulse t o be about 1 m i l l i m e t e r l o n g and gives t h e m a total o f 4 0 M e V o f energy. A l l the dimensions o f the accelerator are set by the chopper. Two

and a h a l f b i l l i o n pulses means that they are separated by

400

picoseconds, and since they have approached the speed o f

l i g h t they are spaced about 12 centimeters apart. Therefore, 12 centimeters is the l e n g t h o f the accelerator cavities. The m a i n accelerator is i n a concrete t u n n e l shaped like a race­ track. I t is 7 / 8 o f a m i l e l o n g and b u r i e d 10 meters u n d e r g r o u n d . I n each straightaway is a linac, or linear accelerator. T h e linac is made u p o f a l o n g series o f h o l l o w cavities, each o f w h i c h is the size and shape o f a slightly squashed grapefruit, w i t h the electron beam passing t h r o u g h the centers o f all o f t h e m . Microwaves are p u m p e d i n t o each cavity, where the waves oscillate back and f o r t h . T h e t r i c k t o accelerating the beam is t o get the beam pulse to enter the cavity just as the microwaves start t o move f o r w a r d , so they push the electrons. W i t h cavity after cavity p u s h i n g the electrons, they end u p effectively r i d i n g a wave d o w n the linac. O n e o f the side effects o f the oscillating microwaves is t h a t they t e n d t o oscillate the electrons i n the metal walls o f the cavity too.

N o r m a l l y this w o u l d lead t o a great deal o f heating, b u t

at Jefferson L a b o r a t o r y the whole accelerator is cooled i n l i q u i d h e l i u m . A t these temperatures, 4 ° K above absolute zero, the cav­ ity walls become superconductors and the heating and energy loss f r o m the electrons sloshing i n the metal vanishes. After one t i m e a r o u n d the accelerator, the beam has gained an additional 8 0 0 t o 1,200 M e V o f energy. I t can n o w be used for an experiment, o r else sent t h r o u g h the accelerator again t o gain an additional 8 0 0 t o 1,200 MeV.

T h i s can be repeated u p t o five

times. I n the fifth and final pass, the electrons have about 100 times the energy they had after the injector, b u t i n b o t h cases their speed is just below the speed o f l i g h t . I n fact, after passing t h r o u g h one linac, a 6 G e V electron w i l l have only gained 5 pico­ seconds, o r 6 milimeters, over a low-energy 4 0 M e V electron.

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Figure 5.2 The two H i g h Resolution Spectrometers i n Hall A . (A) beam pipe, (B) scattering chamber, (C) main magnet, (D) detector hut. T h i s means t h a t they can b o t h ride the same wave d o w n the accelerator. I s n ' t relativity amazing! Finally, after the electrons have been accelerated t o the desired energy, the switchyard w i l l send t h e m t o w a r d their final destina­ t i o n : one o f the three experimental halls. T h e first hall, H a l l A , contains t w o monstrous spectrometers (see figure 5.2). Each one is over four stories tall and 25 meters l o n g . T h e y always re­ m i n d me o f ships w i t h their bows m o o r e d t o a post i n the center o f the experimental hall. Decks, stairs, and railings add t o the nautical image. C o m i n g o u t o f the w a l l , 4 meters u p i n the air, is a s h i n i n g stainless steel beam pipe, 3 inches i n diameter, w h i c h is the only p a r t o f the accelerator an experimentalist n o r m a l l y sees. T h e elec­ trons come o u t o f the accelerator barrel d o w n t h a t beam pipe and i n t o the scattering chamber, w h i c h is a cylindrical b o x o n t o p o f the central post. Inside the chamber is the target, often a t h i n sheet o f carbon, or a waterfall for oxygen and h y d r o g e n studies, or sometimes a glass vial o f gas such as h e l i u m .

MEASURING A RAINBOW - 81

Figure 5.3 The electron's view o f entering Hall C, w i t h the Short Orbit Spectrometer on the left and the H i g h Momentum Spectrometer on the right. Some o f the electrons are scattered o f f the target, possibly t o reach a detector. H o w e v e r , most d o n o t react b u t pass straight t h r o u g h the hall, where they are absorbed i n t o the beam d u m p . O f those w h i c h are scattered, only a small fraction head t o w a r d a spectrometer. I n the spectrometer is a magnet the size o f a city bus. T h i s magnet is the heart o f the spectrometer. I t w i l l bend the trajectory o f a l o w - m o m e n t u m particle m o r e t h a n that o f a h i g h - m o m e n t u m particle, so that particles o f different m o m e n t a are physically spread o u t by the t i m e they reach the detector h u t . The

detector h u t is a garage-size, heavily shielded b u i l d i n g

balanced o n t o p o f a spectrometer. Inside i t , various detection elements can measure w h e n the particle arrived and h o w m u c h the magnet bent its p a t h . A precise measurement o f this bend is also a precise measurement o f the particle's energy and m o m e n ­ tum.

T h e H a l l A spectrometers can measure m o m e n t u m t o an

accuracy o f one p a r t i n four thousand, a t r u l y world-class mea­ surement. I n H a l l C there are also t w o spectrometers, the H i g h M o m e n ­ tum 5.3).

Spectrometer and the Short O r b i t Spectrometer (see

figure

These were the first t w o spectrometers t o come on-line at

Jefferson L a b i n 1995. As the names indicate, the first one is designed for h i g h - m o m e n t u m studies. I n fact, w h e n the energy o f the electron beam was increased b e y o n d the 4 G e V Jefferson

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Figure 5.4 The Hall B CLAS detector unfolded. (A) outer wire chamber—6 meters i n diameter, (B) time-of-flight scintillators, (C) photo multiplier tubes (PMTs), ( D ) calorimeter, (E) catwalksthe detector is four stories tall. (Courtesy o f Thomas Jefferson National Accelerator Facility [Jefferson Lab]) L a b was originally designed for, the H i g h M o m e n t u m Spectrom­ eter was ready and w a i t i n g . T h e Short O r b i t Spectrometer is designed w i t h a curious c o m ­ b i n a t i o n o f magnets t h a t t w i s t the trajectory o f a particle i n t o an S pattern. By d o i n g this i t loses some resolution, b u t the detector h u t is only 10 meters f r o m the target, less t h a n h a l f the distance o f the three other spectrometers. I t was designed this way t o study particles w i t h short lifetimes, such as strange mesons like the lambda, w h i c h w o u l d decay before traveling all the way t h r o u g h a l o n g spectrometer. T h e spectrometers i n these t w o halls can all be p i v o t e d t o vari­ ous angles a r o u n d the scattering chamber. H a l l B , the last experi­ mental hall t o come o n - l i n e , is radically different. H a l l B contains C L A S , the C E B A F Large Acceptance Spectrometer (see

figure

5.4). ( T h e collaboration t h a t b u i l t C L A S chose n o t t o change its name w h e n the laboratory changed f r o m C E B A F t o Jefferson L a b . ) C L A S is a 4n detector. T h e designation 4n for this k i n d o f detector arises because a sphere o f radius one has a surface area

