Neutrinos, Dark Matter and Co.: From the Discovery of Cosmic Radiation to the Latest Results in Astroparticle Physics 365832547X, 9783658325473

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Claus Grupen

Neutrinos, Dark Matter and Co. From the Discovery of Cosmic Radiation to the Latest Results in Astroparticle Physics

essentials

Springer essentials

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Claus Grupen

Neutrinos, Dark Matter and Co. From the Discovery of Cosmic Radiation to the Latest Results in Astroparticle Physics

Claus Grupen Department Physik Universität Siegen Siegen, Germany

ISSN 2197-6708 essentials ISSN 2731-3107 Springer essentials ISBN 978-3-658-32547-3 (eBook) https://doi.org/10.1007/978-3-658-32547-3

ISSN 2197-6716 (electronic) ISSN 2731-3115 (electronic)

© Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2021 The translation was done with the help of artificial intelligence (machine translation by the service DeepL.com). A subsequent human revision was done primarily in terms of content. This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Responsible Editor: Margit Maly This Springer imprint is published by the registered company Springer Fachmedien Wiesbaden GmbH part of Springer Nature. The registered company address is: Abraham-Lincoln-Str. 46, 65189 Wiesbaden, Germany

What You Can Find in This essential

The birth of astroparticle physics is the historic balloon flight of Victor Hess in 1912, when he discovered cosmic radiation with an ionization chamber. This cosmic radiation was studied in many facets on the ground, under the earth, and in the atmosphere. It was soon discovered that cosmic rays were one way of studying elementary particle processes. In order to understand the whole variety of phenomena in cosmic rays, one had to include many subareas of physics: Thermodynamics, nuclear physics, plasma physics, stellar physics, astronomy, and elementary particle processes, to name a few. Astroparticle physics is therefore multidisciplinary in every respect. Today, astroparticle physics is an active, interdisciplinary field of research that includes and combines astronomy, cosmic rays, and elementary particle physics. In this essential, you will find a short historical outline of astroparticle physics and a description of the latest results without going into mathematical detail. This essential should be seen as an introduction to this new field of research. But you will get an overview of what is happening in the sky, between the stars and between the galaxies. By now, many things are quite well understood, but with every solution found, new questions arise. This range of questions with some answers can be found in this essential. A very detailed description of astroparticle physics, including a mathematical description of the relationships, especially in cosmology, can be found in the book “Entry into astroparticle physics” (“Einstieg in die Astroteilchenphysik”) by C. Grupen, published by Springer in 2018.

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What You Can Find in This essential

Muon showers in the ALEPH experiment 125 m below ground. (Picture credits: ALEPH experiment, https://home.cern/about/experiments/aleph, accessed on September 12, 2018. ALEPH experiment goes cosmic, https://cerncourier.com/1999/09/26/ and https://cernco urier.com/aleph-experiments-go-cosmic/, accessed on September 12, 2018. Avati, V. et al. (2003) Astropart. Phys. 19, pp. 513–523, Cosmic multi-muon events observed in the underground CERN-LEP tunnel with the ALEPH experiment; ALEPH experiment goes cosmic, https://www.hep.physik.uni-siegen.de/~grupen/, accessed on September 1, 2018)

Preface

Whatever the final laws of nature may be, there is no reason to suppose that they are designed to make physicists happy (Steven Weinberg).

Cosmic rays have been falling on the Earth since the formation of the planets. The observation of auroras by Gassendi in 1621 and Halley in 1716 as aurora borealis (“northern dawn”) led Mairan to suspect in 1733 that this phenomenon was of extraterrestrial origin. The auroras are produced by electrons coming from the Sun and entering the polar regions along magnetic field lines on helical orbits. Kant correctly assumed in 1775 that the “nebulae” observed in the sky were to be interpreted as an accumulation of individual stars into galaxies. At the beginning of the twentieth century, measurements of the recently discovered radioactivity led to the assumption that not all phenomena of ionizing radiation were of terrestrial origin. The proof of radiation from space was provided by Victor Hess in his historic balloon flight to altitudes of over 5000 m. The “cosmic radiation” discovered in this way turned out to be a treasure trove of elementary particles to be newly discovered: antiparticles (positrons), muons, pions, and kaons were found in cosmic radiation experiments. With the advent of accelerators and storage rings, the particle zoo was greatly expanded until the quark model created an overdue order. The study of high-energy and rare processes led to a renaissance of cosmic rays—now renamed astroparticle physics. The measurement of solar neutrinos and neutrino oscillations, the discovery of gravitational waves and the simultaneous measurement of cosmic catastrophes caused by electromagnetic radiation, particle radiation and gravitational waves (“multi-messenger astronomy”) provided new insights into the universe. Nevertheless, there are still many unanswered questions: Where is dark matter hiding? Is dark energy a property of space? Do

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we live in a higher-dimensional space? And finally: Is our universe embedded in a multiverse? Siegen October 2018

Acknowledgment

I would like to thank Dr. Tilo Stroh for a careful review of the manuscript and problems that especially for his prompt and effective support in solving may arise, which he was always able to solve professionally and quickly.

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Contents

1 Historical Introduction to Astroparticle Physics . . . . . . . . . . . . . . . . . . . 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 The Last Century . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Contributions of Elementary Particle Physics . . . . . . . . . . . . . . . . . . 1.4 Renaissance of Astroparticle Physics and Open Questions . . . . . . .

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2 Charged Component of Primary Cosmic Radiation . . . . . . . . . . . . . . . .

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3 X-ray Astronomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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4 Gamma Astronomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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5 Neutrino Astronomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Solar Neutrinos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Supernova Neutrinos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 High-Energy Neutrinos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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6 Gravitational Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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7 Cosmology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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8 Astrobiology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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9 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Historical Introduction to Astroparticle Physics

1.1

Introduction

Do you know the laws of heaven or can you set up God’s dominion over the earth? (Book of Job 38:33).

The meaning of an essential is—as the word implies—to summarize the essence of astroparticle physics. Of course, it cannot replace the full scope of a book on this subject. On the contrary, it is intended to whet the appetite to read a more comprehensive presentation on this subject. Since the field of astroparticle physics is subject to rapid change, especially in recent times, I think it is useful to present a selection of material that concerns the particularly current results and open questions. So I will gladly do without technical details and mathematical derivations. Although the methods of relativistic mechanics and elementary particle physics are very important for astroparticle physics, the global connections can be presented without much mathematics. Also the complicated facts of cosmology and cosmogony do not seem to me really appropriate for a detailed description in the context of an essential. Nevertheless, the important results of cosmologists and open questions can be presented clearly. The selection of focal points is of course a subjective matter. Especially with high energies, a lot has changed in recent years. This is also due to the fact that particle physics, with its complex detectors and techniques, has had a strong influence on the design of astroparticle physics experiments. Many particle physicists have also developed a taste for the physical goals of astroparticle physics and have changed their field of research. Of course, the experiments in this new field have not only become more complex but also more expensive. Satellites in orbit, experiments on the International Space Station (ISS), at the South Pole in the Antarctic ice, or gigantic Michelson interferometers for measuring gravitational © Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2021 C. Grupen, Neutrinos, Dark Matter and Co., Springer essentials, https://doi.org/10.1007/978-3-658-32547-3_1

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waves can only be realized in large collaborations with the appropriate financial resources. In addition to the important aspects of the historically grown activities in cosmic radiation, experiments in neutrino physics, the measurement of gravitational waves, the search for antimatter and the search for evidence of dark matter will therefore be in the foreground. After the daily new discoveries of extraterrestrial planets, which have already exceeded the 4000 specimens, the question is also interesting whether astrophysical questions are pursued by earthlings alone. After the discovery and successful revival of 250 million year old bacteria in salt crystals or nematodes, which have not eaten a meal since the last ice age in permafrost regions, it is likely that life will develop wherever living conditions are appropriate. Who knows, maybe astroparticle physics is also interesting for extraterrestrials.

1.2

The Last Century

A cosmic paradox is the beginning of all things, a paradox without any key to its significance (Sri Aurobindo).

