Discovery of the Muon
The muon is a lepton which decays to form an electron or positron.
The muon was discovered in 1937 by J. C. Street and E. C. Stevenson in a cloud chamber. The discovery was published in "New Evidence for the Existence of a Particle Intermediate Between the Proton and Electron", Phys. Rev. 52, 1003 (1937). Before this point the fundamental particles were presumed to be electrons, protons and the (then) newly discovered neutron. The discovery brought attention to the prediction by Hideki Yukawa in 1935 that an intermediate mass "meson" might be responsible for the nuclear strong force. Yukawa had predicted a mass of about 100 MeV/c2 and the muon had a mass very close to that. It was soon discovered, however, that the muon did not participate in the strong interaction at all. Hans Bethe and Robert Marshak suggested that the muon might be the decay product of the particle needed in the Yukawa theory, so the search continued. That search led to the discovery of the pion.