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o f 4ft, and this detector is like a sphere, collecting, detecting, and measuring particles that come o u t o f the target i n any d i r e c t i o n . C L A S has the same components as most other major detectors: magnets, w i r e chambers, Cerenkov counters, t i m i n g scintil­ lators, and calorimeters. B u t w h e n y o u are t r y i n g t o catch p a r t i ­ cles i n all directions, the geometry becomes quite different, start­ i n g w i t h the magnets. Since we w a n t all particles traveling i n any d i r e c t i o n t o go t h r o u g h the magnetic field, the field must be wrapped a r o u n d the beam pipe. T o accomplish t h i s , six huge coils are arranged a r o u n d the target, radially f r o m the beam pipe. The coils generate a t o r o i d a l o r doughnut-shaped field. Electrons that come d o w n the beam pipe and d o n ' t scatter w i l l pass t h r o u g h the hole i n the doughnut-shaped field and i n t o the beam d u m p . Those electrons that are scattered, as w e l l as protons, pions, and other particles t h a t are k n o c k e d o u t o f the target, w i l l travel t h r o u g h the magnetic field and have t h e i r trajectories bent. The h i g h - m o m e n t u m particles are only slightly deflected, the l o w - m o m e n t u m particles are dramatically deflected. I t is n o t enough t o scatter electrons and k n o c k o u t protons; we must also detect the particles and extract the i n f o r m a t i o n f r o m t h e i r d i s t o r t e d trajectories. T h e detector components i n C L A S are packaged i n t o six wedges, shaped like giant pieces o f an orange, and inserted i n between the magnets. W h e n completely assembled, C L A S becomes a sphere t h a t encompasses the target. Each wedge contains a n u m b e r o f layers o f detector elements t h a t can measure the trajectory, m o m e n t u m , and energy o f the particle. T h e first three layers t h a t the particle encounters are the w i r e chambers, or d r i f t chambers. T h e i r purpose is t o measure points along the particle's trajectory. By l o c a t i n g the trajectory i n chambers at r o u g h l y 1 , 2 , and 3 meters f r o m the target—be­ fore, i n the m i d d l e of, and after the magnetic field—we can deter­ m i n e where the particle o r i g i n a t e d , w h i c h way i t was g o i n g , and w h a t its m o m e n t u m was (the b e n d i n the field tells us this) (see figure

5.5).

A w i r e chamber is essentially the same as the device designed by Hans Geiger for Ernest R u t h e r f o r d i n the 1910s. A charged

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Figure 5.5 The cross section o f CLAS w i t h a reconstruction o f an event. (A) inner wire chamber, (B) middle wire chamber, (C) outer wire chamber, ( D ) Cerenkov counter (E) time-of-flight scintillators (ToF), (F) calorimeters, (G) track o f an electron, ( H ) track o f a proton, ( I ) track o f an unidentified particle. particle passing t h r o u g h a gas w i l l leave a t r a i l o f ions, atoms that have had electrons k n o c k e d o f f t h e m . T h e ions, w h i c h are positively charged, w i l l d r i f t t o w a r d a negatively charged sense w i r e . T h e charge o f the ions is t h e n deposited o n the sense w i r e w h i c h , after amplification, is seen as an electrical pulse i n o u r computer, o r as a " c l i c k " i n a classical Geiger counter. T h e first major i m p r o v e m e n t over Geiger's o r i g i n a l tube is t h a t we have

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thousands o f sense wires ( 3 4 , 0 0 0 i n C L A S ) . Therefore, by k n o w ­ i n g w h i c h w i r e collected the ions, we can get a r o u g h idea o f where the particle w e n t — t o w i t h i n a centimeter. T h e second i m ­ provement comes f r o m t i m i n g the pulse. B y k n o w i n g w h e n the particle goes by, and h o w fast the ions d r i f t , we can calculate h o w close the particle passes t o the sense w i r e — t o about a t e n t h o f a m i l l i m e t e r . W i t h this i n f o r m a t i o n , C L A S can measure m o m e n ­ t u m t o about h a l f o f a percent. T h e next layer o f C L A S is the Cerenkov counter. Cerenkov l i g h t is p r o d u c e d w h e n a particle t h a t is traveling at nearly the speed o f l i g h t i n air or vacuum enters a material t h a t is optically " t h i c k e r , " where the speed o f l i g h t is m u c h slower. T h e particle must slow d o w n t o c o m p l y w i t h this new speed l i m i t . T o slow V

d o w n , the particle must d u m p energy, w h i c h we see as Cerenkov l i g h t , named after the Russian physicist w h o first identified the process. I n an experiment at Jefferson L a b , the electrons w i l l all be traveling at nearly the speed o f l i g h t , whereas the pions and nucléons w i l l be m u c h slower because they are m u c h more mas­ sive. By selecting a material w i t h the r i g h t optical density, the electrons w i l l all e m i t Cerenkov l i g h t , whereas the other particles w i l l pass t h r o u g h , invisible. Since these experiments are i n i t i a t e d by an electron f r o m the beam, identifying the electron can often be key t o unraveling and deciphering the data read o u t o f C L A S . T h e next layer o u t i n this o n i o n l i k e detector is the time-offlight ( T o F ) scintillators. T h i s layer is about 3 meters f r o m the target and consists o f over 300 "bars." Each bar is made o f scin­ t i l l a t i n g plastic, 2 inches by 6 inches, f r o m 0.5 meter t o nearly 4 meters l o n g and w e i g h i n g u p t o several h u n d r e d pounds. T h e over 300 bars fit together t o f o r m an almost complete shell a r o u n d the inner detectors. As described i n chapter 2 , scintil­ l a t i n g plastic is clear and looks like Plexiglas, b u t scintillates— produces a metallic blue l i g h t — w h e n a particle passes t h r o u g h it. A t each end o f the scintillator bar is a p h o t o m u l t i p l i e r tube ( P M T ) . A P M T is a h i g h l y sensitive l i g h t detector t h a t can sense as l i t t l e as a single p h o t o n . T h i s c o m b i n a t i o n o f fast s c i n t i l l a t i n g plastic and a sensitive l i g h t detector means we have a precise mea-

CHAPTER S

86

sûrement o f the t i m e the particle passed t h r o u g h the time-offlight

scintillators—to w i t h i n 100 t o 2 0 0 picoseconds. T h a t is

r o u g h l y equal t o the a m o u n t o f t i m e i t takes l i g h t t o travel an i n c h or t w o ! T h e last layer is the calorimeter. T h e name "calorimeter" dates back t o devices that measured the heat or energy contents o f a material, t h e i r "calories." I n a detector like C L A S , i t is the energy o f the particle that is measured. T h e calorimeter for C L A S is b u i l t up o f many alternating layers o f lead and s c i n t i l l a t i n g plastic. T h e lead w i l l slow d o w n the particle and cause i t t o d u m p its energy. I t can d u m p its energy either by p r o d u c i n g photons, or by k n o c k i n g protons, neutrons, or pions o u t o f the lead. T h e charged particles w i l l be seen i n the layers o f s c i n t i l l a t i n g plastic, and all the particles can k n o c k o u t more particles f r o m the next layer o f lead. By this mechanism a single high-energy particle that enters the f r o n t o f the calorimeter can create an avalanche o f particles by the t i m e i t reaches the back o f the detector. Because o f t h i s , calorimeters are sometimes also called "shower counters." T h e calorimeter n o t only tells us the energy o f a p a r t i cle, b u t because o f the m u l t i p l e layers o f sensitive scintillator, i t can also tell us w h a t the rate o f energy loss is, and t h a t rate is a u n i q u e signature o f particle type—whether i t is a p r o t o n or 7C , a +