Astroparticle physics was born with the discovery of cosmic radiation by Victor Hess with his spectacular balloon flight in 1912. After the discovery of radioactivity by Henri Becquerel and the new penetrating rays observed by Wilhelm Conrad Röntgen, it was initially assumed that the phenomena that led to electroscopes in the laboratory discharging virtually on their own must have been of terrestrial origin. Victor Hess found residual terrestrial radiation on the ground and at low altitudes, but above 1000 m a radiation appeared that was completely new and surprising. He thus discovered “cosmic radiation,” which is now part of the field of astroparticle physics. His achievement is all the more significant because he suffered from altitude sickness, which made it much more difficult for him to take measurements at altitudes of over 5000 m (Fig. 1.1). However, there were already indications from earlier times that new phenomena were taking place in the sky, because phenomena such as the colorful aurora borealis could certainly not be of terrestrial origin. Now it was known that there was this radiation coming from space, but it was completely unclear what this radiation consisted of. For a long time, penetrating gamma radiation was suspected to be the trigger, but the sources of this radiation were completely unclear, and as far as the very high energies are concerned, the details of the origin of the high-energy cosmic radiation are still a mystery today.

1.2 The Last Century

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Fig.1.1 Victor Hess during a balloon ascent to measure the cosmic radiation. (Picture credits: D. Kuhn, Universität Innsbruck: private communication)

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The extraterrestrial component discovered by Hess was confirmed 2 years later by measurements made by Kohlhörster (1914) even up to higher altitudes. With the discovery of the cloud chamber by Wilson, it was even possible to make penetrating charged particles of cosmic radiation visible at sea level. Due to the progressive detector development, astroparticle physics has numerous experimental possibilities. Measurements with satellites, rockets, airplanes, at mountain altitudes, on the ground, and even deep underground are possible. The multitude of experiments in the hunt for cosmic particles is outlined in Fig. 1.2. In addition to the elementary particles (protons and electrons) known at this time, the positron (1932), the muon (1937), the kaon, the charged pion (1947),

Fig. 1.2 Experimental possibilities in cosmic radiation. (Picture credits: Grupen 2000)

1.2 The Last Century

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and the lambda baryon (1951) were discovered in the first years of cosmic radiation. The particle family was significantly enlarged by discoveries at the emerging accelerators, so that it gradually became clear that the term “elementary” was no longer really appropriate, which eventually led to the quark model. In addition to the many experimental activities in astroparticle physics, Albert Einstein has made major theoretical contributions to our understanding of the processes in the cosmos at the same time. The Special Theory of Relativity (1905) and the General Theory of Relativity (1915/1916) describe the processes at high speeds near the speed of vacuum light and at large masses very precisely. These classical theories have so far survived all experimental tests brilliantly: the correct description of the deflection of light at the Sun (1919), the relativistic time dilation for fast cosmic muons (which otherwise could not reach the sea surface at all), the centuries-old misunderstanding of Mercury’s perihelion rotation, the energy loss in a rotating system of pulsars, the correct description of the effect of gravitational lenses, the discovery of gravitational waves (2015), and the measurement of the gravitational redshift of light from stars near black holes. The validity of these theories is almost unbelievable. Nevertheless, general relativity defies quantization, making it clear that we are still lacking an essential building block for understanding all the forces in the universe. Since space travel to nearby planets is occasionally discussed, another aspect of astroparticle physics is of particular importance. Galactic cosmic radiation represents a special radiation exposure for astronauts. In addition, the Sun is also a source of a solar wind that is highly variable in intensity. These solar particles are particularly dangerous for astronauts when the Sun, in its active phase, emits jets of particles after solar eruptions. Since the cosmic space weather is very difficult to predict and shielding measures are not really practicable, the precise measurement of solar activities is also an important task of astroparticle physics. The solar particle stream naturally also carries a magnetic field, which on the one hand shields the galactic cosmic rays somewhat and on the other hand can become dangerous for terrestrial communication systems and transmission lines due to the high variable magnetic fields and the resulting voltage peaks. In Earth-based detectors, a drastic decrease in galactic particles is observed in the case of a solar eruption, because the flow of charged solar particles with their magnetic field prevents galactic particles from reaching the Earth’s surface (Forbush decrease—on February 18, 2011, see Fig. 1.3). Independently of astroparticle physics, but ultimately not without reference to cosmic questions, there has long been an unsolvable problem in nuclear physics. According to the ideas of the early twentieth century, nuclear beta decay was supposed to occur via a two-body decay into a proton and an electron, but this

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Fig. 1.3 Variation of the intensity of galactic cosmic rays (Forbush decrease) during a solar particle eruption, registered at different measuring stations. Such a solar particle stream can become a serious radiation hazard for astronauts. (Picture credits: Oh, S. Y., Yi, Y. (2012) Solar Phys. 280, pp. 197–204, A Simultaneous Forbush Decrease Associated with an Earthward Coronal Mass Ejection Observed by STEREO)

meant that the two decay products should have fixed, discrete energies. Experiments showed that the electron energies were continuously distributed up to the kinematic limit. But everything was missing: the energy balance was not correct, the conservation of momentum seemed to be violated, as well as the conservation of angular momentum. The despair was so great that Niels Bohr even thought of doubting the correctness of conservation of energy. Wolfgang Pauli made a desperate attempt to save these conservation laws by demanding that, in addition to the proton and electron, another invisible particle is emitted during decay to take over the missing energy, the missing momentum as well as the angular momentum. This particle, later called neutrino, should save the conservation laws mentioned above, but should have at most a small mass. He formulated his ideas in a letter to his fellow physicists who had gathered at a conference in Tübingen in 1930: Dear radioactive ladies and gentlemen, as the bearer of these lines, whom I respectfully ask you to listen to, will tell you in more detail, I have, in the face of … the continuous beta spectrum, fallen on a desperate way out to … save the energy conservation law. Namely, the possibility that electrically neutral particles, which I will call neutrinos, could exist in nuclei that have

1.3 Contributions of Elementary Particle Physics

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spin 1/2 and follow the exclusion principle, and which also differ from light quanta in that they do not travel at the speed of light. The mass of the neutrinos could be of the same order of magnitude as the electron mass and in any case not greater than 0.01 proton masses. The continuous beta spectrum would then be understandable under the assumption that during beta decay, one neutrino is emitted with the electron at a time, such that the sum of the energies of the neutrino and the electron is constant.1

Pauli believed that nobody would ever be able to prove such an enigmatic particle. However, astronomer Walter Baade had great respect for experimental physicists and believed that the neutrino would be discovered at some point, and he offered Pauli a wager. When in 1956 the neutrino was experimentally proven by Cowan and Reines at a nuclear reactor, Pauli fulfilled his bet and sent a case of champagne. Reines also confirmed the gift with the champagne, but at the same time complained that the theorists had drunk the champagne all by themselves and that Cowan and he himself had not got a drop of it. The neutrino has played a prominent role in elementary particle physics and astroparticle physics up to the present time.

1.3

Contributions of Elementary Particle Physics

Energy is in fact the material from which all elementary particles, all atoms, and therefore all things in general are made, and at the same time energy is also that which is moved (Werner Heisenberg).

Now that so many elementary particles had been discovered in cosmic radiation, at accelerators and storage rings, it was time to bring a little order and systematics into this rapidly growing particle zoo. The world around us consists practically only of the “up” and “down” quarks. In interaction experiments, this first generation of quarks was supplemented by “strange” and “charm” quarks, both of which were first found in cosmic radiation. The third generation of quarks (“bottom” and “top”) was discovered at storage rings. So there are six quarks and six antiquarks (see Fig. 1.4). Since there are baryons with three identical quarks, each with parallel spin, which must differ in some quantum number according to the Pauli principle, the color quantum number had to be introduced. This new color quantum number was also confirmed in experiments on hadron generation in electron–positron interactions. 1 Since in 1930, the neutron had not yet been directly detected, Pauli called the required particle

then still neutron and not yet neutrino.

