Astroparticle Physics

8 1 Historical Introduction1.2 Discoveries of New Elementary Particles“If I would remember the names of allthese particles, I’d be a botanist.”Enrico Fermidiscovery of the positronneutrino postulateneutrino astronomyUp to the thirties, only electrons, protons (as part of thenucleus), and photons were known as elementary particles.The positron was discovered in a cloud chamber by Andersonin 1932 (Nobel Prize 1936). This was the antiparticleof the electron, which was predicted by Dirac in 1928 (NobelPrize 1933). This, and the discovery of the neutron byChadwick in 1932 (Nobel Prize 1935), started a new chapterin elementary particle and astroparticle physics. Additionallyin 1930, Pauli postulated the existence of a neutral,massless spin- 1 2particle to restore the validity of the energy,momentum, and angular-momentum conservation laws thatappeared to be violated in nuclear beta decay (Nobel Prize1945). This hypothetical enigmatic particle, the neutrino,could only be shown to exist in a reactor experiment in 1956(Cowan & Reines, Nobel Prize 1995). It eventually lead to acompletely new branch of astronomy; neutrino astronomy isa classic example of a perfect interplay between elementaryparticle physics and astronomy.It was reported that Landau (Nobel Prize 1962), withinseveral hours of hearing about the discovery of the neutron,predicted the existence of cold, dense stars which consistedmainly of neutrons. In 1967, the existence of rotating neu-tron stars (pulsars) was confirmed by observing radio signals(Hewish and Bell, Nobel Prize for Hewish 1975).Neutrons in a neutron star do not decay. This is becausethe Pauli exclusion principle (1925) forbids neutrons to decayinto occupied electron states. The Fermi energy of remnantelectrons in a neutron star is at around several 100 MeV,while the maximum energy transferred to electrons in neutrondecay is 0.77 MeV. There are therefore no vacant electronlevels available.After discovering the neutron, the second building blockof the nucleus, the question of how atomic nuclei could sticktogether arose. Although neutrons are electrically neutral,the protons would electrostatically repel each other. Basedon the range of the nuclear force and Heisenberg’s uncertaintyprinciple (1927, Nobel Prize 1932), Yukawa conjecturedin 1935 that unstable mesons of 200-fold electron masscould possibly mediate nuclear forces (Nobel Prize 1949).neutron starspulsarsstability of a neutron starYukawa particle