p h o t o n or n e u t r o n , an electron o r 7T. A l t h o u g h the particle has n o w vanished, absorbed i n t o the calo r i m e t e r or the concrete walls o f the experimental hall, the inform a t i o n has yet t o reach its final f o r m . C L A S has about 3 4 , 0 0 0 wires and 3,000 P M T s t o read o u t . Each one can be d i g i t i z e d i n t o pulse height and t i m i n g i n f o r m a t i o n . I f C L A S was t o l i g h t up w i t h a complex event for an instant, w i t h particles i n all directions, i t c o u l d potentially produce a h a l f a megabyte o f data! N o r mally, events are an order o f magnitude smaller, b u t they happen at a rate o f 1,500 times each second for eight m o n t h s a year, maki n g C L A S one o f the largest producers o f raw data i n the w o r l d . Yet o u t o f the dozens o f kilobytes o f data f r o m each event, all we w o u l d really like t o k n o w is w h a t k i n d o f particle w e n t i n w h i c h d i r e c t i o n and w i t h w h a t energy. T h a t is a factor o f 1,000

MEASURING A RAINBOW - 87 r e d u c t i o n i n the v o l u m e o f data—but i t s t i l l has the same infor­ m a t i o n . T o start w i t h , some clever circuits w i t h i n the experimen­ tal hall itself reduce the accidental data: there is n o need t o record and analyze the pulse height o f wires that d i d n ' t fire, or wires that fired by themselves due t o electronic noise or static. T h e r e m a i n i n g data are shipped o u t o f the experimental hall over a high-speed data l i n k t o a "computer f a r m , " w h i c h can d o some i n i t i a l analysis and repackaging and t h e n forwards the data t o a "data s i l o " u n t i l i t can be f u l l y analyzed later. T h e data silo is a tape drive and a robotics a r m that can shuffle tapes f r o m storage racks t o the tape drive and back again. Tapes may seem o l d fashioned i n this age o f C D and D V D data, b u t w h e n calculating dollars per gigabyte o f data, tape is very c o m p e t i t i v e — a n d w i t h C L A S p r o d u c i n g terabytes o f data each year, that is very i m ­ portant. After the experiment we can p e r f o r m o u r analysis at a more leisurely pace. T h e raw data is a l o n g list o f w i r e I D numbers and hit times, as w e l l as P M T I D numbers, times, and pulse ampli­ tudes. Some parts o f the analysis are straightforward. For exam­ ple, i f l i g h t is detected at b o t h ends o f a time-of- flight scintillator at the same m o m e n t , t h e n we k n o w that the particle passed t h r o u g h the center o f the scintillator bar. Also, since we k n o w the speed o f l i g h t i n plastic, we k n o w at w h a t t i m e i t passed t h r o u g h . O t h e r parts o f the analysis, like the conversion f r o m w h i c h w i r e was h i t t o m o m e n t u m , is more complex. I n this anal­ ysis we make o u r best guess as t o the i n i t i a l m o m e n t u m o f the particle. We t h e n simulate o u r guess and compare the hits i n o u r simulation w i t h the hits f r o m the real event i n the real detector. We t h e n repeat the simulation w i t h a better guess u n t i l the simu­ lated hits are nearly identical t o the real hits, i n w h i c h case the m o m e n t u m o f o u r last and best s i m u l a t i o n is essentially the m o ­ m e n t u m o f the real event. Clearly o u r analysis depends i n a c r i t i ­ cal way o n h o w w e l l we understand o u r detector, b u t that has just been part o f the historical t r e n d over the past century. Each layer by itself can tell us s o m e t h i n g about the particle that raced t h r o u g h i t , b u t by c o m b i n i n g i n f o r m a t i o n f r o m all the

88 - CHAPTER 5 layers we can d o better t h a n the sum o f the parts. F o r instance, by itself, the w i r e chamber can t e l l us where the particle traveled to w i t h i n one w i r e spacing (about a centimeter). H o w e v e r , i f we also have the t i m i n g i n f o r m a t i o n f r o m the t i m e - o f - f l i g h t scintil­ lators, we can use the d r i f t t i m e o f the ions and calculate where a particle traveled t o w i t h i n a t e n t h o f a m i l l i m e t e r — 1 0 0 times better. W i t h particle type and energy f r o m the calorimeter and m o m e n t u m f r o m the w i r e chamber, we can infer velocity. W i t h velocity and the t i m e f r o m the t i m e - o f - f l i g h t scintillators, we can trace back the particle t o the o r i g i n a l t i m e o f the scattering. W i t h this s t a r t i n g t i m e , we can d o t w o t h i n g s : we can reiterate all calculations w i t h this precise s t a r t i n g t i m e and i m p r o v e o u r precision, and we can correlate a particle detected i n one side o f C L A S w i t h a particle t h a t came f r o m the same beam m i c r o pulse b u t was detected o n the other side o f C L A S . I t was t h i s , the measurement o f correlated particles, w h i c h really drove the design and b u i l d i n g o f this detector. Finally, we have reduced t h a t d a u n t i n g h a l f a megabyte o f data t o a few simple numbers: w h a t k i n d o f particles were they (electrons? protons? pions?), i n w h i c h directions d i d they start, w h a t were their m o m e n t a and energy? U p t o this stage we have dealt w i t h a general purpose analysis. For each experiment a u n i q u e q u a n t i t y w h i c h is o f special interest w i l l have been identified. I n the case o f the decay o f the A particle described i n the previous chapter, i t was a spectrum o f counts versus the sum o f the p r o t o n and p i o n energy. I n other cases i t w i l l be a c o m b i n a t i o n o f angles and energies. K n o w i n g the best c o m b i n a t i o n for testing models o f nucléons and quarks, m u c h as w h e n Bjorken convinced the S L A C people i n the late 1960s t o p l o t t h e i r data as a f u n c t i o n o f the " B j 0 r k e n - x " (see chapter 2 ) , is a signature o f a healthy and dynamic interplay between experi­ mentalist and theorist. T h e road f r o m conceiving an experiment t o m o u n t i n g the ef­ f o r t , f r o m the h o t w i r e at the start o f the accelerator t o the data at the other end, is a l o n g one, b u t i t is one that is u n d e r s t o o d i n m i n u t e and meticulous detail. A n d i t is these meticulous de-

MEASURING A RAINBOW - 89 tails that give us confidence i n the results: p r o b i n g s o m e t h i n g one q u a d r i l l i o n t h the size o f a h u m a n is n o t r i v i a l task. T h e data, w h e n i t is p l o t t e d as a curve i n a j o u r n a l , or perhaps even as a single data p o i n t , have had a l o n g , complex, and involved history. A single p o i n t or set o f points can carry w i t h i t years o f labor. Yet i t can also carry w i t h i t the careful and j o i n t intelligence o f a team o f physicists and technical support. M o r e i m p o r t a n t i n the drive t o decipher the w o r l d o f quarks inside neutrons and protons, an experimental p o i n t c o u l d potentially d i s t i n g u i s h be­ tween t w o c o m p e t i n g models. H o w e v e r , as often as n o t , an ex­ p e r i m e n t w i l l n o t completely reject one t h e o r y or m o d e l and back another; rather, i t w i l l p o i n t t o a m i d d l e g r o u n d , perhaps a c o m b i n a t i o n o r h y b r i d o f the t w o models. O r , o n those rawer and exciting occasions, i t w i l l find s o m e t h i n g new beyond the scope o f either m o d e l .

