Astroparticle Physics

1.2 Discoveries of New Elementary Particles 9Initially it appeared that the muon discovered by Andersonand Neddermeyer in a cloud chamber in 1937, had the requiredproperties of the hypothetical Yukawa particle. Themuon, however, has no strong interactions with matter, andit soon became clear that the muon was a heavy counterpartof the electron. The fact that another electron-like particleexisted caused Rabi (Nobel Prize 1944) to remark: “Whoordered this?” Rabi’s question remains unanswered to thisday. The situation became even more critical when Perl (NobelPrize 1995) discovered another, even heavier lepton, thetau, in 1975.The discovery of the strongly interacting charged pions(π ± ) in 1947 by Lattes, Occhialini, Powell, and Muirhead,using nuclear emulsions exposed to cosmic rays at mountainaltitudes, solved the puzzle about the Yukawa particles(Nobel Prize 1950 to Cecil Powell for his development ofthe photographic method of studying nuclear processes andhis discoveries regarding mesons made with this method).The pion family was supplemented in 1950 by the discoveryof the neutral pion (π 0 ). Since 1949, pions can also beproduced in particle accelerators.Up to this time, elementary particles were predominantlydiscovered in cosmic rays. In addition to the muon(µ ± ) and the pions (π + ,π − ,π 0 ), tracks of charged andneutral kaons were observed in cloud-chamber events. Neutralkaons revealed themselves through their decay into twocharged particles. This made the K 0 appear as an upsidedown ‘V’, because only the ionization tracks of the chargeddecay products of the K 0 were visible in the cloud chamber(Rochester & Butler 1947, Fig. 1.11).In 1951, part of the Vs were recognized as Lambdabaryons, which also decayed relatively quickly into twocharged secondaries (Λ 0 → p + π − ). In addition, the Ξand Σ hyperons were discovered in cosmic rays (Ξ: Armenteroset al., 1952; Σ: Tomasini et al., 1953).Apart from studying local interactions of cosmic-rayparticles, their global properties were also investigated. Theshowers observed under lead plates by Rossi were alsofound in the atmosphere (Pfotzer, 1936). The interactions ofprimary cosmic rays in the atmosphere initiate extensive airshowers, see Sect. 7.4, (Auger, 1938). These showers leadto a maximum intensity of cosmic rays at altitudes of 15 kmabove sea level (‘Pfotzer maximum’, Fig. 1.12).One year earlier (1937), Bethe and Heitler, and at thesame time Carlson and Oppenheimer, developed the theorydiscovery of muonsdiscovery of the taudiscovery of pionsFig. 1.11Decays of neutral kaons in a cloudchamber {4}V particlesbaryons and hyperonsFig. 1.12Intensity profile of cosmic particlesin the atmosphere