nootrēˈnō [Ital.,=little neutral (particle)], elementary particle with no electric charge and a very small or zero mass emitted during the decay of certain other particles. The neutrino was first postulated in 1930 by Wolfgang Pauli in order to maintain the law of conservation of energy during beta decay (see conservation laws; radioactivity). When a radioactive nucleus emits a beta particle (electron), the electron may have any energy from zero up to a certain maximum. Pauli suggested that when the electron has less than the maximum possible value, the remaining energy is carried away by an undetected particle, the neutrino. Its charge must be zero because a charged particle would easily be detected. Moreover, if it were charged, the law of conservation of charge would be violated during beta decay. The neutrino was named by Enrico Fermi. Further studies showed that the neutrino was also necessary to maintain the conservation laws of momentum and spin. Like the electron, the neutrino is a lepton; it participates only in the weak decay of nuclear particles and has no role in the strong force binding nuclei together. Neutrinos are also emitted when a pion decays into a muon and in the decays of a number of other elementary particles. Neutrinos are stable and can be absorbed only by the same weak interactions through which they are created; an energetic neutrino can induce the reverse of the decay that produced it.

The neutrino was not detected directly until 1956, when American physicists Frederick Reines and Clyde L. Cowan recognized them by their impact with subnuclear particles in mineral water. In 1962 it was found that the neutrino associated with the muon (the muon neutrino) is distinct from that associated with the electron (the electron neutrino). A third type, the tau neutrino, associated with the tau particle, was identiified in the mid-1970s but not detected until 2000. Each type of neutrino has its own antiparticle.

According to the so-called oscillation theory, neutrinos can change from one type to another as they travel through space. If the theory is valid, to make these transformations neutrinos would have to have a tiny amount of mass. In 1995 and again in 1996 a team at the Los Alamos National Laboratory claimed to have detected the oscillation of muon antineutrinos into electron antineutrinos. In 2001 researchers at the Sudbury Neutron Observatory in Ontario, Canada, found evidence that the electron neutrinos produced by fusion reactions within the sun can change into tau and muon neutrinos as they travel to the earth. If these discoveries are validated, it would be a clear indication that neutrinos are not massless. This is significant because of its implications for the composition and evolution of the universe, including the rate of the universe's expansion. Neutrinos would exert gravitational effects and thus could account for some of the dark matter in the universe.

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