Particle Taxonomy and Quark Soup

I

F Y O U ' V E E V E R played the o l d parlor game " t w e n t y ques­ t i o n s , " y o u ' l l remember t h a t the object is t o guess some secret t h i n g that one o f y o u r opponents has i n m i n d . Y o u can ask n o more t h a n t w e n t y yes-or-no questions t o n a r r o w the possibilities before y o u make y o u r guess. Traditionally, t h o u g h , the player w i t h the secret begins by answering w h a t is, supposedly, the most p r i m a r y question o f all: " A n i m a l , vegetable, or mineral?" Whenever I played t w e n t y questions as a c h i l d , I always f o u n d myself w o n d e r i n g about t h i n g s that seemed t o fall between the cracks—or t o bridge all three categories. Is the calcium i n bone " a n i m a l " or "mineral"? Where does a virus fit in—especially a computer virus? H o w about a plastic house plant made f r o m hy­ drocarbons that come, after all, f r o m the remains o f real a l t h o u g h ancient plants? Yet however m u c h I m i g h t t r y d e r a i l i n g the game my aunts and uncles were teaching me, they c o u l d always pigeon­ hole whatever I m i g h t come up w i t h . I never asked t h e m where quarks w o u l d fit i n t o this t r i f u r c a t e d w o r l d — b u t I ' m sure that, w i t h o u t hesitation, they w o u l d have called t h e m "minerals." O u r impulse t o classify a n y t h i n g instantly, even i f we really haven't g o t a clue, reflects o u r desire t o see the w o r l d as an or­ derly place. A n d perhaps cataloging t h i n g s is essential t o o u r way o f understanding: relational knowledge o r comprehension by as­ sociation. W h e n we encounter s o m e t h i n g new, we need t o label

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it, even i f the label is whimsical or misguided. I f we d o n ' t have a tag or pointer or index for the new object or idea, and we become distracted, o u r brains m i g h t n o t be able t o find o u r way back t o it, losing i t forever. As the eighteenth-century Swedish naturalist Carolus Linnaeus t o l d us: The first step o f science is to know one thing from another. This knowledge consists i n their specific distinctions; but i n order that it may be fixed and permanent distinct names must be recorded and remembered. W i t h o u t the r e c o r d i n g and r e m e m b e r i n g o f names, things get lost. I t is like t r y i n g t o find a single p o e m i n a vast library w i t h o u t a catalog. H o w d i d we ever find things w i t h o u t Dewey Decimal? I n fact, the n a m i n g o f objects is a t r u l y ancient activity. I n Genesis ( 2 : 1 9 - 2 0 ) A d a m is given the a u t h o r i t y t o name all animals. Aris­ totle t r i e d t o teach us h o w t o organize o u r understanding o f the w o r l d i n his b o o k Categories. I t was this passion for labeling and organizing the w o r l d that lay b e h i n d the intricate circles, rings, and terraces o f Dante's Divine Comedy. B u t Dante's gazetteer d i d more t h a n just name and place things as A d a m had done. H e also imposed an o r d e r i n g , a hierarchy, o n his w o r l d . Circles are subdivided i n t o rings. T h e lower the r i n g is i n the Inferno, the more heinous. The higher the terrace i n Purgatory or o r b i t i n Paradise, the more exalted. This mania for organizing the natural w o r l d f o u n d an all-time champion i n Carolus Linnaeus ( 1 7 0 7 - 7 8 ) . Linnaeus developed the b i n o m i a l system for the classification o f all l i v i n g things, a system that is essentially i n use today. " B i n o m i a l " implies a t w o part name for every type o f flora and fauna, a genus and a species, a general and a specific name, Homo sapiens, Felis catus, and Sterna paradisaes—the h u m a n , the house cat, and the arctic tern. Yet this is just the surface o f biological taxonomy. I n the f u l l b i n o m i a l system the w o r l d is d i v i d e d i n t o " k i n g ­ doms," such as vegetable and animal. These are subdivided i n t o "phyla," based o n the most significant physical traits. Phyla are further subdivided i n t o "class," "order," "family," "genus," and

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"species." F o r example, consider the classification o f o u r most familiar species—the h u m a n being: kingdom: phylum: (subphylum class: order: family: genus: species:

Animalia Chordata Vertebrata) Mammalia Primates Hominidae Homo H o m o sapiens

B u t i f we viewed the b i n o m i a l system as only pigeonholes for species, "a place for everything, and e v e r y t h i n g i n its place," t h a t w o u l d trivialize the role o f a g o o d cataloging system. I n this system, catalogers understood t h a t i m p o r t a n t traits come i n the higher levels o f the hierarchy. For instance, Linnaeus recognized that "chordata" is an i m p o r t a n t trait that d i s t i n ­ guishes a large g r o u p o f animals—those w i t h a n o t o c h o r d , or central nerve, r u n n i n g along the back. Hence, chordata is a phy­ l u m and very near the t o p o f the system. I n contrast, the differ­ ence between an arctic t e r n (Sterna paradisaes) and a c o m m o n t e r n (Sterna hirundo)—as Roger T o r y Peterson i n his field guide tells us—is a small band o f w h i t e near the bird's beak. T h i s small d i s t i n c t i o n , or "field m a r k , " is the only visible trait that stands between the t w o species. Therefore, the t w o terns are classified as part o f the same genus. A second feature o f a g o o d classification system is that i t gives us a glimpse o f the u n d e r l y i n g axioms o f the system. I n chapter 2 we saw that the periodic table o f the elements o f D m i t r i M e n ­ deleev had its structure—unbeknownst t o Mendeleev—because o f the way electrons can occupy atomic orbits. T h e first r o w o f the periodic table has t w o elements because the first o r b i t o f an a t o m can have t w o electrons. T h e elements o f the first c o l u m n , the alkali metals, have one electron i n t h e i r outermost orbit. T h e noble elements, i n the last c o l u m n , have their outermost orbits completely filled.

