The neutrino was originally proposed as a solution to a serious problem observed in the radioactive decays of certain atoms. In a process known as nuclear beta decay, one of the neutrons in the decaying nucleus is transformed into a proton, accompanied by the emission of an electron. Somehow energy seems to have been destroyed. Later, the existence of neutrinos was directly confirmed by Frederick Reines and Clyde L. Cowan, Jr., who detected the inverse reaction, in which a neutrino is captured by a proton, producing a neutron. Subsequent experiments demonstrated that there are three different kinds of neutrinos, associated not only with electrons but also with muons and tau particles, more massive cousins of the electron.
Neutrinos have at most a tiny mass and only feebly interact with ordinary matter.
That situation is starting to change. Strong indications of neutrino oscillations, which can occur only if neutrinos have a mass, are now seen in the observations of solar neutrinos, in studies of atmospheric neutrinos and in some recent particle accelerator experiments. At about the time when Pauli postulated the existence of the neutrino, other researchers deduced the basic mechanism by which the sun shines. The sun's energy derives from nuclear fusion reactions taking place near its center. Most of this energy emerges as light and other forms of electromagnetic radiation from the solar surface, but about 3 percent of the energy is emitted as neutrinos.