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Early developments: Particle physics aspects of cosmic rays
Astroparticle Physics 53 (2014) 86–90
Contents lists available at SciVerse ScienceDirect
Astroparticle Physics journal homepage: www.elsevier.com/locate/astropart
Early developments: Particle physics aspects of cosmic rays Claus Grupen Faculty of Science and Technology, University of Siegen, Walter-Flex-Str. 3, D-57068 Siegen, Germany
a r t i c l e
i n f o
Article history: Available online 12 January 2013 Keywords: Cosmic rays Particle physics History
a b s t r a c t Cosmic rays is the birthplace of elementary particle physics. The 1936 Nobel prize was shared between Victor Hess and Carl Anderson. Anderson discovered the positron in a cloud chamber. The positron was predicted by Dirac several years earlier. In subsequent cloud chamber investigations Anderson and Neddermeyer saw the muon, which for some time was considered to be a candidate for the Yukawa particle responsible for nuclear binding. Measurements with nuclear emulsions by Lattes, Powell, Occhialini and Muirhead clarified the situation by the discovery of the charged pions in cosmic rays. The cloud chamber continued to be a powerful instrument in cosmic ray studies. Rochester and Butler found V’s, which turned out to be shortlived neutral kaons decaying into a pair of charged pions. Also K’s, R’s, and N’s were found in cosmic rays. But after that accelerators and storage rings took over. The unexpected renaissance of cosmic rays started with the search for solar neutrinos and the observation of the supernova 1987A. Cosmic ray neutrino results were best explained by the assumption of neutrino oscillations opening a view beyond the standard model of elementary particles. After 100 years of cosmic ray research we are again at the beginning of a new era, and cosmic rays may contribute to solve the many open questions, like dark matter and dark energy, by providing energies well beyond those of accelerators. Ó 2013 Elsevier B.V. All rights reserved.
1. Some historical remarks Initially it was not clear whether cosmic rays were really ‘rays’, that is gamma rays or charged particles. There was a fierce dispute between Millikan and Compton about the nature of cosmic rays. Gerhard Hoffmann showed in 1926 that cosmic rays could produce showers in absorber layers (Hoffmannsche Stöße; bursts), but this could have been due to either energetic photons as well as particles [1–3]. The open question was settled by Jacob Clay in 1927 who used an ionisation chamber on a sea voyage from Java to the Netherlands demonstrating that the intensity of cosmic rays depends on the geomagnetic latitude, a clear evidence for the particle nature of cosmic rays [4]. The discovery of Hess of the extraterretrial origin of cosmic rays started a period of prosperity for elementary particle physics with huge potential for the detection of new subatomic particles. 2. The golden years of cosmic rays The discovery of the positron by Carl Anderson in 1932 was certainly the first highlight contribution of cosmic rays to particle physics. Anderson was not aware of the relativistic Dirac equation predicting the existence of anti-electrons. He scanned his cloud chamber events when he saw a particle, curiously coming from below, which showed the ionisation density of an electron, but with a E-mail address: [email protected] 0927-6505/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.astropartphys.2013.01.002
curvature in the strong magnetic field, clearly indicating a positive charge [5]. In Fig. 1 the first positron observed in cosmic rays is seen. In subsequent studies, together with Neddermeyer, he also found a particle with ionisation between that of an electron and a proton in 1937 [7], (see also Fig. 2, [6]). Actually such a particle was already seen earlier (1932) by Paul Kunze from Rostock in Germany, also in a cloud chamber. But Kunze was not aware of this important observation [8]. It was suspected that the new particle of intermediate mass could have been the particle postutated by Yukawa in 1935 to mediate strong interactions [9]. The predicted mass was about 200 times the electron mass and the expected lifetime several hundred nanoseconds. There were esentially two problems with this identification: firstly, the new particle had no strong interactions, as it should have because it was assumed to mediate the strong interactions, and secondly the corrected lifetime (corrected from the value given by Yukawa of 0.25 microseconds to 1.6 ns by L.W. Nordheim in 1939, [10]) was too short for the new particle with a lifetime of 2.2 microseconds. Therefore the particle initially called the mu–meson was not the Yukawa particle, but it was a new lepton, the muon, a member of the second family of leptonic fermions! Emulsion measurements at mountain altitudes by Lattes, Powell, Occhialini and Muirhead clarified the situation in 1947 by the discovery of charged pions in cosmic rays, which were the real Yukawa particles [11], (see also Fig. 3, [12]).
