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The neutrino is an elementary particle. It has spin 1/2 and so it is a fermion. Its mass is very small, although recent experiments have shown it to be above zero. It feels neither the strong nor the electromagnetic force, so it only interacts through the weak force and gravitation.

Because the neutrino only interacts weakly, when moving through ordinary matter its chance of interacting with it is very small. It would take a light year (10 Pm) of lead to block half the neutrinos flowing through it. Neutrino detectors therefore typically contain hundreds of tons of a material constructed so that a few atoms per day would interact with the incoming neutrinos.

The neutrino was first postulated in 1931 by Wolfgang Pauli to explain the continuous spectrum of beta decay, the decay of a neutron into a proton and an electron. Pauli theorized that an undetected particle was carrying away the observed difference between the energy and angular momentum of the initial and final particles. Because of their "ghostly" properties, the first experimental detection of neutrinos had to wait until about 25 years after they were first discussed. In 1956 Clyde Cowan, Frederick Reines, F. B. Harrison, H. W. Kruse, and A. D. McGuire published the article "Detection of the Free Neutrino: a Confirmation" in Science, a result that was rewarded with the 1995 Nobel Prize. The name neutrino was coined by Enrico Fermi as a word play on neutrone, the Italian name of the neutron particle. (Neutrone in Italian also means big and neutral, and neutrino means small and neutral.) In 1962 Leon M. Lederman, Melvin Schwartz and Jack Steinberger found out that more than one type of neutrino exists.

There are three known flavors of neutrinos: the electron neutrino ν_e, the muon neutrino ν_μ and the tau neutrino ν_τ, named after their partner leptons in the Standard Model. Experiments probing energy scales thousands of times larger have not revealed the existence of any additional neutrinos therefore it is widely believed that there are only three. Also the anecdotal correspondance between the six quarks in the Standard Model and the six leptons, including three neutrinos, provides a hint that serves as further proof to some that there can be only three neutrinos. Nevertheless, proof that there are only three neutrinos remains an elusive goal of particle physics.

In a phenomenon known as neutrino oscillation neutrinos are observed to take a well defined mass only after a corresponding measurement. In fact, before measurement the flavor of the neutrino is considered to exist as a superposition of the multiple, generation 1 through generation 3 states. Whereas conventional wisdom implies that there are separate neutrino and antineutrino states, observation implies otherwise. Although, it cannot presently be ruled out that neutrinos have zero mass, the phenomenon of neutrino oscillation provides a compelling solution to the solar neutrino problem and seems to be experimentally validated. A neutrino with zero mass would not exhibit observable oscillation and therefore would not provide an explaination to the solar neutrino problem.