PARTICLE TAXONOMY AND QUARK SOUP

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I n a similar manner, Linnaeus's b i n o m i a l system has its struc­ ture dictated by that all-encompassing t h e o r y o f biology: evolu­ t i o n . T h e tree structure o f the f u l l taxonomic system is a shadow o f the tree o f e v o l u t i o n . By this I mean that the evolutionary p a t h o f t w o species o f different phyla diverged eons ago, whereas the separation between the arctic t e r n and the c o m m o n t e r n is an event o f yesterday. Generally speaking, similar traits arise f r o m similar genetics and speak o f a similar history. So now, w h e n we t u r n f r o m the vernal w o r l d o f b i o l o g y t o the quiet and deep d o m a i n o f the nucléons and quarks, we m i g h t be t e m p t e d t o lay o u t a " b i n o m i a l system"—or a "periodic table"— o f particles. B u t learning a list o f particular traits instead o f start­ ing w i t h the u n d e r l y i n g elements or axioms is a l o n g road. I f i t is done w i t h any detail, i t is a r e c o u n t i n g o f the h i s t o r y o f particle physics, complete w i t h false starts and b l i n d alleys. I t w o u l d be like learning every physical t r a i t o f a m y r i a d o f species before learning about D a r w i n ' s t h e o r y o f e v o l u t i o n . O r like learning all the properties o f all chemical elements—what every alkali and transition metal l o o k like—before discussing the electron and the p r o t o n . F o l l o w i n g this p a t h w o u l d take us along the same route as G e l l - M a n n , w h o m we discussed i n chapter 2 . A n d s t i l l , i n the end we w o u l d end up w i t h the patterns that G e l l - M a n n observed and named the " e i g h t - f o l d way." Instead, let us start w i t h the u n d e r l y i n g t h e o r y o f quarks and show h o w we can derive the particular traits o f the observed particles. S t a r t i n g w i t h h a l f a dozen different types o f quarks and a few rules o f combinatorics, we can cook up the hundreds o f observed particles. I n the end, we must be able t o reproduce the "eight-fold way," as w e l l as the vastly more complex and ex­ tended patterns that arise f r o m new particles discovered i n the three and a h a l f decades since G e l l - M a n n ' s paper. There are six ingredients t o quark soup: the " d o w n " and " u p " quarks, the "strange" and " c h a r m " quarks, and the " b o t t o m " and " t o p " quarks. They all have spin | , an electrical charge o f +

| or - | , and a new and curious q u a n t i t y called color charge.

Each o f the six types, or "flavors" as they are generally called, has

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a d i s t i n c t mass and a preferred decay pattern. A n d that is all the properties o f quarks—they are very simple objects. T h e labeling o f the types o f quarks as flavors may seem p a r t i c u ­ larly whimsical, especially w h e n compared t o the classical names used by previous generations o f physicists, such as " a t o m , " "elec­ t r o n , " " p r o t o n , " and " n e u t r o n . " Even the particles such as the T , the A, the p, and the A , have names that tie themselves t o the w o r l d o f D e m o c r i t u s . B u t the nomenclature o f quarks is quite different. Maybe i t was an artifact o f the 1960s and 1970s, o r perhaps i t was merely meant t o emphasize the fact t h a t there is n o t h i n g i n o u r c o m m o n and macroscopic experience t o guide us. H o w e v e r , the names are n o t completely a r b i t r a r y T h e y are designed t o help us remember w h i c h quarks t e n d t o be paired together. We call the p a i r i n g a "family." T h e up and d o w n quarks, or u and d quarks i n the physicist's shorthand, are the first and most familiar family, and account for the most c o m m o n particles, the p r o t o n , n e u t r o n , and p i o n . As we progress along the list, the quarks are m o r e massive. Therefore, they require m o r e energy t o be f o r m e d , they have shorter lifetimes, and they become m o r e and m o r e rare and elusive. T h e second family is made up o f the c h a r m and strange quarks. Finally, the t h i r d fam­ ily, the most massive and the most recently "discovered" quarks, are the t o p and b o t t o m quarks. T h e names " u p q u a r k " and " d o w n q u a r k " f o l l o w f r o m the older nuclear physics models i n w h i c h the p r o t o n and the neu­ t r o n are treated as i f they are the same particle (the nucléon) w i t h a new quality called isospin d i s t i n g u i s h i n g the t w o . T h u s , the p r o t o n is described as a nucléon w i t h "isospin-up" and the n e u t r o n is a nucléon w i t h " i s o s p i n - d o w n . " T h e advantage t o this m o d e l was t h a t nuclear physics c o u l d t h e n use all the mathe­ matics that atomic physics had developed for electrons w i t h spin. Therefore, w h e n the quark m o d e l was b e i n g developed i t was realized that the p r o t o n had t o have m o r e "up-ness" and the n e u t r o n more "down-ness," so these quark names naturally followed.

PARTICLE TAXONOMY AND QUARK SOUP

95

T h e case o f the strange quark also evolved o u t o f the preex­ i s t i n g conventions. T h e kaon (K) and the lambda ( A ) had been observed i n the 1950s and called strange because o f t h e i r unex­ plained appearance and semistability. I n fact, many o f the p a r t i ­ cles made up o f the three lightest quarks were all w e l l k n o w n i n 1964,

and G e l l - M a n n even used the notations

and s i n his

first paper o n quarks. However, the idea o f families was n o t at all developed, and the name "strange" was n o t designed as part o f the doublet. I n 1 9 6 4 , Bjorken and Glashow came u p w i t h the name " c h a r m , " t o complement strangeness. "Top"

and " b o t t o m " were originally proposed as " t r u t h " and

"beauty," b u t these names seem t o have fallen i n t o general disuse. Perhaps " t r u t h " and "beauty" were just t o o whimsical, or per­ haps they really d i d n o t f o r m a pair o f opposites: Beauty is truth, truth beauty—that is all ye know on earth, and all ye need to know. Keats: Ode on a Grecian Urn There was even a movement at one t i m e t o replace the names w i t h only the letters t and b, b u t " t o p " and " b o t t o m " seem t o be the stable m i d d l e g r o u n d . Yet names are merely labels and n o t the "specific d i s t i n c t i o n s " Linnaeus w o u l d have us seek. For these distinctions we t u r n t o the physical traits: the masses and the decay patterns. The masses o f the different flavors are d i s t i n c t , b u t extremely difficult t o measure. T h e usual m e t h o d for measuring the mass o f a particle involves isolating i t and measuring its m o m e n t u m , energy, or velocity. By k n o w i n g t w o o f these three quantities, mass can be derived. T h i s m e t h o d works w e l l for measuring the mass o f a p i o n or p r o t o n or kaon, b u t i t doesn't w o r k so w e l l for a quark, for we cannot isolate a single quark. T h i s is the unique p r o b l e m i n s t u d y i n g the h i d d e n w o r l d o f quarks inside protons and neutrons. We must depend o n theories. We

start o u t w i t h a quark t h e o r y and t h e n calculate experi­

mental observables, such as the cross sections or masses o f macro-

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scopic particles like the p r o t o n o r the p i o n . We t h e n

adjust

the masses o f the quarks and various other parameters i n the t h e o r y u n t i l o u r calculations agree w i t h the laboratory measure­ ments. T h i s sounds like a technique that guarantees success, as a c o o k b o o k m i g h t tell us, "add seasonings t o taste." A d d and ad­ just parameters u n t i l the t h e o r y describes nature. B u t this is where "Occam's razor" comes i n t o play: the simplest t h e o r y that describes the widest range o f observations is the preferred theory. Also, there is a sense that the parameters o f the t h e o r y must be associated w i t h s o m e t h i n g i n nature itself. A t h e o r y whose parameters are masses and charges w o u l d be preferred t o a t h e o r y o f nonphysical parameters

