A neutrino is a subatomic particle that is very similar to an electron, but has no electrical charge and a very small mass, which might even be zero.

Both the muon and the tau, like the electron, have accompanying neutrinos, which are called the muon-neutrino and tau-neutrino. The three neutrino types appear to be distinct: For instance, when muon-neutrinos interact with a target, they will always produce muons, and never taus or electrons. In particle interactions, although electrons and electron-neutrinos can be created and destroyed, the sum of the number of electrons and electron-neutrinos is conserved. This fact leads to dividing the leptons into three families, each with a charged lepton and its accompanying neutrino.

To detect neutrinos, very large and very sensitive detectors are required. Typically, a low-energy neutrino will travel through many light-years of normal matter before interacting with anything. Consequently, all terrestrial neutrino experiments rely on measuring the tiny fraction of neutrinos that interact in reasonably sized detectors. For example, in the , a 1000 ton  solar-neutrino detector picks up about 1012 neutrinos each second. About 30 neutrinos per day are detected.

 first postulated the existance of the neutrino in 1930. At that time, a problem arose because it seemed that both energy and angular momentum were not conserved in . But Pauli pointed out that if a non-interacting, neutral particle--a neutrino--were emitted, one could recover the . The  of neutrinos did not occur until 1955, when  recorded anti-neutrinos emitted by a nuclear reactor.

FAINT EVIDENCE. In this solar neutrino event, 75 of the 9,600 light sensors in the detector observed a photon of light. Lines trace the path from the neutrino's impact with heavy water to the light sensors.