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Fig. 1.4 Periodic table of elementary particles. (Picture credits: Cartoon Grupen 2013)

The interaction and binding of the quarks is accomplished by gluons. Gluons each carry a color and an anti-color. Because of the different combinations of three degrees of color freedom, the gluons carrying colors form a color octet. Parallel to hadronic quarks, there are six fundamental leptons: charged electrons, muons, and tauons, each with their associated neutrinos. For each lepton, there is also a corresponding type of antiparticle. So there are three different types of neutrinos. The three quark generations are opposed by the three lepton generations:       u c t , , s b d      · νe νμ ντ , , e− μ− τ−

(1.1)

1.4 Renaissance of Astroparticle Physics and Open Questions

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In 1989, the electron–positron storage ring LEP (Large Electron–Positron Collider) demonstrated that there are exactly three generations of neutrino with light neutrinos. Possible heavy neutrinos are discussed in some theories as candidates for dark matter, but have not yet been found. In the Standard Model of particle physics, all fundamental fermions (particles with half-integer spin) are massless. According to current theories, they obtain their observed masses through the mechanism of spontaneous symmetry breaking. However, this Higgs mechanism requires the existence of a neutral boson (particle with integer spin), which was actually detected at CERN in 2012. The Standard Model has thus been rounded off to a certain extent. At the same time, however, it is clear that the Standard Model cannot be the ultimate wisdom, because the model still contains far too many free parameters that have to be adapted to the experimental observations.

1.4

Renaissance of Astroparticle Physics and Open Questions

My goal is simple. It is the complete understanding of the universe: why it is as it is and why it exists at all (Stephen Hawking).

However, the interim dominance of accelerators and storage rings has been replaced by a renaissance in astroparticle physics through a series of spectacular discoveries: • The discovery of cosmological blackbody radiation by Penzias and Wilson in 1965, which led to the confirmation of the classical Big-Bang model and refuted the idea of a stationary universe. • The observation of the supernova explosion SN 1987A in the Large Magellanic Cloud in the visible spectral range and by neutrinos in large-volume underground detectors. • The solution of the solar neutrino problem with the far-reaching discovery of neutrino oscillations. • The discovery of the accelerated expansion of the universe for several billion years by studying the brightness of distant Type Ia supernovae. • The discovery of the unexpected surplus of primary positrons at energies between 10 and 1000 GeV. • The measurement of the first extragalactic neutrinos with the ICECUBE experiment.

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• The observation of intense gamma sources in the high-energy range in the Milky Way and at extragalactic distances. • The observation of air showers caused by high-energy cosmic particles through the radio emission they produce. • The measurement of the interactions of photons of blackbody radiation with high-energy protons, which lose their energy in this way and lead to the termination of the primary spectrum. • The discovery of extragalactic gamma-ray bursts (GRBs), the exact explanation of which is still being sought. • The observation of particle jets from active galactic nuclei (AGNs), in which high-energy particles are accelerated, where the details of the acceleration need to be better understood. • The multi-messenger observation of cosmic catastrophes such as the collision of neutron stars (kilonova). However, there are still many unanswered questions concerning dark matter, dark energy, and the unification of quantum mechanics and general relativity. In cosmology, inflation may solve some of the problems of the classical Big-Bang model, but it looks difficult with experimental tests. Since the expansion of the universe has been increasing again for a few billion years, the question is whether the cosmological constant is perhaps dynamic. Summary The birth of astroparticle physics was the discovery of cosmic rays during the historic balloon flight of Victor Hess in 1912. In the early days of cosmic rays—the name astroparticle physics was not yet familiar—many discoveries of elementary particles were made. Positrons, muons, and pions were the first new elementary particles. With the advent of accelerators, however, the field of elementary particles shifted to the earthbound accelerators. It was not until the 1970s that cosmic accelerators became interesting again: the measurement of solar neutrinos, the discovery of the supernova 1987A with the measurement of neutrinos from this source and the discovery of neutrino oscillations led to a renaissance of cosmic rays. Today, astroparticle physics is an active, interdisciplinary field of research that includes and combines astronomy, cosmic rays, and elementary particle physics. The future of astroparticle physics will lie in the simultaneous, detailed observation of cosmic events with different techniques in different spectral ranges (multi-messenger experiments).

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Charged Component of Primary Cosmic Radiation

The universe is a symphony of strings, and the Mind of God that Einstein eloquently wrote about for thirty years would be cosmic music resonating through eleven-dimensional hyperspace. Michio Kaku

The measurement of primary cosmic rays, i.e., the radiation that reaches the Earth from the Milky Way and possibly extragalactic distances, can only be carried out with detectors outside the Earth’s atmosphere, i.e., at altitudes of more than 40 km. The spectrum of the main components of primary cosmic rays from direct measurements is shown in Fig. 2.1. The various data therefore come from balloon measurements, satellites, and an experiment on the ISS. As expected, the frequency of the various atomic nuclei decreases with increasing nuclear charge number. Direct measurements require a good identification of the different elements. An essential detector of such an arrangement for particle identification is a tracking chamber system in the strongest possible magnetic field for momentum determination. Because of the equilibrium between centrifugal and Lorentz force (v⊥ B assumed), the charged nuclei experience a deflection of the particle trajectory, which is mv2 = z·e·v· B ρ

(2.1)

which gives the momentum for singly charged particles: p =e·ρ· B © Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2021 C. Grupen, Neutrinos, Dark Matter and Co., Springer essentials, https://doi.org/10.1007/978-3-658-32547-3_2

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Charged Component of Primary Cosmic Radiation

Fig. 2.1 Spectrum of the main components of primary cosmic radiation from direct measurements. The different data are from balloon measurements, from satellites and from an experiment on the International Space Station (ISS). The intensity shown on the ordinate is per unit area (m2 ), per solid angle (in steradian), per second and per energy (in GeV). (Picture credits: Boyle, P., Müller, D. (2018), Major components of the primary cosmic radiation; based on individual publications from Beatty, J. J., Matthews J., Wakely, S. P. in the section ‘Cosmics Rays’; The Review of Particle Physics, Berkeley; https://pdg.lbl.gov/; accessed on September 12, 2018. Tanabashi, M. et al. (Particle Data Group) (2018) Phys. Rev. D 98, p. 030,001, Review of Particle Physics, Fig. 29.1; https://pdg.lbl.gov/, accessed on September 1, 2018)

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(p—momentum, B—magnetic field, v—particle velocity, m—particle mass, ρ— radius of curvature). In cosmic radiation, the momentum related to the charge z, called magnetic rigidity R, is often used: R=

pc . ze

(2.2)

The identification of particles in such a detector means that the mass and charge of a particle must be determined. Any interaction process of particles or radiation can in principle be used to identify particles. Because of p = mv, the momentum of a particle of deflection radius ρ in the magnetic field can be represented as follows: ρ∝

p γ m 0 βc = ; z z

where z is the charge of the particle, m 0 its mass at rest, β =   1 γ the Lorentz factor γ = √ 2 .

(2.3) v c

its velocity, and

1−β

In addition to momentum measurement, the experimental setup also uses timeof-flight measurements with scintillation counters that determine the velocity β. A measurement of the specific ionization of the charged particles provides information on the nuclear charge z and also on β. The energy of the nuclei can be determined with an electromagnetic and a hadronic calorimeter according to E kin = (γ − 1)m 0 c2 .

(2.4)

For redundancy reasons, Cherenkov counters and transition radiation detectors are also frequently used, which are useful for reliable particle identification. Due to the limited size of detectors on balloons and in satellites, only primary particles up to some 1000 GeV can be detected in this direct way for intensity reasons. For higher energies, indirect measurement techniques must be used via the generation of large air showers by high-energy particles in the atmosphere. However, optical observations using the air Cherenkov technique require moonless, clear nights. Such air showers can also be detected around the clock by their radio emission. With these techniques, the energy resolution is less accurate compared to direct measurements, and the identification of nuclei is quite problematic. The periodic table of the incoming particles at energies above 1018 eV consists— for metrological reasons—practically only of protons and iron nuclei; i.e., a finer nuclear structure is very difficult to resolve with current measurement techniques.

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The shape of the primary spectrum is modulated by solar activity at energies up to the GeV range. The active Sun prevents low-energy galactic primary particles from reaching the Earth because the magnetic field accompanying the solar wind partially shields galactic particles. The relatively weak galactic magnetic field cannot store all types of particles equally well. Light particles such as hydrogen and helium nuclei begin to leave the Milky Way at energies of a few PeV. This causes the energy spectrum to become steeper at these energies (“knee of primary cosmic rays”). When the relatively frequent iron nuclei (z = 26) leave the galaxy because of the limited magnetic storage capacity of the Milky Way, the spectrum becomes even steeper (“iron knee”). At even higher energies, primary protons, for example, lose energy through interactions with the cosmic background radiation due to pion production (Greisen–Zatsepin–Kuzmin cut-off): γ + p → p + π 0, γ + p → n + π +.