2 ) , a l t h o u g h , ad­

m i t t e d l y , the whole quark t h e o r y was i n i t i a l l y susceptible t o this criticism. S t i l l , all o f these criteria d o n o t u n i q u e l y identify a single quark theory. I n fact, there are t w o different theories i n general use, and the masses o f the quarks derived depend o n the t h e o r y b e i n g used. T h e t w o theories are Q C D and the constituent quark m o d e l . Q C D is almost certainly the best t h e o r y we have o f quark dynamics i n terms o f the ultramicroscopic structure o f the subnucleon w o r l d , b u t i t is a complex and difficult t h e o r y w i t h w h i c h t o w o r k . The quark masses used i n Q C D are called bare masses, or current masses. I n contrast, the constituent quark m o d e l is m u c h simpler, using equations similar t o Schrôdinger's e q u a t i o n for the atom. H o w e v e r , the constituent quark m o d e l has a m u c h lower status t h a n Q C D , as indicated by its name. Q C D is a the­ o r y w i t h a t r u l y fundamental f o u n d a t i o n , whereas the constit­ uent quark m o d e l is merely a model—useful for calculations, b u t n o t a fundamental t h e o r y o f matter. I t is n o t that we are so i g n o r a n t about quarks as t o entertain t w o opposing theories. Rather, they are complementary theories w i t h their o w n appropriate applications. I t is like c o m p a r i n g the a t o m seen by a chemist w i t h that seen by a physicist. T h e chemist m i g h t see the a t o m as a solid object w i t h the ability t o b i n d t o its n e i g h b o r i n g atoms t o f o r m molecules. A physicist m i g h t see the a t o m as electrons and a nucleus w i t h a l o t o f space

PARTICLE TAXONOMY AND QUARK SOUP - 97 i n between, and w o u l d also describe the atoms i n terms o f orbits or energy levels. A d m i t t e d l y , the orbits have a great deal t o d o w i t h w h a t chemical bonds are f o r m e d , b u t the t w o views o f the a t o m are b o t h useful and have t h e i r d i s t i n c t and preferred scales and scopes. T h e same is true o f Q C D and the constituent quark m o d e l . A l l qualifiers aside, the constituent quark m o d e l is s t i l l very useful. I n this m o d e l the effects o f the gluons (we w i l l talk about gluons i n detail later) that swarm a r o u n d each Q C D bare quark are r o l l e d up w i t h that bare quark t o f o r m a larger quark—a "constituent q u a r k , " w i t h a "constituent-quark mass." We can estimate the constituent masses i n a simple way. First, the p r o t o n is b u i l t up o f t w o up quarks and one d o w n quark (p =

uud),

whereas the n e u t r o n is made o f one up quark and t w o d o w n quarks (n = udd). So the difference i n masses o f the t w o nucléons must be related t o the differences o f the masses o f the up and d o w n quarks. I n fact, since the n e u t r o n and the p r o t o n have al­ most the same mass, the constituent masses o f the up and d o w n quarks must be almost identical. We can then also conclude that each quark must have r o u g h l y a t h i r d o f the mass o f the nucléons. T h e masses f r o m Q C D are m u c h lighter, since i n Q C D a great deal o f energy is a t t r i b u t e d t o the gluons, t o the b i n d i n g between quarks, and especially the b i n d i n g o f quarks t o themselves. There are more quarks t h a n just the up and d o w n quarks, and fortunately nature has p r o v i d e d us w i t h a great many more p a r t i ­ cles and masses t h a n just the n e u t r o n and the p r o t o n t o guide us. W i t h dozens o f particles observed, we can estimate the mass o f the quarks as: constituent-mass ( G e V / c )

bare-mass ( Q C D ) ( G e V / c )

up

~ 0.3

0.001-0.005

down

~ 0.3

0.003-0.009

charm

-0.7

1.15-1.35

strange

~ 0.5

0.075-0.170

—

169-179

- 1.2

4.0-4.4

2

top bottom

2

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The fact that we can start w i t h only six quarks and b u i l d essen­ tially all the observed particles is a c o m p e l l i n g argument i n sup­ p o r t o f a quark t h e o r y — b u t i t is o n l y part o f the macroscopic evidence, the evidence we see f r o m the outside particles. I n a d d i t i o n t o mass, each quark has a unique charge and a distinct decay pattern. T h e peculiar fractional charges o f quarks were one o f the o r i g i ­ nal unpalatable features o f the quark m o d e l . T h e u p , charm, and t o p quarks have a charge o f +|# ( - 1 * is the charge o f an electron) and the d o w n , strange, and b o t t o m quarks have a charge of-^e. I n the early 1960s, this contradicted c o m m o n sense. Ever since M i l l i k a n ' s water and o i l - d r o p experiments we had viewed the w o r l d as being made o f integer charges, + 1 ^ , 0, and -le, and n o t h i n g else. B u t we had never, and s t i l l have never, directly seen a fractional charge. We n o w understand this i n terms o f quarks always c o m i n g i n groups o f three or i n quark-antiquark pairs. Given that they must be b o u n d i n t o these combinations, per­ haps i t is not so strange that these fractional charges remain con­ cealed i n the stable, observed particles, b u t what is less clear is w h y fractional charges should remain elusive after particles fall apart and decay. T o understand the inner mechanics o f a decay let us t u r n t o the quintessential weak decay, the beta decay o f a n e u t r o n i n t o a p r o t o n , electron, and n e u t r i n o . O n the quark level the decay looks like the diagram shown i n figure 6 . 1 . T h e decay o f the macroscopic particle, the n e u t r o n , is a result o f the decay o f one quark, the d o w n quark. T h e other t w o quarks are referred t o as "spectators." ( I n the m u c h older t e r m i n o l o g y for describing the o r d i n a r y chemistry o f substances dissolved i n water, ions that d o n o t participate i n a reaction are called "spectator" ions.) T h i s simple n e u t r o n beta decay is perhaps the most c o m m o n o f de­ cays, yet b u r i e d i n i t is a wealth o f quark physics, i f o n l y we can unravel i t . First, charge is conserved. T h a t is one o f the great canons o f physics: charge is always conserved. Also, w h e n a quark decays, i t always decays i n t o one quark and a short-lived particle called the W-boson, w h i c h t h e n immediately decays i n t o some-

PARTICLE TAXONOMY AND QUARK SOUP - 99

Figure 6.1 Beta decay—the quintessential weak decay. t h i n g else ( i n this case, an electron and n e u t r i n o c o m b i n a t i o n ) . T h i s means that w h e n a quark decays i n t o another flavor, the magnitude o f its charge can change by only the charge o f a Wboson, that is, by an a m o u n t equal t o o r opposite that o f the charge o f an electron (there are b o t h positive and negative ver­ sions o f the W-boson). As a result o f this chain o f reasoning, we find that i t is n o t the charge o f the quarks that must be propor­ t i o n a l t o the electron charge; rather, i t is the difference between the quark charges that is ± le. O n e o f the curious properties o f a beta decay is that i t is m o d i ­ fied by its e n v i r o n m e n t . T h e rate o f decay o f the d o w n quark i n t o an up quark is slowed because the quarks are embedded inside a nucléon. We w o u l d like t o l o o k at an even simpler case, the decay o f a free quark. B u t o f course there is n o such t h i n g . N a t u r e , however, does provide us w i t h a t e l l i n g phenomenon: the beta decay o f a m u o n (figure 6.2). T h e m u o n is just like an electron, except i t is 2 0 0 times heavier. Its beta decay is very clean, and the theoretical predictions and experimental results are i n ex­ ceedingly g o o d agreement. Experimentally and theoretically, the lifetime o f the m u o n is 2 x 10" second, or 2 milliseconds. T h a t may seem fast, b u t i t is forever compared t o the strong decays, w h i c h l i m i t the lifetime o f a A particle t o 5 x 1 0 second—the 6

