A neutrino is an elementary particle that usually travels close to the speed of light and is able to pass through ordinary matter almost unaffected. Although Neutrinos are similar to the more familiar electron, they have one important difference: neutrinos do not carry electric charge. This means neutrinos are electrically neutral; thus they are not affected by the electromagnetic forces which act on electrons. A neutrino also differentiates from a neutron by the fact that it is a massless particle.
The neutrino was first postulated in 1930 by Wolfgang Pauli to explain why the electrons in beta decay were not emitted with the full reaction energy of the nuclear transition. The apparent violation of conservation of energy and momentum was most easily avoided by postulating another particle. Enrico Fermi called the particle a neutrino and developed a theory of beta decay based on it, but it was not experimentally observed until 1956. This elusive particle, with no charge and almost no mass, could penetrate vast thicknesses of material without interaction.