C. Grupen / Astroparticle Physics 53 (2014) 86–90
Fig. 1. Observation of the positron in a cloud chamber operated in a strong magnetic field. The positron enters the chamber from below, as can be seen from the stronger bending of the track after having passed through the lead plate in the middle of the chamber. Also the increased ionisation at the end of the track clearly shows that the particle is coming from below [5].
Emulsion chambers exposed to cosmic rays at high altitudes turned out to be an ideal instrument to record cosmic ray events. Pioneering results with this technique of photographic plates were obtained by Blau and Wambacher. The discovery of ‘stars’, i.e.
Fig. 2. A penetrating cosmic ray muon photographed in a multiplate spark chamber [6], private communication by V.S. Kaftanov. Photo Credit Vitali S. Kaftanov.
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Fig. 3. Pion–muon–electron decay chain resulting from an antiproton annihilation with a proton in a streamer chamber filled with neon gas. The photo has been modified, so that only the pi–mu–e decay chain is visible. The photo was taken with the Experiment PS 179 at CERN; Photo credit CERN [12].
interactions of primary cosmic rays, constituted a breakthrough of this method [13]. However, also the cloud chamber continued to be a powerful instrument in cosmic ray studies. Rochester and Butler using the cloud chamber of the Blackett group in Manchester found V’s, which turned out to be shortlived neutral kaons decaying into a pair of charged pions [14], (see Fig. 4, [15]). Also K’s (see Fig. 5),
Fig. 4. Observation of a kaon produced in a cloud chamber, which decays into a pair of charged pions. The photo was taken from P. Galison, Images and Logic, University of Chicago Press (1997) [15].
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Fig. 6. Pair production and decay of X particles created in a cosmic–ray interaction observed in an emulsion chamber by Niu et al. in 1971. Particle B decayed at point B into a charged particle B0 and a p0 . Two c rays, daughters of the p0 , initiated electron showers, seen as wide ‘tracks’. Particle C decayed at C into a charged particle C 0 and unseen neutral hadron(s). Shown is the z projection of the event [22].
Fig. 5. First observation of a hypernucleus containing a lambda particle which has replaced a neutron. A cosmic ray, probably a proton, coming in from the top right has created a big star of tracks after collision with a nucleus in the emulsion. The total energy released in the disintegration is consistent with the decay of a lambda particle in the hypernucleus, [16].
R’s, and the N were found in cosmic rays [16–19]. K’s were around in the early fifties, and it is difficult to do justice to all those who contributed to it. There are even hints that the particle found by Yehuda Eisenberg in an emulsion stack exposed to cosmic rays at 100 000 feet altitude (about 30.5 km) in 1954 could have been the X , the missing link of the quark model [20]. Teucher et al. saw a possible example of the production and annihilation of an antiproton in an emulsion stack flown at an altitude of 29 km in Texas in January 1955, [21]. Also, the charm discovery in 1974 by Samuel C.C. Ting at Brookhaven and by Burt Richter at Stanford may have been anticipated by the observation of an X particle in a nuclear emulsion by Niu et al. [22] in 1971. The observed meson had a mass of about 2 GeV and a lifetime consistent with that of a charged D-meson. However, there was only one event and there was no estimate of the background (see Fig. 6). One must also take into account that occasionally particles were ‘observed’ in cosmic rays which later could not be confirmed, just to name a few, like free quarks in air shower cores [23], proton decay in the Kolar Gold Fields [24], and the Centauros [25]. This is certainly the reason why some results from cosmic rays were met with scepticism from the upcoming accelerator community. The era of accelerators, of electron–positron colliders, and of proton–proton or proton–antiproton storage rings took now over with the discovery of a plethora of new particles establishing the standard model of electroweak and strong interactions [26,27].
3. Interlude on accelerators The electron antineutrino was discovered by Cowan and Reines at a nuclear reactor [28] confirming Pauli’s conjecture from the
thirtees of the last century which he published in an open letter to the ‘‘Group of Radioactives’’ [29]. Lederman, Schwartz and Steinberger showed in 1962 in an accelerator experiment at Brookhaven that the neutrino from pion decay did not produce electrons but rather muons thereby demonstrating that it was different from the electron-type neutrino [30]. A third lepton generation was found at the Stanford electron–positron storage ring by Perl [31]. In the hadronic sector many new states were discovered and it became obvious that not all of these were elementary. The standard model of elementary particles with its three generations of quarks and leptons finally combined the particles seen in cosmic rays and at accelerators. Also the observation of the gauge bosons mediating the strong (gluons seen at DESY in 1979), electromagnetic (photons, with the fine structure constant a), and weak interactions (W þ ; W , and Z discovered at CERN in 1983) rounded up the common framework (for more details see [27]).