2 1

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Figure 6.2 M u o n decay—the simplest o f all weak decays. t i m e i t takes l i g h t t o cross t w o protons. T o t r y and understand these scales, i f we expand the lifetime o f the A particle t o a single second, the m u o n w o u l d prosper for 13 m i l l i o n years! We r e t u r n n o w t o the n e u t r o n , w i t h w h i c h we started o u r dis­ cussion o f decays. I t has a lifetime o f 14 minutes. W h e n we scale the A's lifetime t o one second, g i v i n g the m u o n a ripe o l d age o f 13 m i l l i o n years, the n e u t r o n becomes m o r e ancient t h a n the stars, w i t h a lifetime o f 5 q u a d r i l l i o n years—about h a l f a m i l l i o n times the age o f the universe! T h i s longevity arises p r i m a r i l y f r o m the fact that the masses o f the d o w n and up quarks are so similar, w h i c h means that there is l i t t l e energy available t o drive the decay. A second factor t h a t determines lifetime is t h a t quarks d o n o t decay i n t o all other flavors w i t h equal probability, even after we have adjusted decay rates for different masses and allowed ener­ gies. Instead, a quark w i l l t e n d t o decay i n t o the flavor o f its sibling, the flavor o f the other quark i n the same family. Quarks can also decay i n t o flavors o f the adjacent family, and o n rare occasions there may even be decays between the first and t h i r d families. Schematically we can lay o u t the allowed decays as i n figure 6.3. The decay between family members is the preferred d i r e c t i o n ; between adjacent families is possible; and between dis­ tant families is a rarity.

PARTICLE TAXONOMY AND QUARK SOUP

101

Figure 6.3 The allowed weak decays. The thick arrows indicate the most likely decays: (u-d), (c-s)y and (t-b). The t h i n arrows indicate decays that take place occasionally: (u-s)y (d-c)y (c-b), and (s-t). The dotted arrow indicates the rarest of decays (u-b) and (d-t). One last subtle detail is that w h e n the d o w n quark i n the neu­ t r o n decays i n t o an up quark i n a p r o t o n , the quark must be i n an allowed o r b i t i n

both situations, w h e n

i t is i n the n e u t r o n and

w h e n i t is i n the p r o t o n . B u t where those orbits are and h o w the quarks arrange themselves w i t h i n nucléons we w i l l leave for the f o l l o w i n g chapter. Since we n o w k n o w the quark charge, mass, allowed decays, and decay rates, let us t r y t o b u i l d up a particle and see h o w w e l l i t compares t o nature. For example, i f we combine the three lightest quarks, the u p , d o w n , and strange quarks, t h e i r masses add up t o 1.1 G e V / c . T h e n o r m a l decay m o d e w i l l be for the 2

strange quark t o decay i n t o an up quark and a W-boson. T h e boson w i l l t h e n decay i n t o a anti-up quark ( u) and a d o w n quark (d). Finally, the four quarks and one antiquark that come o u t o f the decay can pair themselves up i n t w o different ways, g i v i n g rise t o t w o d i s t i n c t sets o f daughter particles, a (71° n) or (n~ p) pair. T h e Feynman diagram for these t w o alternative decays is shown i n figure 6.4. I n fact i n nature we find a particle w i t h a mass o f 1.115 G e V / c , w h i c h decays i n t o (n~ p) or (n° n). I t is called the lambda ( A ) . 2

But t o understand the decay o f the A i n detail we must i n t r o d u c e the last p r o p e r t y that quarks possess: " c o l o r . " I f electrical charge

(+e/-e) is the

charge that binds electrons and the nucleus t o f o r m

atoms, t h e n the three color charges ( r / f ,

b/b,g/g) are

what bind

quarks together t o f o r m a nucléon, o r any hadron. As we saw i n

102 - CHAPTER 6

Figure 6.4 Lambda decay. The A can decay into a (n° n) or a ( T T p) pair. The second combination is more likely, since the quarks i n the 7 T can have any color. chapter 3, the rule is that a b a r y o n (three-quark c o m b i n a t i o n ) w i l l be made up o f the three colors ( r % ) , whereas mesons (quarkantiquark pairs) w i l l be made o f color-anticolor pairs ( r f o r bb o r £ $ ) . The effect o n the decay diagrammed above is curious. L e t us i n i t i a l l y r a n d o m l y assign colors t o the o r i g i n a l three quarks. Say that the strange quark is red, the up quark is blue, the d o w n quark is green. I n the decay i n t o the 71° and

all the colors o f

the final quark are n o w determined. Since a quark keeps its color w h e n i t decays, the up quark i n the n° is red, its partner must be anti-red, and the new d o w n quark is also red. B u t i n the second diagram, the colors o f the M and d i n the 7T are completely arbi­ trary. We should replace the second diagram w i t h three different colored diagrams, for the %~ c o u l d be made up o f r f , bb, o r gg. N o w the p r o b a b i l i t y o f a lambda decaying i n t o a p %~ is n o t just 3 times larger t h a n the p r o b a b i l i t y o f decaying i n t o a

mt . Q

Rather, i t is enhanced by only the square r o o t o f 3, for the same reason that probabilities are the square o f wavefunctions i n q u a n t u m mechanics. Finally, f r o m o u r color c o u n t i n g we have calculated that the " b r a n c h i n g r a t i o " o f lambda is given by:

PARTICLE TAXONOMY AND QUARK SOUP A

p TT

A -> n

_

_ 65% ^

71°

103

34%

Lambdas decay i n t o a jfr TT 65 percent o f the t i m e and a w 7C° 36 percent o f the t i m e , i n excellent agreement w i t h experimental results. The p o i n t o f this digression i n t o the decay o f the lambda is that we can take any c o m b i n a t i o n o f quarks and t h e n predict the particle's mass, spin, charge, lifetime, and b r a n c h i n g ratios o f the combinations. We can also apply this sort o f quark a d d i t i o n to the mesons w i t h great success. A quark and an antiquark can combine i n t o a meson. W i t h six flavors o f quarks, and six antiquarks, we w o u l d expect thirty-six basic mesons, b u t the c o m b i nations i n v o l v i n g the t o p quark are so short-lived that they have n o t been observed or named. T h e other twenty-five are listed below:

d (-1/3)

î(+l/3)

«(-2/3)

J(+l/3)

£(-2/3)

n°

TT

K°

D-

B>

n°

IC

D>

&

•

u (+2/3)

£(+1/3)

s (-1/3)

IC

IC

c (+2/3)

rr

jy

//¥

B

b (-1/3)

B°

B-

B

Y

f(-2/3)

B°

s

s

+ c

t(+2/3)

A n anomaly that immediately catches the eye is that b o t h the (uu)

and the (dd) combine t o make up a 7t°. I n reality, i t is more

the other way a r o u n d , that is, the n° is made up o f a c o m b i n a t i o n o f (uu) and (dd) pairs. T h i s is because ups and downs have nearly identical constituent masses and therefore they can "decay" i n t o each other—a (uu) pair can become a (dd) pair and likewise a (dd)

pair can become a (uu) pair. We can imagine that inside a

7C°, a series o f decays is always t a k i n g place, as shown i n figure 6.5. Also, even i f the uu were n o t "entangled" w i t h the dd, there is n o experimental way o f d i s t i n g u i s h i n g the t w o . There is n o u n i q u e observable, n o field marks t o delineate the t w o .

