NEUTRINO

type of fundamental particle with no electric charge, little or no mass, and one-half unit of spin. Neutrinos belong to the family of particles called leptons, which are not subject to the strong nuclear force. There are three types of neutrino, each associated with a charged leptoni.e., the electron, muon, and tau. The electron-neutrino was proposed in 1930 by the Austrian physicist Wolfgang Pauli to explain the apparent loss of energy in the process of beta decay, a form of radioactivity. The Italian-born physicist Enrico Fermi further elaborated (1934) the proposal and gave the particle its name. An electron-neutrino is emitted along with a positron in positive beta decay, while an electron-antineutrino is emitted with an electron in negative beta decay. Neutrinos are the most penetrating of subatomic particles because they react with matter only through the weak interaction. Neutrinos do not cause ionization, because they are not electrically charged. Only 1 in 10 billion, traveling through matter a distance equal to the Earth's diameter, reacts with a proton or neutron. Electron-neutrinos were first experimentally observed in 1956, when a beam of antineutrinos from a nuclear reactor produced neutrons and positrons by reacting with protons. Another type of neutrino, produced when pi mesons (pions) decay, was conclusively shown (1962) to be a different species: the muon-neutrino. Although they are as unreactive as the other neutrinos, muon-neutrinos were found to produce muons but never electrons when they react with protons and neutrons. The American physicists Leon Lederman, Melvin Schwartz, and Jack Steinberger received the 1988 Nobel Prize for Physics for having established the identity of muon neutrinos. In the mid-1970s, particle physicists discovered yet another variety of charged lepton, the tau. A tau-neutrino and tau-antineutrino are associated with this third charged lepton. All types of neutrino have masses much smaller than those of their charged partners, if indeed they have any mass at all. For example, experiments show that the mass of the electron-neutrino must be less than 0.0004 that of the electron. There is, however, no compelling theoretical reason for the mass of the neutrino to be exactly zero. Indeed, the shortfall in the number of neutrinos detected on Earth from the nuclear reactions in the core of the Sun (the so-called mystery of the missing solar neutrinos) may well be explicable if one or more of the neutrino types has a small mass. See also lepton.