(2.5)

Therefore, the primary spectrum becomes even steeper in this way. It even seems to break off significantly by this process (“ankle” of cosmic rays, see Fig. 2.2). Many experiments have tried to find the sources of cosmic radiation. With charged particles, this is almost impossible because the irregular galactic and extragalactic magnetic fields randomize the original direction of the charged particles. Only at the highest energies, is it hoped that the charged particles will also retain their original direction to some extent. The Auger experiment actually finds a certain accumulation along the supergalactic plane for primary particles with energies above 57 EeV, but this is not statistically very significant. The interaction of the primary cosmic particles leads to the fact that the universe is no longer transparent for high-energy protons and ultimately also for heavier nuclei due to the Greisen–Zatsepin–Kuzmin cut-off. A typical attenuation length for particle energies above 6 · 1019 eV is about 100 Mpc; i.e., a very large part of the universe cannot be explored with charged primary particles. However, for other cosmic messengers such as neutrinos or even gravitational waves, the universe is transparent.

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Fig. 2.2 Representation of the energy spectrum of primary cosmic radiation with an intensity scaled by E 3 . The data come from the Auger experiment and clearly show the bending of the spectrum above 6 · 1019 eV. (SD stands for the data from the Cherenkov surface detectors and “hybrid” includes both the surface detectors and the fluorescence telescopes). (Photo credits: Highlights from the Pierre Auger Observatory, Karl-Heinz Kampert for the Pierre Auger Collaboration, Proceedings of Highlight talk presented at ICRC 2011, Beijing; arXiv:1207.4823 [astro-ph.HE]. https://www.auger.org/, accessed on September 1, 2018. With kind permission of K.-H. Kampert)

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X-ray Astronomy

The universe is popping all over the place. Riccardo Giacconi

After the great successes of astronomy in the optical field, it is not surprising that attempts would be made to pursue astronomy in other spectral regions as well. In addition to radio astronomy and infrared astronomy, X-ray and gamma astronomy were also suitable. The only problem is that although it is possible to operate astronomy from the surface of the Earth from the radar to the optical. However, the Earth’s atmosphere is too thick for extraterrestrial X-rays to have a chance of reaching the Earth’s surface. In the keV range, where most X-ray sources have the highest luminosity, the range of X-rays in air is only about 10 cm. In order to observe X-rays from celestial objects, it is therefore necessary to operate detectors at the edge of the atmosphere or in space. Therefore, only balloon experiments, rocket flights, or satellite missions come into consideration. The first direct detection of X-rays from the Sun was in 1962, during a flight with a V2 rocket captured after the Second World War. In the same year, the X-ray source Scorpius X-1 was discovered more by chance when an American rocket was looking for X-rays from the Moon. This was extremely surprising, because it was already known that our Sun emits a small fraction of its energy in the X-ray range. However, X-rays from other celestial objects were not expected. After all, the nearest stars were several 100,000 times further away than our Sun. Such sources would have had to have enormous luminosity in the X-ray range compared to the Sun in order to be detected with the detectors of the 1960s using simple Geiger counters. This raised the question of which mechanisms are responsible for the generation of X-rays. © Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2021 C. Grupen, Neutrinos, Dark Matter and Co., Springer essentials, https://doi.org/10.1007/978-3-658-32547-3_3

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In astronomy, X-rays are the spectral range of energies from 0.1 to several hundred keV. Possible generation mechanisms are blackbody radiation from hot stars, bremsstrahlung, synchrotron radiation from electrons in magnetic fields and the inverse Compton effect in interactions of relativistic electrons with strong photon fields. In 1971, the UHURU (“Freedom” in Swahili) satellite carried out a first Xray survey of the sky. In the process, 339 X-ray sources were discovered. A surprising result was the determination of the extremely strong magnetic field of Hercules X-1 of 500 million T as a result of measuring the hard X-rays from this source with a balloon experiment. The German–British–American joint project with the X-ray satellite ROSAT was particularly successful. ROSAT had a much higher geometric acceptance, angular and energy resolution, and an enormously increased signal-to-noise ratio compared to previous X-ray satellites. During the ROSAT sky survey, 150,000 X-ray sources were found. Virtually, all stars emit X-rays. ROSAT also found diffuse X-ray radiation from large gas clouds, which allowed an estimate of the density of matter in the universe. With Chandra and the X-ray satellite XMM (X-ray Multi-Mirror Mission; renamed XMM-Newton resp. Newton Observatory in 2000), which was also launched in 1999, it is possible to achieve some improvements in resolution compared to ROSAT and to gain further insights into the components of matter that is not optically luminous. With improved detectors on board, the Chandra and XMMNewton satellites have discovered a large number of new sources and achieved spectacular results in the X-ray range. With new X-ray detectors (consisting of high-resolution CCD cameras, multichannel photomultipliers and special highly integrated silicon detectors), a number of astrophysically interesting objects have been put under the (X-ray) magnifying glass in the truest sense of the word. Figure 3.1 shows an image of the Type Ia supernova named after Tycho Brahe in the constellation Cassiopeia. Type Ia supernovae are formed when white dwarfs exceed a critical mass limit and the star collapses. In a thermonuclear explosion of the star and by the energy released in the process, the white dwarf is completely disrupted. X-ray astronomy has provided important insights into phenomena at the end of the lifetime of stars. These include supernova explosions, neutron stars and pulsars, and stellar black holes. At extragalactic distances, AGNs and mass-accumulating black holes dominate the emission of X-ray radiation. Spectacular—though not surprising—was the detection of X-rays from the Moon. However, the Moon does not emit this X-ray radiation itself. This is reflected corona radiation from the Sun, because just as the Moon does not glow in the optical sense, it only partially reflects solar X-rays (Fig. 3.2).

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Fig. 3.1 X-ray image of Type Ia supernova SN 1572 (Tycho), taken with the Chandra telescope. (Picture credits: Chandra X-ray Observatory, https://chandra.si.edu/, accessed on September 1, 2018. Tycho’s Supernova Remnant: A New View of Tycho’s Supernova Remnant, https://chandra.harvard.edu/photo/2009/tycho/, accessed on September 1, 2018)

It is also interesting to note that the development of novel X-ray and gamma detectors has led to important advances in materials research and mainly in medical imaging techniques (positron emission tomography [PET], single photon emission computed tomography [SPECT], computed tomography, etc.).

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Fig. 3.2 ROSAT X-ray image of the Moon. (Picture credits: Max Planck Institute for Extraterrestrial Physics, The ROSAT Satellite, X-rays from the Moon, https://www.mpe.mpg.de/ xray/wave/rosat/publications/highlights/moon.php, accessed on September 1, 2018)

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Gamma Astronomy

Let there be light. Bible, Genesis

Cosmic gamma radiation opens a window into the highest-energy processes in the universe. Basically, the production mechanisms for cosmic gamma radiation are the same as for X-rays. However, processes such as the π 0 decay and disintegration of antimatter are added. In supernova explosions, among other things, radioactive elements are also synthesized, so that gamma lines are also expected in the decay of these nuclei. The detection methods used in the range up to energies of some GeV are methods already known from the measurement of X-rays. In the high-energy range at energies above 100 GeV, imaging air Cherenkov telescopes are used. However, the gamma sky for energies above the PeV range remains limited to galactic distances, because from 1015 eV on, absorptive processes start via gamma–gamma interactions, e.g., with cosmic blackbody radiation. Figure 4.1 shows a sky survey in the light of gamma rays with energies above 100 MeV recorded by the Energetic Gamma Ray Experiment Telescope (EGRET) detector on board the Compton Gamma Ray Observatory (CGRO). Sources of gamma radiation are supernovae, neutron stars and pulsars, black holes, and AGNs. The gamma emission of AGNs is thought to be produced by matter falling into the central supermassive black hole. The plasma jets formed in this process are emitted perpendicular to the accretion disk, although the exact production mechanism of high-energy radiation still needs to be better understood in detail.

© Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2021 C. Grupen, Neutrinos, Dark Matter and Co., Springer essentials, https://doi.org/10.1007/978-3-658-32547-3_4

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Fig. 4.1 Complete survey of the sky (All Sky Survey) in the light of gamma radiation with energies above 100 MeV (data from the EGRET detector on board CGRO). Various sources such as Cygnus X3, Vela, Geminga, and the Crab Nebula as well as the Galactic Center can be identified. In addition, some extragalactic sources outside the galactic plane can be seen. (Picture credits: The Energetic Gamma Ray Experiment Telescope (EGRET); NASA: EGRET Data; CGRO EGRET Team; Steve Drake for the HEASARC, https://heasarc.gsfc. nasa.gov/docs/cgro/egret/, accessed on September 1, 2018. Hartman, R. C. et al. (1999) ApJS 123, pp. 79–202, The Third EGRET Catalog of High-Energy Gamma-Ray Sources. https:// heasarc.gsfc.nasa.gov/docs/cgro/images/egret/EGRET_All_Sky.jpg, accessed on September 1, 2018)

A particularly interesting event was the collision of two neutron stars in 2017, a “kilonova,” which led to the emission of gravitational waves and electromagnetic radiation ranging from radio waves to visible light and gamma radiation. Such multi-messenger observations are extremely valuable for understanding the origin of the high-energy cosmic processes. A surprising discovery of unique short bursts of gamma radiation was made by the American Vela reconnaissance satellites in the early 1970s. These satellites

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were actually intended to monitor compliance with an agreement to stop nuclear weapon tests in the atmosphere. However, the registered gamma radiation did not come from the Earth or atmosphere, but from sources outside the Earth, and therefore had nothing to do with nuclear weapon explosions, which are also a source of gamma radiation. The spatial distribution of these gamma-ray bursters showed that they were definitely extragalactic. They could be supernovae or hypernovae. But neutron stars, pulsars, magnetars, hypothetical quark stars, or black holes could also be the cause. Gamma-ray bursters also offer an opportunity to test the Lorentz invariance: electromagnetic radiation should always propagate at the speed of light in a vacuum, regardless of the energy of the photons. Because of the large distances of GRBs, the arrival times of photons of different energies could be used to test whether the speed of light depends on the photon energy. The gamma-burst monitor of the FERMI satellite has measured the arrival times of burst GRB 090510 in the energy range from a few hundred keV to 30 GeV and found no deviations from the Lorentz invariance, thus already ruling out certain models for theories of quantum gravity.

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Neutrino Astronomy

Neutrino physics is largely an art of learning a great deal by observing nothing. Haim Harari

The starting signal for neutrino physics almost would have been the explosion of an atomic bomb. To detect the weakly interacting neutrinos at tiny cross sections, a large neutrino flux was needed. The strongest neutrino fluxes are obtained immediately after a nuclear explosion. Nuclear fission bombs produce an intensive flow of electron antineutrinos via neutron decay n → p + e− + ν¯ e . The antineutrinos from this decay are exactly the particles that Pauli postulated for nuclear beta decay in 1930. But an explosion of a nuclear bomb near a sensitive neutrino detector would probably produce a strong background of interfering signals. In discussions with Fermi and Bethe, among others, Cowan and Reines suggested using the inverse nuclear beta decay for neutrino detection ν¯ e + p → n + e+ . The subsequent disintegration of the positron e+ + e− → γ + γ could significantly suppress a possible background via a coincidence of the two high-energy gamma quanta. The final-state neutron could also be captured by a nucleus, which is excited by it and subsequently emits gamma quanta. In this way, a strong nuclear reactor would also be suitable as a neutrino source for neutrino detection instead of a nuclear fission bomb. Such a detection was then also successfully carried out in 1955/1956 at the Savannah River Reactor in an experiment with the sensible name “Poltergeist” by Cowan and Reines.

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Solar Neutrinos

The first neutrino experiment of astrophysical interest then immediately led to the solar neutrino problem. Ray Davis wanted to detect with a radiochemical experiment the solar neutrinos from the proton–proton fusion p + p → d + e+ + νe , and especially the neutrinos from the following fusion reactions to helium. The solar neutrinos are— in contrast to those from nuclear beta decay—not electron antineutrinos. They can be trapped in a huge tank with 380,000 l of perchloroethylene by the reaction νe + 37 Cl →

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Ar + e−

(5.1)

and can be measured via the electron-capture reaction of 37 Ar. The solar astrophysicists agreed on the expected enormous neutrino flux on the order of magnitude of 1011 per square centimeter and second. When the Davis experiment found only a fraction of the predicted rate, they initially believed that the radiochemical experiment was flawed, measuring only a handful of neutrinos per month. Moreover, the experiment was only sensitive to the relatively rare high-energy neutrinos from boron decay (7 Be + p → 8 B + γ with subsequent 8 B → 8 Be + e+ + νe ). An extreme assumption was the presupposition that the solar fire in the Sun might have been extinguished. This would only be noticed after many 10,000 years via the emission of photons, but in neutrino light, it would be noticed practically immediately (more precisely: after 8 min). However, the result of the Davis experiment was confirmed by other radiochemical experiments that were also sensitive to the low-energy neutrinos from the proton–proton chain. The solution came from an experiment on atmospheric neutrinos measured in the large water Cherenkov detector of Kamiokande and its successor SuperKamiokande. The number of measured muon neutrinos was significantly lower than expected. Furthermore, the muon-neutrino rate depended on the zenith angle. The flow of muon neutrinos that had passed through the whole Earth was also greatly reduced (see Fig. 5.1). After theoretical speculations about particle oscillations in the neutrino sector, which had already been carried out by Pontecorvo in the 1950s, it was concluded that the three neutrinos νe , νμ , and ντ , which were generated by weak interactions, were eigenstates of this weak interaction, but they turned out to be mixed states of three different mass eigenstates. Such mixtures were already known from the quark sector. Although the solar neutrinos are born as electron neutrinos in the proton–proton fusion and other processes during the production of helium, on

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Fig.5.1 The muon neutrinos measured in Super-Kamiokande. In cyan are the expected fluxes under the assumption that there are no neutrino oscillations. The expected assumption with neutrino oscillations is shown in red. The data points (in black) best match the idea of neutrino oscillations. (Picture credits: Super-Kamiokande homepage, https://www-sk.icrr.u-tokyo.ac. jp/sk/index-e.html, accessed on September 1, 2018. Hyper-Kamiokande homepage, Neutrinos and Neutrino Oscillation, https://www.hyper-k.org/en/neutrino.html, accessed on September 1, 2018; actual picture from https://pdg.lbl.gov/2020/reviews/rpp2020-rev-neutrino-mix ing.pdf resp. https://pdg.lbl.gov/2020/figures/numix/figures/zenith_SK1-4.eps, accessed on August 10, 2020; with kind permission of Masayuki Nakahata, ICRR Tokyo)

their way from the Sun to Earth some of them change into other neutrino types (νμ and ντ ) to which the radiochemical experiments were not sensitive. The solution finally came from a Canadian experiment with a target of heavy water. Electron neutrinos interact with the deuterium of heavy water via a charged current according to νe + d → p + p + e− , but all three types of neutrinos can decompose the deuterium into a proton and a neutron: νx + d → p + n + νx (x = e, μ, τ ). The neutron can then be detected in the detector. When the two reactions are taken together, the neutrino rate was in line with the predictions of solar astrophysicists. Neutrino oscillations are only possible if neutrinos have a mass. But the oscillation experiments only provide a difference of mass squares of the different neutrino flavors.

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The measurement of neutrino masses is currently a great challenge, because the masses are expected to be below the electron-volt range. The huge KATRIN experiment at the Karlsruhe Institute of Technology (KIT) hopes to find an answer to this question within the next 5 years. This expensive neutrino scale must be able to suppress a strong background of cosmic radiation and ambient radioactivity. In the Standard Model of particle physics, neutrinos are massless. If one adds the lepton oscillations, the observation of the cosmic neutrino oscillations forces an extension of the Standard Model!