4. The renaissance of cosmic rays 4.1. Neutrinos Soon after the discovery of the muon neutrino in 1962 it was also seen in the Kolar Gold field [32]. But the renaissance of cosmic rays started with the observation of solar neutrinos [33]. The fact that the measured flux of solar neutrinos was too low became known as the solar neutrino problem. The proposal that the weak eigenstates (me ; ml ; ms ) were in fact superpositions of three different mass eigenstates (m1 ; m2 ; m3 ) lead to the idea of neutrino oscillations. This was clearly underlined by the observation of the Super-Kamiokande detector of a deficit of atmospheric muon neutrinos which had passed through the earth (see Fig. 7). The collaboration announced the first evidence of neutrino oscillations in 1998 [34]. The idea of neutrino oscillations was pioneered by Pontecorvo already in the fifties of the last century. Pontecorvo proposed that flavour oscillations may occur in analogy to neutral kaon mixing [35].
C. Grupen / Astroparticle Physics 53 (2014) 86–90
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Fig. 9. The rotation curves in the galaxy NGC 3198 require the existence of gravitational matter, after [42]. The curve labelled ‘disk’ describes the expectation of rotational velocities based on baryonic matter. Dark matter in the halo is needed to understand the measured orbital velocities. Fig. 7. A muon neutrino deficit is observed for muon neutrinos having passed through the Earth: a clear indication for neutrino oscillations, after [36].
set a significant limit on the electron neutrino mass of at most 16 (eV/c2) [39]. 4.2. Dark matter and dark energy
Fig. 8. Arrival times of the supernova SN1987A neutrinos in the Kamiokande, IMB and Baksan detectors, after [40].
Just as atmospheric muon neutrinos the low energy solar electron neutrinos had obviously been transformed into other neutrino flavours to which the detectors were insensitive. The final solution to the missing solar neutrinos in terms of oscillations was demonstrated by the Sudbury Neutrino Observatory [37]. This experiment was able to record all types of neutrino flavours from the sun by charged and neutral current processes. SNO showed that the solar neutrino flux was a mixture of the three neutral lepton flavours and nothing was missing. Neutrino oscillations imply non-zero neutrino masses which represents a discovery of physics beyond the standard model by cosmic rays. Also the observation of the supernova explosion SN1987A in the Large Magellanic Cloud contributed significantly to particle physics. Kamiokande detected 11 antineutrinos, the Irvine–Michigan– Brookhaven collaboration (IMB) 8 antineutrinos, and Baksan 5 antineutrinos in a burst lasting less than 13 s (see Fig. 8). Approximately three hours earlier, the Mont Blanc liquid scintillator detected a five-neutrino burst, but this is generally not believed to be associated with SN 1987A [38]. It could be shown that the arrival time distributions of neutrinos from SN1987A allowed to
Up to the thirties of the 20th century it was assumed that we know the main ingredients of the universe. However, the orbital velocities of stars in galaxies, in particular in our Milky Way lead Jan Oort in 1932 to postulate the existence of dark matter which was also proposed by Fritz Zwicky in 1933 to account for evidence of ‘missing mass’ in the orbital velocities of galaxies in clusters [41]. Zwicky argued that without additional gravitational material the stars in galaxies and galaxies in clusters would simply fly apart. The observed rotation curve, e.g. in the galaxy NGC 3198 clearly demonstrates that extra gravitational matter is required to explain the dynamics of galaxies, (see Fig. 9, [42]). A strong argument for dark matter also comes from the observation of gravitational lensing [43]. Gravitational lensing results and the rotation curves indicating almost constant velocities also at large distances from the galactic centres have prompted a large number of direct searches for dark matter, so