neutrino W. Pauli (in 1930) proposed the existence of neutrino, a massless neutral particle with spin 1/2 , to account for the conservation of energy and momentum in beta decay*. It remained undetected for a long time, because of its extremely low cross section of interaction with matter. It was finally detected in 1956, using a nuclear reactor, which produces a large flux of neutrinos.

The weak decay of pions* also produce neutrinos along with muons,

p ^- ® m ^- + n m

The neutrinos accompanying muons have been found to be different from those accompanying electrons (as in beta decay ). It has been proved conclusively by the famous ‘two neutrino experiment’ conducted by Lederman and co workers that neutrinos, created together with muons in decay of pions can only produce muons and no electrons. With the discovery of tau lepton in 1976, the expected number of neutrino types has gone up to three, namely n _e , n m , and n t .

The neutrinos are classified under leptons and have spin 1/2. So far only upper limits of their masses have been established ( see elementary particles Table EII ).