5.2

Supernova Neutrinos

The brightest supernova since the Kepler supernova of 1604 was discovered on February 23, 1987 by Ian Shelton at the Las Campanas Observatory in the Tarantula Nebula in the Large Magellanic Cloud at a distance of 170,000 light years in Chile. The same region of the sky in which the supernova exploded was routinely photographed 20 h earlier by Robert McNaught in Australia. However, the image that already contained the supernova was evaluated too late by McNaught. Ian Shelton had already noticed the brightness of the supernova with the naked eye during an observation break outside the observatory. A bright blue supergiant, Sanduleak, could be identified as the precursor of the supernova on the basis of older images. Sanduleak was originally an inconspicuous star with 10 times the mass of the Sun. During the hydrogen burning, it increased its luminosity significantly. After the hydrogen burning, the star expanded into a red giant until the temperature and pressure in its center allowed the He-burning. In a relatively short time, the helium was also consumed, and during a subsequent gravitational contraction, the carbon in the core ignited at high temperatures. Through further fusion phases with the burning of oxygen, neon, silicon, and sulfur, the star finally reached iron, the element with the highest binding energy per nucleon. After the formation of iron, the star could no longer gain energy from further fusion processes. Therefore, the stability of Sanduleak could not be maintained any longer: It collapsed under its own gravity. This gravitational collapse led to a gigantic explosion in which 1058 neutrinos were emitted democratically, i.e., in all three neutrino flavors, within a period of about 10 s. Of this multitude of neutrinos, 20 were found in the large water Cherenkov detectors of Kamiokande and the IMB experiment (Irvine–Michigan– Brookhaven). The Kamiokande collaboration had a bit of luck, because shortly before the supernova explosion, there was a maintenance-related data-taking

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break. If the neutrinos from the supernova had arrived in this metrological interruption, Kamiokande would have missed the supernova explosion. The Baksan experiment in the Caucasus was also able to measure some neutrino events. For metrological reasons, only neutrinos with the electron flavor could be detected. In total, SN 1987A released an energy of E total = (6 ± 2) · 1046 J

(5.2)

in the form of neutrinos. For comparison: the world energy consumption is about 600 Exa-Joule (6·1020 J). From the distribution of the arrival times of the neutrino events, a limit for the electron-neutrino mass of m νe ≤ 10 eV

(5.3)

could be derived. A comparable supernova at a distance of 5–10 light years would probably wipe out all life on Earth. However, such a supernova can only be expected on statistical average every 500 million years. A possible close candidate for a supernova explosion is the red giant Betelgeuse in the constellation Orion. When this star will collapse spectacularly is difficult to estimate. Betelgeuse, however, is at a relatively safe distance of about 600 light years.

5.3

High-Energy Neutrinos

Neutrinos fulfill many requirements for an effective astronomy: 1. Neutrinos are not affected by homogeneous or irregular magnetic fields. 2. Neutrinos do not decay on their way from the source to Earth. 3. Neutrinos and antineutrinos are distinguishable; so in principle, one can find out whether the particle originates from a matter or antimatter source. 4. Neutrinos are sufficiently penetrating. That is why you can use them to look inside the sources. 5. Neutrinos are not absorbed by interstellar or intergalactic dust or by infrared or blackbody photons. One disadvantage is certainly the difficult detection of neutrinos because of their very low probability of interaction. This makes it necessary to use large-volume detectors with good energy, time, and angular resolution. However, detectors such

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as ice or water Cherenkov detectors meet these requirements. At the moment, the main focus is on ongoing experiments such as ICECUBE at the South Pole and ANTARES in the Mediterranean. The detectors of the next generation such as Hyper-Kamiokande, DUNE (Deep Underground Neutrino Experiment) in the USA or the extensions of ICECUBE (ICECUBE-Gen2) with a volume increase of a factor of about 10, or the large planned detector KM3Net in the Mediterranean would allow to extend the energy range far beyond the PeV range if statistics are good. The cosmic neutrino spectrum extends over many decades of energy and intensity, see Fig. 5.2. The solar, atmospheric, and supernova neutrinos have already been presented. The measurement of the neutrinos of energies in the milli-electron-volt range produced in the Big Bang is a great challenge, but at the moment seems to be metrologically hopeless. The geoneutrinos not shown in the figure are actually not of interest for astroparticle physics. However, these geoneutrinos from the beta decays of radioisotopes in the Earth’s crust (e.g., 238 U, 232 Th, and 40 K), which are measured as a side effect in astroparticle physics experiments, can lead geologists to important conclusions about the structure of the Earth. In a similar way, the high-energy neutrinos measured in ICECUBE can, due to the energydependent neutrino–nucleon cross section, provide information about the internal structure of the Earth by measuring the direction-dependent rate of high-energy neutrinos.

Neutrino warning system. (Picture credits: Cartoon Grupen 2018)

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Fig. 5.2 Comparison of the fluxes of cosmic neutrinos in different energy ranges. (Picture credits: Grupen (2000); s. a. Katz, U. F., Spiering, Ch. (2012) Prog. Part. Nucl. Phys. 67, pp. 651–704, High-energy neutrino astrophysics: status and perspectives; https://inspirehep. net/record/944151/plots, accessed on September 3, 2018. Neutrino Astronomy, Rosa Poggiani in High Energy Astrophysical Techniques: pp. 115–121, Springer, Heidelberg 2017)

However, the investigation of high-energy neutrinos at energies beyond a few TeV is a highly exciting astrophysical discipline. Possible sources for such high-energy neutrinos are AGNs, hypernovae, GRBs, and black-hole mergers. The ICECUBE experiment (see Fig. 5.3) has measured several high-energy neutrinos, recently one of them of about 300 TeV (see Fig. 5.4), which seems to be correlated with high-energy gamma fluxes from detectors such as the FERMI satellite and the air-shower Cherenkov detector MAGIC. A random coincidence is excluded with a significance of ≈ 3σ .

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Fig. 5.3 Setup of the ICECUBE experiment at the South Pole. (Picture credits: ICECUBE homepage, https://icecube.wisc.edu/, accessed on September 1, 2018; ICECUBE detector, https://bub.fysik.su.se/english/IceCube/, accessed on September 1, 2018; Courtesy of Francis Halzen)

The correlation with high-energy gamma quanta and the angular resolution of the muon in the ICECUBE detector also make it possible to determine the origin of the event. With great certainty, this event is of extragalactic origin. Such experiments are of great importance for the investigation of the origin of highenergy cosmic processes and the search for sources in which high-energy particles are accelerated. The fact that neutrinos of this energy are measured from extragalactic distances suggests the hadronic origin of these interaction processes. Neutrinos of these energies are generally considered to indicate hadronic processes in the source (“smoking gun”). Previous measurements in the X-ray and gamma range

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Fig. 5.4 ICECUBE representation of a neutrino event registered from the direction of the TXS 0506+056 quasar. The event came from the bottom right. The energy deposited in the ICECUBE detector is estimated to be about 300 TeV. A high-energy gamma shower from the same source was also seen simultaneously by the FERMI telescope and the MAGIC air-shower Cherenkov detector. (Picture credits: Halzen, F. (2018). The IceCube Collaboration, Fermi-LAT, MAGIC, AGILE, ASAS-SN, HAWC, H.E.S.S., INTEGRAL, Kanata, Kiso, Kapteyn, Liverpool Telescope, Subaru, Swift/NuSTAR, VERITAS, VLA/17B-403 teams, ...; Science 12 July (2018): eaat1378, Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A; DOI: 10.1126/science.aat1378; http://science. sciencemag.org/content/361/6398/eaat1378, accessed on September 1, 2018. The IceCube Collaboration, https://icecube.wisc.edu/, accessed on September 1, 2018. Courtesy of Francis Halzen)

are compatible with electromagnetic acceleration mechanisms such as synchrotron radiation, bremsstrahlung, or inverse Compton effect. High-energy neutrinos at these energies are difficult to understand with electromagnetic acceleration models. High-energy neutrinos also limit the models for the sources of such neutrinos. Simultaneously measured gamma radiation helps to determine the properties of the sources and their propagation. As with the high-energy event in the case of the TXS 0506+056 quasar, which is located at a distance of 5.7 billion light years and whose relativistic jet is pointing precisely in the direction of Earth, simultaneous observations in different spectral ranges help to understand more about the origin of high-energy cosmic rays. Therefore, multi-messenger experiments are extremely valuable for these investigations.