far without conclusive result (see e.g. [44]). A solution to the mystery of dark matter might come from accelerators (supersymmetric particles?) or by cosmic ray experiments studying the interaction of ordinary cosmic rays with dark matter particles in the halo of our galaxy possibly leading to signatures like correlated showers on earth [45]. The COBE, WMAP and high z supernova experiments can only be consistently understood by assuming a large amount of dark energy in the universe [46]. Already Einstein discussed the possibility of repulsive gravitation around 1916. The nature of dark energy (cosmological constant? energy of the vacuum?,. . .) remains at present completely in the dark. 5. Conclusions Cosmic rays was the birthplace of elementary particle physics. The discovery of the positron, of the muon, pion, kaon and many baryons paved the way for the development of the standard model of particle physics. The observation of neutrino oscillations in cosmic rays, which implied non-zero neutrino masses lead to physics beyond the standard model. Whether cosmic rays can contribute to the understanding of the nature of the dominating non-baryonic
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dark matter or dark energy in our universe (23% dark matter, 73% dark energy) remains to be seen. It is certainly a challenge for accelerator as well as cosmic ray physics. Acknowledgement It is a pleasure to thank Prof. Dr. Markus Risse from Siegen for a careful and critical reading of the manuscript. I thank Prof. Dr. Hinrich Meyer from DESY/Wuppertal for pointing out the possible discovery of the antiproton by Lohrmann et al. to me, and many thanks to Dr. Christine Sutton, CERN for finding the original paper of Armenteros [19] showing the first observation of the N . References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
[14] [15] [16] [17] [18] [19]
[20] [21] [22]
[23] [24]
G. Hoffmann, F. Lindholm, Gerl. Beitr. Geophys. 20 (1928) 12. G. Hoffmann, W.S. Pforte, Phys. Z. 31 (1930) 347. O.C. Allkofer, Introduction to Cosmic Radiation, Verlag Karl Thiemig, 1975. Per Carlson, Phys. Today 65 (2) (2012) 30. C.D. Anderson, Phys. Rev. 43 (6) (1933) 491–494. V.S. Kaftanov, V.A. Liubimov, Nucl. Instrum. Methods 20 (1963) 195–202. C.D. Anderson, S.H. Neddermeyer, Phys. Rev. 51 (1937) 884; J.C. Street, E.C. Stevenson, Phys. Rev. 52 (1937) 1003. P. Kunze, Z. Phys. 83 (1933) 1. H. Yukawa, Proc. Phys. Math. Soc. Japan 17 (1935) 48. L.W. Nordheim, Phys. Rev. 55 (1939) 506. C.M.G. Lattes, H. Muirhead, G.P.S. Occhialini, C.F. Powell, Nature 159 (4047) (1947) 694–697., Credit: Prof. G. Piragino/ Science photo library. M. Blau, H. Wambacher, Nature 140 (1937) 585; M. Blau, H. Wambacher, Mitt. Inst. Radiumforsch. 409 (1937); H. Wambacher, Phys. Z. 23/24 (1938) 883. G.D. Rochester, C.C. Butler, Nature 160 (1947) 855. Peter Louis Galison, Image and Logic: A Material Culture of Microphysics, University Of Chicago Press, 1997. M. Danysz, J. Pniewski, Phil. Mag. 44 (1953) 348. R. Armenteros et al., Phil. Mag. 42 (1951) 1113. A. Bonetti et al., Nuovo Cimento 10 (1953) 345. R. Armenteros et al., Phil. Mag. Ser. 7 43 (341) (1952) 597; F. Close, M. Marten, Chr. Sutton, The Particle Explosion, Oxford University Press (1987). Y. Eisenberg, Phys. Rev. 96 (1954) 541. M. Teucher, H. Winzeler, E. Lohrmann, Nuovo Cimento, Serie X 3 (1956) 228. K. Niu, E. Mikumo, Y. Maeda, Prog. Theor. Phys. 46 (1971) 1644–1646; K. Niu, E. Mikumo, Y. Maeda, in: Conf. Paper, 12th Int. Cos. Ray Conf., Hobart, 1971, pp. 2792–2798.; K. Niu, Proc. Japan. Acad. Ser. B 84 (2008) 1–16. C.B.A. McCusker, I. Cairns, Phys. Rev. Lett. 23 (1969) 658–659. M.R. Krishnaswamy et al., Am. Inst. Phys. Conf. Proc. 96 (1) (1983) 168–174; M.R. Krishnaswamy, M.G.K. Menon, N.K. Mondal, V.S. Narasimham, B.V. Sreekantan, Y. Hayashi, N. Ito, S. Kawakami, S. Miyake, Nuovo Cimento C 9 (2) (1986) 167–181; H. Adarkar, S.R. Dugad, M.R. Krishnaswamy, M.G.K. Menon, B.V. Sreekantan, Y. Hayashi, N. Ito, S. Kawakami, S. Miyake, Y. Uchihori 2000. .
