The neutrino is the most inaccessible elementary particle we know of. It has no electrical charge and, as far as we know, no rest mass. The neutrino almost does not intereact with matter at all. It may travel through ten thousand billion kilometers (a light year) of lead without interacting.

To explain the conservation of energy in nuclear decays (beta decays), Pauli introduced, in 1930, the neutral particle, which Fermi, a couple of years later, gave the name "neutrino", the small neutron. "Today, I have done something a theoretician never should do," Pauli wrote. "I have introduced a new particle, which cannot be discovered." The neutrino is almost impossible to detect. It is the only known particle that only interacts weakly. Since it may travel through light years of lead without interacting, it is also most often passing through detctors without leaving a trace.

Frederick Reines did pioneering work during the 50's together with Clyde L. Cowan, Jr., which made it possible to show the existence of the anti-neutrino belonging to the electron.

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