neutrino, type of fundamental elementary subatomic particle with no electric charge, very little or no mass, and one-half 12 unit of spin. Neutrinos belong to the family of particles called leptons, which are not subject to the strong nuclear force. Rather, neutrinos are subject to the weak force that underlies certain processes of radioactive decay. There are three types of neutrino, each associated with a charged lepton—ilepton—i.e., the electron, the muon, and the tau—and therefore given the corresponding names electron-neutrino, muon-neutrino, and tau-neutrino. Each type of neutrino also has an antimatter component, called an antineutrino; the term neutrino is sometimes used in a general sense to refer to both the neutrino and its antiparticle.

The basic properties of the electron-neutrino was proposed neutrino—no electric charge and little mass—were predicted in 1930 by the Austrian physicist Wolfgang Pauli to explain the apparent loss of energy in the process of radioactive beta decay, a form of radioactivity. The Italian-born physicist Enrico Fermi further elaborated (1934) the proposal theory of beta decay and gave the “ghost” 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 Despite such predictions, neutrinos were not detected experimentally for 20 years, owing to the weakness of their interactions with matter. Because they are not electrically charged, neutrinos do not experience the electromagnetic force and thus do not cause ionization of matter. Furthermore, they react with matter only through the very weak interaction . Neutrinos do not cause ionization, because they are not electrically chargedof the weak force. Neutrinos are therefore the most penetrating of subatomic particles, capable of passing through an enormous number of atoms without causing any reaction. Only 1 in 10 billion of these particles, traveling through matter for a distance equal to the Earth’s diameter, reacts with a proton or a neutron. Electron-neutrinos were first experimentally observed Finally, 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. team of American physicists led by Frederick Reines reported the discovery of the electron-antineutrino. In their experiments antineutrinos emitted in a nuclear reactor were allowed to react with protons to produce neutrons and positrons. The unique (and rare) energy signatures of the fates of these latter by-products provided the evidence for the existence of the electron-antineutrino.



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 002 percent that of the electron. There is, however, For many years it seemed that neutrinos’ masses might be exactly zero, although there was 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 why this should be so. Then in 2002 the Sudbury Neutrino Observatory (SNO), in Ontario, Canada, found the first direct evidence that electron-neutrinos emitted by nuclear reactions in the core of the Sun (the so-called mystery of the missing solar neutrinos) may well be explicable change type as they travel through the Sun. Such neutrino “oscillations” are possible only if one or more of the neutrino types has a some small mass. See also leptonStudies of neutrinos produced in the interactions of cosmic rays in the Earth’s atmosphere also indicate that neutrinos may have mass, but further experiments are needed to understand the exact masses involved.