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Gravitational Waves

Imagination is more important than knowledge, because knowledge is limited. Albert Einstein

One hundred years after Einstein’s prediction, the LIGO experiment with the two Michelson interferometers in Hanford and Livingston discovered the first gravitational-wave signal from the fusion of two black holes in 2015 (see Fig. 6.1). Just as Pauli thought in 1930 that “his” neutrino could never be measured experimentally, Einstein believed that the slight rippling of space-time by gravitational waves could never be detected. With this discovery, made possible by an extremely vibration-free positioning of the interferometer mirrors and the use of “squeezed” laser light (with a phase-dependent reduced blur), a new method for studying cataclysmic processes in the universe has emerged in gravitational-wave astronomy. The first indirect indication of the correctness of Einstein’s prediction came from a close observation of the change in the periastron time of a double pulsar system measured by astronomers Taylor and Hulse over a period of 30 years since 1974. Since the discovery of the first gravitational-wave signal, more than half a dozen such events have now been detected, and other detectors such as VIRGO in Italy have been added. Gravitational-wave detectors are also planned in India (IndIGO, LIGO-India), in Japan (Kamioka Gravitational-Wave Detector KAGRA; for 2019), and the European Einstein telescope (after 2021). Already three detectors distributed worldwide allow a good determination of the origin of gravitational waves. Similarly, multi-messenger observations of events seen in many

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Fig. 6.1 The fusion of two black holes measured by the laser interferometer GravitationalWave Observatory LIGO, shown in a computer simulation. (Photo credits: Caltech/MIT/LIGO Laboratory; photo credits: LIGO homepage, https://www.ligo.caltech.edu/, accessed on September 1, 2018. GW1509014: LIGO Detects Gravitational Waves, https://www.black-holes. org/gw150914, accessed on September 1, 2018)

spectral ranges are particularly useful for determining position and distance, as in the collision of two neutron stars in 2017 (“kilonova”). In order to achieve a really good spatial resolution and to cover the widest possible frequency range, a gravitational-wave detector in space would be the best solution. LISA (Laser Interferometer Space Antenna), a triangular American– European interferometer in space (arm length 5 million km), was planned for this purpose. Since NASA is not pursuing this project for cost reasons, the Europeans want to realize a reduced version (eLISA, e—evolved) with an arm length of 1 million km (from 2034). With the LISA Pathfinder, ESA has already shown that the realization of such a space telescope is technically feasible.

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A real challenge would be to measure the gravitational waves from the very first fractions of seconds after the Big Bang, when the structure of the universe emerged from the quantum nebula. However, this would present a similar difficulty as to measure primordial neutrinos. Summary of Chapters 2–6 The particles that fall to Earth from space are charged particles (nuclei and electrons), neutral particles (neutrinos), and electromagnetic radiation in various spectral ranges (gamma- and X-rays). All these messengers from the Milky Way and other galaxies provide different information. Charged particles can be used to learn something about the chemical composition of primary cosmic radiation. Gamma- and X-rays allow the sources to be identified in the high-energy range, but due to absorption effects, essentially only the surfaces of the cosmic sources can be studied. With neutrinos, however, it is possible to look into the interior of the sources, but at the expense of the difficult detection of neutrinos, which therefore requires very large detectors. For the more distant future, it would also be interesting to use gravitational waves in astronomy. But there is still a long way to go, even though gravitational waves were first detected in 2015 when two black holes merged in a binary system.

7

Cosmology

As far as the laws of mathematics refer to reality, they are not certain; as far as they are certain, they do not refer to reality. Albert Einstein 1921

When Einstein formulated his general theory of relativity in 1915/1916, it was believed that the universe was stationary. His field equations, however, predicted a dynamic universe, and similar to Newton’s theory, the gravitational attraction could cause the universe to clump together. To counteract this, Einstein introduced the cosmological constant into his field equations, which corresponded to a repulsive gravitation. With the discovery of the expansion of the universe by Hubble (1929), the cosmological constant would actually have become superfluous, but observations of distant supernovae in the 1990s show that the universe has apparently been regaining gas for a few billion years. This accelerated expansion is best described by a repulsive gravitation, which is realized by the so-called dark energy. With these ingredients, the extended Friedmann equation can describe the dynamic behavior of the universe quite well. The detailed modeling of primordial nucleosynthesis succeeds very well in the standard Big Bang model. The calculated elemental abundances of the elements hydrogen, helium, lithium, and beryllium are in excellent agreement with the experimentally determined values. However, the temporal evolution of the universe depends crucially on the total energy/matter density of the universe. The evolution of the universe can be described by representing the energy content normalized to the critical matter density with the parameters of the normalized mass density m or the corresponding vacuum energy density V (see Fig. 7.1). The critical matter density corresponds © Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2021 C. Grupen, Neutrinos, Dark Matter and Co., Springer essentials, https://doi.org/10.1007/978-3-658-32547-3_7

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Fig. 7.1 Representation of the relative size of the universe as a function of time for various assumptions about its energy content. (Picture credits: NASA Official: Wollack, E. J., NASA/WMAP Science Team (2015), The Physics of the Universe, https://www.physicsof theuniverse.com/topics_bigbang_accelerating.html, accessed on September 1, 2018; https:// map.gsfc.nasa.gov/media/990350/990350s.jpg, accessed on September 1, 2018)

to a flat universe (i.e., total = 1). The values of the parameters derived from the data of the COBE, WMAP, and Planck satellites favour an expanding universe with visible matter densities of only about 5% and dominant proportions of dark matter (27%) and dark energy (68%). If these values turn out to be correct, and there is no doubt about this at the moment, the universe will expand forever, and at an accelerated rate (red curve in Fig. 7.1). In the long run, all structures will be torn apart by the dark energy that increases with the volume (“Big Rip”). The COBE, WMAP, and Planck satellites have very accurately measured the cosmic blackbody radiation, which is consistent with an average temperature of about 2.7 K. Important cosmological parameters have been derived from the spatial fluctuations of this temperature, which act as seeds for galaxy formation. According to the latest evaluations of the Planck data, the age of the universe is

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approximately 13.8 billion years. The exact determination of the Hubble constant, which describes the expansion of the universe and thus also the age of the universe, is currently a hotly debated research topic. Different measurements yield slightly different values at the level of three standard deviations. The results on dark matter and dark energy are a great mystery for cosmology. Fritz Zwicky had already suspected in the 1930s that the dynamics of stars in galaxies and the dynamics of galaxies were not compatible with normal matter. The measured rotation curves of stars in galaxies cannot be described by Kepler’s motions alone, assuming that a massive black hole resides at the center of each galaxy (see Fig. 7.2). Although it is unknown what this gravity-only interacting matter consists of, it can at least be localized by its gravitational lensing effect. However, the direct search for dark-matter particles has so far been disappointingly unsuccessful. But when it comes to dark energy, one is completely in the dark. After this unsatisfactory situation about the lack of knowledge about the energy content of the universe, another question concerning the evolution of the universe must be solved. It seems that the large-scale structure of the universe is characterized by homogeneity and isotropy. However, the structure of the early universe seems to be rather lumpy. What caused the universe to change to such a relatively smooth state? The common answer to this question is the model of inflationary expansion. It seems that in the inflationary phase from about 10−38 to 10−36 s after the Big Bang, the universe expanded by a huge factor of e100 , thus “straightening out” all the irregularities. The model of inflation thus solves the flatness problem (i.e., total = 1), it answers the question of why the universe has the same temperature everywhere, and the question of why magnetic monopoles, if they ever existed, have been so infinitely diluted so that not a single one can be found. A crucial test of inflation would be to prove the existence of gravitational waves predicted by it as a result of the Big Bang, which would provide fossil evidence of the Big Bang, so to speak. However, this proof is extremely difficult to provide. There is therefore no experimental confirmation of the model of the inflationary universe as yet, but it seems to be the only model at the moment that correctly describes the difficult transition from the early universe to what we currently find. However, it must be pointed out that there is no unique model of the inflationary universe. In fact, there are a variety of different concepts of inflation, between which one can perhaps distinguish experimentally.