[25] C.M.G. Lattes, Prog. Theor. Phys. Suppl. 47 (1971) 1–125; C.M.G. Lattes et al., Phys. Rev. 65 (1980) 151; S. Hasegawa et al., Nucl. Phys. 141 (1996) B474; V. Kopenkin, Y. Fujimoto, T. Sinzi, Phys. Rev. D 68 (2003) 052007; V. Kopenkin, Y. Fujimoto, T. Sinci, Phys. Rev. D 68 (2003) 052007. and references therein. [26] J.W. Cronin, Eur. Phys. J. H 36 (2011) 183 (The 1953 Cosmic Ray Conference at Bagnères de Bigorre). [27] J. Beringer et al., Particle data group, Phys. Rev. D 86 (2012) 010001. [28] F. Reines, C.L. Cowan Jr., Nature 178 (4531) (1956) 446. [29] ETH – Wolfgang Pauli – Neutrino;. [30] G. Danby, J.-M. Gaillard, K. Goulianos, L.M. Lederman, N.B. Mistry, M. Schwartz, J. Steinberger, Phys. Rev. Lett. 9 (1962) 36. [31] M.L. Perl et al., Phys. Rev. Lett. 35 (1975) 1489–1492. [32] C.V. Achar, M.G.K. Menon, V.S. Narasimham, P.V. Ramana Murthy, B.V. Sreekantan, K. Hinotani, S. Miyake, D.R. Creed, J.L. Osborne, J.B.M. Pattison, A.W. Wolfendale, Phys. Lett. 18 (1965) 196–199. [33] B.T. Cleveland et al., Astrophys. J. 496 (1998) 505–526; P. Anselmann et al., GALLEX, Phys. Lett. B 285 (1992) 376–389; W.C. Haxton, Annu. Rev. Astron. Astrophys. 33 (1995) 459–503. [34] Y. Fukuda et al., Phys. Rev. Lett. 81 (8) (1998) 1562–1567. . . [35] B. Pontecorvo, Zh. Eksp. Teor. Fiz. 33 (1957) 549–551 (reproduced and translated in Sov. Phys. JETP 6 (1957) 429); B. Pontecorvo, Zh. Eksp. Teor. Fiz. 53 (1967) 1717 (reproduced and translated in Sov. Phys. JETP, 26 (1968) 984); V. Gribov, B. Pontecorvo, Phys. Lett. B 28 (1969) 493. [36] Super-Kamiokande Collaboration. . [41] J. Oort. .; F. Zwicky, Helv. Phys. Acta 6 (1933) 110–127; F. Zwicky, Astrophys. J. 86 (1937) 217. [42] K.G. Begeman, Astron. Astrophys. 223 (1989) 47–60. [43] P. Schneider, J. Ehlers, E.E. Falco, Gravitational Lenses (Astronomy and Astrophysics Library), Springer, 1999; Peter Schneider, Christopher Kochanek, Joachim Wambsganss, Gravitational Lensing: Strong, Weak and Micro: Saas-Fee Advanced Course 33, Springer, 2006. [44] N.J. Spooner, J. Phys. Soc. Japan 76 (2007) 111016; T.J. Sumner, J. Phys.: Conf. Ser. 312 (2011) 072003. [45] O. Carrel, M. Martin, Phys. Lett. B 325 (1994) 526; T. Kitamura et al., Astroparticle Phys. 6 (1997) 279. [46] S. Perlmutter, Phys. Today (2003) 53–60. and references therein.
Contents lists available at SciVerse ScienceDirect
Astroparticle Physics journal homepage: www.elsevier.com/locate/astropart
Early developments: Particle physics aspects of cosmic rays Claus Grupen Faculty of Science and Technology, University of Siegen, Walter-Flex-Str. 3, D-57068 Siegen, Germany
a r t i c l e
i n f o
Article history: Available online 12 January 2013 Keywords: Cosmic rays Particle physics History
a b s t r a c t Cosmic rays is the birthplace of elementary particle physics. The 1936 Nobel prize was shared between Victor Hess and Carl Anderson. Anderson discovered the positron in a cloud chamber. The positron was predicted by Dirac several years earlier. In subsequent cloud chamber investigations Anderson and Neddermeyer saw the muon, which for some time was considered to be a candidate for the Yukawa particle responsible for nuclear binding. Measurements with nuclear emulsions by Lattes, Powell, Occhialini and Muirhead clarified the situation by the discovery of the charged pions in cosmic rays. The cloud chamber continued to be a powerful instrument in cosmic ray studies. Rochester and Butler found V’s, which turned out to be shortlived neutral kaons decaying into a pair of charged pions. Also K’s, R’s, and N’s were found in cosmic rays. But after that accelerators and storage rings took over. The unexpected renaissance of cosmic rays started with the search for solar neutrinos and the observation of the supernova 1987A. Cosmic ray neutrino results were best explained by the assumption of neutrino oscillations opening a view beyond the standard model of elementary particles. After 100 years of cosmic ray research we are again at the beginning of a new era, and cosmic rays may contribute to solve the many open questions, like dark matter and dark energy, by providing energies well beyond those of accelerators. Ó 2013 Elsevier B.V. All rights reserved.
