A lepton (see also elementary particles) that exists in three forms, one in association with the electron, one with the muon, and one with the tau particle. Each form has its own antiparticle. The neutrino, which was postulated in 1930 to account for the ‘missing’ energy in beta decay, was identified tentatively in 1953 and, more definitely, in 1956. Neutrinos have no charge and very low masses, and travel very close to the speed of light. In some grand unified theories they are predicted to have nonzero masses. There is now much indirect evidence that neutrinos have nonzero masses, but the values have not been determined.

Neutrinos can change from one type to another type, this process being known as neutrino oscillation or neutrino mixing. This is thought to be the explanation for the solar neutrino problem. Since neutrino mixing was established in 1998 there has been a great deal of activity in measuring the parameters associated with this mixing and in relating these quantities to grand unified theories and family symmetry. It is thought that neutrino mixing should be associated with violation of CP invariance, but this has not yet been experimentally detected.

http://neutrino.phys.washington.edu/~superk/ The US website for the Super-Kamiokande neutrino observatory