ABSTRACT
Following the Reines-Cowan experiment, physicists in the late 1950s were faced with two further problems concerning the neutrino. The first—the question of whether the neutrino and antineutrino were identical or distinct particles—was thought, wrongly, to have been answered. The second question was whether there was only one kind of neutrino. It was clear that neutrinos were producing muons. The muon and its neutrino had already added complexity to the system of elementary particles. Measuring, or setting a limit on, the mass of the tau neutrino is even more difficult than doing so for the electron neutrino or muon neutrino. There was seemingly convincing evidence that the neutrino and the antineutrino were distinct particles. The discovery of the muon neutrino led physicists to formulate two separate conservation laws, one for electron family members and one for muon family members. Electrons, muons, and their respective neutrinos were called leptons, or light particles.