1. Some historical remarks Initially it was not clear whether cosmic rays were really ‘rays’, that is gamma rays or charged particles. There was a fierce dispute between Millikan and Compton about the nature of cosmic rays. Gerhard Hoffmann showed in 1926 that cosmic rays could produce showers in absorber layers (Hoffmannsche Stöße; bursts), but this could have been due to either energetic photons as well as particles [1–3]. The open question was settled by Jacob Clay in 1927 who used an ionisation chamber on a sea voyage from Java to the Netherlands demonstrating that the intensity of cosmic rays depends on the geomagnetic latitude, a clear evidence for the particle nature of cosmic rays [4]. The discovery of Hess of the extraterretrial origin of cosmic rays started a period of prosperity for elementary particle physics with huge potential for the detection of new subatomic particles. 2. The golden years of cosmic rays The discovery of the positron by Carl Anderson in 1932 was certainly the first highlight contribution of cosmic rays to particle physics. Anderson was not aware of the relativistic Dirac equation predicting the existence of anti-electrons. He scanned his cloud chamber events when he saw a particle, curiously coming from below, which showed the ionisation density of an electron, but with a E-mail address: [email protected] 0927-6505/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.astropartphys.2013.01.002
curvature in the strong magnetic field, clearly indicating a positive charge [5]. In Fig. 1 the first positron observed in cosmic rays is seen. In subsequent studies, together with Neddermeyer, he also found a particle with ionisation between that of an electron and a proton in 1937 [7], (see also Fig. 2, [6]). Actually such a particle was already seen earlier (1932) by Paul Kunze from Rostock in Germany, also in a cloud chamber. But Kunze was not aware of this important observation [8]. It was suspected that the new particle of intermediate mass could have been the particle postutated by Yukawa in 1935 to mediate strong interactions [9]. The predicted mass was about 200 times the electron mass and the expected lifetime several hundred nanoseconds. There were esentially two problems with this identification: firstly, the new particle had no strong interactions, as it should have because it was assumed to mediate the strong interactions, and secondly the corrected lifetime (corrected from the value given by Yukawa of 0.25 microseconds to 1.6 ns by L.W. Nordheim in 1939, [10]) was too short for the new particle with a lifetime of 2.2 microseconds. Therefore the particle initially called the mu–meson was not the Yukawa particle, but it was a new lepton, the muon, a member of the second family of leptonic fermions! Emulsion measurements at mountain altitudes by Lattes, Powell, Occhialini and Muirhead clarified the situation in 1947 by the discovery of charged pions in cosmic rays, which were the real Yukawa particles [11], (see also Fig. 3, [12]).
C. Grupen / Astroparticle Physics 53 (2014) 86–90
Fig. 1. Observation of the positron in a cloud chamber operated in a strong magnetic field. The positron enters the chamber from below, as can be seen from the stronger bending of the track after having passed through the lead plate in the middle of the chamber. Also the increased ionisation at the end of the track clearly shows that the particle is coming from below [5].
Emulsion chambers exposed to cosmic rays at high altitudes turned out to be an ideal instrument to record cosmic ray events. Pioneering results with this technique of photographic plates were obtained by Blau and Wambacher. The discovery of ‘stars’, i.e.
Fig. 2. A penetrating cosmic ray muon photographed in a multiplate spark chamber [6], private communication by V.S. Kaftanov. Photo Credit Vitali S. Kaftanov.