104

CHAPTER 6

Figure 6.5 The life o f a 7t°. The 7t° can oscillate between being a combination o f a uU or a dd pair o f quarks.

So far we have discussed such field marks as charge, mass, and weak decays, b u t there is one more observable p e r m u t a t i o n o f quarks that we encountered i n earlier chapters: spin. W h e n the spin o f a quark i n a nucléon flips, the particle gains mass and we call i t a A particle. T h e possibility o f spin flips are universal i n the quark w o r l d , and the process can be applied t o any o f the quarkantiquark pairs listed above. I n the mesons we have listed, the spins o f the quark and antiquark are antiparallel, that is, they p o i n t i n the opposite direction. This means that w h e n we combine the spins o f the quarks t o get the overall spin o f the meson, the spins cancel. So the spin o f the meson is zero, and therefore the particle has n o preferred direction. These spin-zero mesons are collectively referred t o as pseudoscalar mesons. S t i l l , there is the possibility o f aligning the spins o f the t w o quarks t o add up t o spin 1. This creates a more massive meson, called a vector meson. Each pseudoscalar meson we have listed has its heavier alter-ego vector meson: 71(140)

p ( 7 7 0 )

JC(500)

it(890)

D ( 1 8 6 0 )

D*(2000)

5(5280)

#(5325)

T h e name "pseudoscalar" describes the observable consequence o f h a v i n g spin 0. W i t h n o spin, these mesons have n o preferred d i r e c t i o n . I f the meson scatters o f f s o m e t h i n g , i t scatters equally

PARTICLE TAXONOMY AND QUARK SOUP

105

i n all directions. I f i t decays, its daughter particles fly o f f i n all directions uniformly. Vector mesons have an o r i e n t a t i o n , a pre­ ferred d i r e c t i o n given by their spin o r i e n t a t i o n . So w h e n a vector meson scatters or decays, the angular d i s t r i b u t i o n o f the scat­ tered particles, o r daughter particles, is a unique signature, a field mark, o f its "vectorness." I t seems that every t i m e we t h i n k we have a handle o n the number o f ways t o add up quarks, and thus the n u m b e r o f c o m b i ­ nations, or macroscopic particles, I yell o u t , " B u t w a i t , there is m o r e . " First i t was m o r e flavors—each w i t h its special mass, elec­ t r i c charge, and decay pattern. T h e n we added i n color charge, w h i c h changed o u r c o u n t i n g rules. N e x t we added i n spin, w h i c h d o u b l e d the n u m b e r o f particles. B u t s t i l l , we have n o t described the most basic and familiar particles, the n e u t r o n and the p r o t o n . So let us finally t u r n t o the last p e r m u t a t i o n o f the quark a d d i n g rules, the a d d i t i o n o f three quarks t o make a " b a r y o n . " A t first glance we m i g h t expect that there are 6 x 6 x 6 = 2 1 6 different baryons, since each quark can have one o f six flavors. B u t the mathematics is n o t only about combinations, i t is also about per­ mutations. For example, a p r o t o n is n o t just a (uud)

y

(udu)

and (duu).

b u t also

T h e n the original list o f 2 1 6 combinations re­

duces t o fifty-six real combinations o f quarks. W i t h what we learned f r o m mesons, and paying careful a t t e n t i o n t o the Pauli exclusion p r i n c i p l e , we can essentially derive all the stable bary­ ons that physicists have observed. S t i l l , t o w o r k t h r o u g h fifty-six combinations o f quarks is a great deal more drudgery t h a n necessary, and we can learn every­ t h i n g we need t o k n o w about three-quark combinations by con­ sidering o n l y the three lightest and most c o m m o n quarks: the up, the d o w n , and the strange quarks. First, there are t e n u n i q u e combinations o f these three quarks (as listed b e l o w ) . Also, as we p o i n t e d o u t w h e n t r y i n g t o understand the A particle, there are t w o u n i q u e spin combinations: spin i , w h e n one quark is antiparallel w i t h respect t o the t w o other quarks, or spin | , where the spins o f all the quarks are aligned. We can t h e n tabulate the bary­ ons made up o f the three lightest quarks and spin - or - :

106 - CHAPTER 6 combinations

spin 1 / 2

ddd ddu uud uuu dds uds uus dss uss sss

spin 3 / 2 AA" A A E-* E»*

n

+

P

++

Ir Z° A E H" H

I * E~* H*

+

+

+

+

a

There are three things about this table that grab o u r a t t e n t i o n . First, the (uuu), (ddd), and (sss) spin | combinations are missing. Second, there are t w o particles for the spin | (uds) c o m b i n a t i o n . T h i r d , these are the baryons that showed up the G e l l - M a n n ' s "eight-fold way" paper, w h i c h I m e n t i o n e d i n chapter 2. T h e (uuu), (ddd), and (sss) spin | combinations are missing because i f these particles existed they w o u l d be fermions, b u t w o u l d n o t satisfy all the criteria required for fermions. Fermions are particles that have half-integer spins ( i , | , | , . . .) and quan­ t u m mechanics requires that fermions must have "antisymmetric total wavefunctions." I n a sense, this is the generalization o f the Pauli exclusion principle. B u t w h y are these combinations " a n t i ­ symmetric"? T h e reason goes back t o color. W h e n we i n t r o d u c e d an antisymmetric "color w a v e f u n c t i o n " o n t o p o f flavor and spin, the c o m b i n a t i o n ( uuu) ( w h i c h w o u l d be a p —a d o u b l y charged p r o t o n ) w o u l d be antisymmetric w i t h respect t o b o t h spin and color, w h i c h means symmetric w i t h respect t o the p r o d u c t o f flavor, spin, and color, and so therefore n o t allowed. A l t h o u g h i t was the observation o f the A and the absence o f the p that i n t r o d u c e d the q u a n t i t y color, its presence is far-reaching, f r o m such a phenomenon as the decay o f the A t o the fact that baryons are always made up o f three quarks. B u t what color is and h o w i t combines w i l l have t o w a i t for the f o l l o w i n g chapter. ++

+ +

++

PARTICLE TAXONOMY AND QUARK SOUP

107

T h e final p h e n o m e n o n we must explain is the fact that the three-quark c o m b i n a t i o n (uds) can f o r m either a A or a £° p a r t i cle. T o understand t h i s , let us b u i l d i t u p , piece by piece. We can build it out

ofuidîsloru'îdisîoruîdîsl.

The up

and d o w n quarks are so similar that t o understand the symmetry o f the w h o l e particle, we can start o u t by just l o o k i n g at the symmetry o f these t w o . I n the first t w o combinations we can combine i n an antisymmetric way t o get the A particle :

A=[(ul

dî)--(uî

dl)]sî

This is " a n t i s y m m e t r i c " because i f we exchange the u p quark and the d o w n quark we change the total sign o f the c o m b i n a t i o n ([u