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Fig. 7.2 Rotation curves of the spiral galaxy NGC 6503. The contributions of the galactic disk, the gas and the halo are marked separately. (Picture credits: Freese, K. (2009), EAS Publ. Ser. 36, pp. 113–126, Review of Observational Evidence for Dark Matter in the Universe and in upcoming searches for Dark Stars; arXiv:0812.4005 [astro-ph]; https://arxiv.org/pdf/ 0812.4005.pdf, accessed on September 1, 2018. https://www-personal.umich.edu/~ktfreese/, accessed on September 1, 2018)

Another problem that is still open is the unexpected dominance of matter over antimatter, which cannot be explained in the Standard Model of elementary particles. Although violations of charge and parity symmetry conservation occur in the Standard Model, their size is not sufficient to solve the problem. Andrei Sakharov formulated in 1967 three necessary conditions for a dynamic generation of baryon asymmetry in the universe during baryogenesis:

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1. Violation of the baryon number conservation 2. Violation of C and CP invariance 3. Thermodynamic non-equilibrium However, in all experimental studies in particle physics, the baryon number seems to be preserved. The third condition is needed to obtain unequal population densities for matter and antimatter particles. However, it is completely unclear how such a theory, which fulfills the Sakharov conditions, should look like. Although antiparticles are found in astroparticle physics, in most cases their rate can be explained by secondary production. One exception is the unexpectedly high proportion of positrons in the energy range between 10 and 1000 GeV in primary cosmic radiation. Although pulsars could also be responsible for this surplus, the question is currently open. At the moment we have to take the baryon density of the universe or the baryon-to-photon ratio from the observation as free parameters to understand models of the universe. Summary Cosmology deals with the solutions of Einstein’s field equations and tries to describe the evolution of our universe. The Friedmann equation can adequately reflect the behavior of the universe. The formation of the light elements in the first 3 min after the Big Bang is well described in primordial nucleosynthesis. It is a strong support of the classical Big Bang model. One way to correctly describe difficulties in the evolution of the early universe up to the present state is through the model of inflation. But there are enough open questions in cosmology and cosmogony. The understanding of the matter content of the universe (dark matter, dark energy) and the dominance of matter over antimatter are, among other things, two sets of questions that defy explanation in the Standard Model of particle physics.

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Astrobiology

I believe in aliens. I think it would be way too selfish of us as mankind to believe we are the only lifeforms in the universe. Demi Lovato

In the deep-frozen permafrost soil in Siberia, cryobiotically preserved nematodes were found during thawing, which had not moved about since the last ice age. For the first time since 40,000 years, after being given a chance to absorb food from a nutrient solution, they lived on! In a salt crystal inside an old rock layer near Carlsbad in New Mexico, a bacillus was found in a bubble filled with brine, which started to live again after being placed in a nutrient solution. The bacillus was inactive for 250 million years. It could easily survive a journey from the Andromeda galaxy. Certain microorganisms can survive extremely high doses of ionizing radiation. The bacterium Deinococcus radiodurans can survive radiation doses of up to about 20,000 Sv (for humans, 4.5 Sv is already lethal). Deinococcus radiophilus even prefers high doses of radiation and can heal radiation damage with up to 10,000 breaks in its DNA. With the discovery of extrasolar planets since 1995, this discipline has gained much interest. With the help of new observation methods and modern satellite technology, more and more planets have been discovered in other solar systems in recent years. In the meantime, more than 4000 exoplanets are known, of which over 100 are considered habitable. It seems that every star also has several planets. So we must reckon with about 100 billion planets in our Milky Way galaxy alone. The problem of possible contact is the large, almost insurmountable distances and the possibly limited life span of civilizations. Life as we know it seems © Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2021 C. Grupen, Neutrinos, Dark Matter and Co., Springer essentials, https://doi.org/10.1007/978-3-658-32547-3_8

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to thrive wherever the chemical conditions are right. There are enough known examples of life forms that can live under extreme conditions and can survive long periods of time in a state without metabolism and without oxygen and water. With such cryptobiotic life forms, the definition of life and death must be reconsidered. However, in order to allow life to develop on planets, a number of astrophysical and elementary particle physics conditions must be fulfilled, but these conditions are present in our Milky Way and probably in other galaxies as well. Some of these parameters can be verified with earthbound accelerators, but other relevant parameters, especially the behavior of cosmic particles at the very highest energies or processes that can contribute information to dark matter or dark energy, fall within the domain of astroparticle physics. However, neither accelerators nor astroparticles can provide information about life in other universes. Astrophysically relevant parameters are, for example, the values of the quark masses. Furthermore, they include the exact value of the coupling of the strong interaction, the  parameter, i.e., the density parameter that determines the expansion behavior of the universe, the cosmological constant , and the number of space dimensions. Our universe could randomly be the result of a selection process from the multitude of possible universes in a multiverse. It is quite conceivable that there is a great variety of physical laws in other universes. Only in those universes in which the origin and development of life is possible can questions be asked as to why the parameters have such special, life-enabling properties. As a consequence of this anthropic principle, it follows that it is not mysterious that we find such special values in our universe. It could just be that we happen to live in a universe where the development of life is possible. Summary There is much speculation about extraterrestrial life. Since hydrocarbon compounds have been found in comets and meteorites, it is considered likely that there are life forms that exist under space conditions and can survive for a long time. If these life forms colonize planets and find favorable chemical conditions, evolution to higher life is possible. However, many astrophysical boundary conditions must be fulfilled for such an evolution. But if you consider that there are 1022 stars in our universe, for example, and that practically every star has planets, then a probability of 10−16 is sufficient to allow millions of life forms to develop. Many religions are convinced of the uniqueness of life on Earth. However, this assumption could turn out to be as much of an error as the prevailing view in classical antiquity that the Earth is the center of the universe.

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Outlook

My goal is simple. It is the complete understanding of the universe: why it is the way it is and why it exists at all. Stephen Hawking

Especially recently, there have been many advances in astroparticle physics: discovery of gravitational waves and measurement of extragalactic neutrinos. But there are also numerous, serious problems for which no solutions are in sight at the moment. Below are some astrophysical homework for the future: • Quantum mechanics and general relativity are brilliantly confirmed in their fields of application, but these grandiose theories cannot be presented within the framework of a uniform description. Are string theories or quantum loop gravity a solution? • The dominant content of the universe seems to be dark energy. We have absolutely no idea what it could be. • Besides, we need the dark matter. We know approximately where it is, and there are also assumptions and approaches as to what it could consist of (supersymmetric particles, heavy neutrinos, …?), but there is still no clarity. • How do cosmic accelerators manage to accelerate particles to 1020 eV? And where are these accelerators located? • The theory of inflation can explain many cosmological mysteries. However, there is still no decisive experimental test to confirm it. Are there alternatives to inflation? • How to understand the mystery of the matter–antimatter imbalance? Does the solution lie in elementary particle physics? © Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2021 C. Grupen, Neutrinos, Dark Matter and Co., Springer essentials, https://doi.org/10.1007/978-3-658-32547-3_9

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• Although much is now understood in the neutrino sector, there are still a few things that are unclear. Are neutrinos really Dirac or Majorana particles and are there sterile neutrinos? How can primordial neutrinos be detected? How do neutrinos get their masses? The solution to all these problems will certainly require a broad accelerator program and a multi-messenger approach in astroparticle physics.

What You Learned From This essential

You can take with you knowledge on the following topics: • • • • • • • •

Basics about cosmic radiation Detection of astroparticles Neutrino interactions Dark matter and dark energy Gravitational waves Cosmic antimatter Cosmology Extraterrestrial planets and astrobiology

© Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2021 C. Grupen, Neutrinos, Dark Matter and Co., Springer essentials, https://doi.org/10.1007/978-3-658-32547-3

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Literature

De Angelis A, Pimenta M (2018) Introduction to particle and astroparticle physics: multimessenger astronomy and its particle physics foundations, 2nd edn. Springer, Heidelberg Gaisser TK, Engel R, Resconi E (2016) Cosmic rays and particle physics, 2nd edn. Cambridge University Press, Cambridge Grupen C (2018) Einstieg in die Astroteilchenphysik, 2nd edn. Springer, Heidelberg Grupen C (2020) Astroparticle physics, 2nd edn. Springer Nature Switzerland AG, Switzerland Klapdor-Kleingrothaus HV, Zuber K (1999) Particle astrophysics (Studies in high energy physics, cosmology, and gravitation). Institute of Physics Publishing, Bristol Lesch H, Gaßner J (2017) Urknall, Weltall und das Leben, 4th edn. Komplett-Media, Grünwald Peacock JA (1999) Cosmological physics. Cambridge University Press, Cambridge Spurio M (2015) Particles and astrophysics: a multi-messenger approach. Springer, Heidelberg

© Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2021 C. Grupen, Neutrinos, Dark Matter and Co., Springer essentials, https://doi.org/10.1007/978-3-658-32547-3

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