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Fig. 3. Pion–muon–electron decay chain resulting from an antiproton annihilation with a proton in a streamer chamber filled with neon gas. The photo has been modified, so that only the pi–mu–e decay chain is visible. The photo was taken with the Experiment PS 179 at CERN; Photo credit CERN [12].
interactions of primary cosmic rays, constituted a breakthrough of this method [13]. However, also the cloud chamber continued to be a powerful instrument in cosmic ray studies. Rochester and Butler using the cloud chamber of the Blackett group in Manchester found V’s, which turned out to be shortlived neutral kaons decaying into a pair of charged pions [14], (see Fig. 4, [15]). Also K’s (see Fig. 5),
Fig. 4. Observation of a kaon produced in a cloud chamber, which decays into a pair of charged pions. The photo was taken from P. Galison, Images and Logic, University of Chicago Press (1997) [15].
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C. Grupen / Astroparticle Physics 53 (2014) 86–90
Fig. 6. Pair production and decay of X particles created in a cosmic–ray interaction observed in an emulsion chamber by Niu et al. in 1971. Particle B decayed at point B into a charged particle B0 and a p0 . Two c rays, daughters of the p0 , initiated electron showers, seen as wide ‘tracks’. Particle C decayed at C into a charged particle C 0 and unseen neutral hadron(s). Shown is the z projection of the event [22].
Fig. 5. First observation of a hypernucleus containing a lambda particle which has replaced a neutron. A cosmic ray, probably a proton, coming in from the top right has created a big star of tracks after collision with a nucleus in the emulsion. The total energy released in the disintegration is consistent with the decay of a lambda particle in the hypernucleus, [16].
R’s, and the N were found in cosmic rays [16–19]. K’s were around in the early fifties, and it is difficult to do justice to all those who contributed to it. There are even hints that the particle found by Yehuda Eisenberg in an emulsion stack exposed to cosmic rays at 100 000 feet altitude (about 30.5 km) in 1954 could have been the X , the missing link of the quark model [20]. Teucher et al. saw a possible example of the production and annihilation of an antiproton in an emulsion stack flown at an altitude of 29 km in Texas in January 1955, [21]. Also, the charm discovery in 1974 by Samuel C.C. Ting at Brookhaven and by Burt Richter at Stanford may have been anticipated by the observation of an X particle in a nuclear emulsion by Niu et al. [22] in 1971. The observed meson had a mass of about 2 GeV and a lifetime consistent with that of a charged D-meson. However, there was only one event and there was no estimate of the background (see Fig. 6). One must also take into account that occasionally particles were ‘observed’ in cosmic rays which later could not be confirmed, just to name a few, like free quarks in air shower cores [23], proton decay in the Kolar Gold Fields [24], and the Centauros [25]. This is certainly the reason why some results from cosmic rays were met with scepticism from the upcoming accelerator community. The era of accelerators, of electron–positron colliders, and of proton–proton or proton–antiproton storage rings took now over with the discovery of a plethora of new particles establishing the standard model of electroweak and strong interactions [26,27].
3. Interlude on accelerators The electron antineutrino was discovered by Cowan and Reines at a nuclear reactor [28] confirming Pauli’s conjecture from the
thirtees of the last century which he published in an open letter to the ‘‘Group of Radioactives’’ [29]. Lederman, Schwartz and Steinberger showed in 1962 in an accelerator experiment at Brookhaven that the neutrino from pion decay did not produce electrons but rather muons thereby demonstrating that it was different from the electron-type neutrino [30]. A third lepton generation was found at the Stanford electron–positron storage ring by Perl [31]. In the hadronic sector many new states were discovered and it became obvious that not all of these were elementary. The standard model of elementary particles with its three generations of quarks and leptons finally combined the particles seen in cosmic rays and at accelerators. Also the observation of the gauge bosons mediating the strong (gluons seen at DESY in 1979), electromagnetic (photons, with the fine structure constant a), and weak interactions (W þ ; W , and Z discovered at CERN in 1983) rounded up the common framework (for more details see [27]).
4. The renaissance of cosmic rays 4.1. Neutrinos Soon after the discovery of the muon neutrino in 1962 it was also seen in the Kolar Gold field [32]. But the renaissance of cosmic rays started with the observation of solar neutrinos [33]. The fact that the measured flux of solar neutrinos was too low became known as the solar neutrino problem. The proposal that the weak eigenstates (me ; ml ; ms ) were in fact superpositions of three different mass eigenstates (m1 ; m2 ; m3 ) lead to the idea of neutrino oscillations. This was clearly underlined by the observation of the Super-Kamiokande detector of a deficit of atmospheric muon neutrinos which had passed through the earth (see Fig. 7). The collaboration announced the first evidence of neutrino oscillations in 1998 [34]. The idea of neutrino oscillations was pioneered by Pontecorvo already in the fifties of the last century. Pontecorvo proposed that flavour oscillations may occur in analogy to neutral kaon mixing [35].
C. Grupen / Astroparticle Physics 53 (2014) 86–90
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Fig. 9. The rotation curves in the galaxy NGC 3198 require the existence of gravitational matter, after [42]. The curve labelled ‘disk’ describes the expectation of rotational velocities based on baryonic matter. Dark matter in the halo is needed to understand the measured orbital velocities. Fig. 7. A muon neutrino deficit is observed for muon neutrinos having passed through the Earth: a clear indication for neutrino oscillations, after [36].
set a significant limit on the electron neutrino mass of at most 16 (eV/c2) [39]. 4.2. Dark matter and dark energy
Fig. 8. Arrival times of the supernova SN1987A neutrinos in the Kamiokande, IMB and Baksan detectors, after [40].
Just as atmospheric muon neutrinos the low energy solar electron neutrinos had obviously been transformed into other neutrino flavours to which the detectors were insensitive. The final solution to the missing solar neutrinos in terms of oscillations was demonstrated by the Sudbury Neutrino Observatory [37]. This experiment was able to record all types of neutrino flavours from the sun by charged and neutral current processes. SNO showed that the solar neutrino flux was a mixture of the three neutral lepton flavours and nothing was missing. Neutrino oscillations imply non-zero neutrino masses which represents a discovery of physics beyond the standard model by cosmic rays. Also the observation of the supernova explosion SN1987A in the Large Magellanic Cloud contributed significantly to particle physics. Kamiokande detected 11 antineutrinos, the Irvine–Michigan– Brookhaven collaboration (IMB) 8 antineutrinos, and Baksan 5 antineutrinos in a burst lasting less than 13 s (see Fig. 8). Approximately three hours earlier, the Mont Blanc liquid scintillator detected a five-neutrino burst, but this is generally not believed to be associated with SN 1987A [38]. It could be shown that the arrival time distributions of neutrinos from SN1987A allowed to
Up to the thirties of the 20th century it was assumed that we know the main ingredients of the universe. However, the orbital velocities of stars in galaxies, in particular in our Milky Way lead Jan Oort in 1932 to postulate the existence of dark matter which was also proposed by Fritz Zwicky in 1933 to account for evidence of ‘missing mass’ in the orbital velocities of galaxies in clusters [41]. Zwicky argued that without additional gravitational material the stars in galaxies and galaxies in clusters would simply fly apart. The observed rotation curve, e.g. in the galaxy NGC 3198 clearly demonstrates that extra gravitational matter is required to explain the dynamics of galaxies, (see Fig. 9, [42]). A strong argument for dark matter also comes from the observation of gravitational lensing [43]. Gravitational lensing results and the rotation curves indicating almost constant velocities also at large distances from the galactic centres have prompted a large number of direct searches for dark matter, so far without conclusive result (see e.g. [44]). A solution to the mystery of dark matter might come from accelerators (supersymmetric particles?) or by cosmic ray experiments studying the interaction of ordinary cosmic rays with dark matter particles in the halo of our galaxy possibly leading to signatures like correlated showers on earth [45]. The COBE, WMAP and high z supernova experiments can only be consistently understood by assuming a large amount of dark energy in the universe [46]. Already Einstein discussed the possibility of repulsive gravitation around 1916. The nature of dark energy (cosmological constant? energy of the vacuum?,. . .) remains at present completely in the dark. 5. Conclusions Cosmic rays was the birthplace of elementary particle physics. The discovery of the positron, of the muon, pion, kaon and many baryons paved the way for the development of the standard model of particle physics. The observation of neutrino oscillations in cosmic rays, which implied non-zero neutrino masses lead to physics beyond the standard model. Whether cosmic rays can contribute to the understanding of the nature of the dominating non-baryonic
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dark matter or dark energy in our universe (23% dark matter, 73% dark energy) remains to be seen. It is certainly a challenge for accelerator as well as cosmic ray physics. Acknowledgement It is a pleasure to thank Prof. Dr. Markus Risse from Siegen for a careful and critical reading of the manuscript. I thank Prof. Dr. Hinrich Meyer from DESY/Wuppertal for pointing out the possible discovery of the antiproton by Lohrmann et al. to me, and many thanks to Dr. Christine Sutton, CERN for finding the original paper of Armenteros [19] showing the first observation of the N . References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
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