Wolfgang Pauli proposed the neutrino in 1930 to explain missing energy in beta decay.
In beta decay, a neutron transforms into a proton while emitting an electron. Early measurements suggested that the electron carried a continuous range of energies, apparently conflicting with conservation of energy and momentum. Pauli suggested an unseen, electrically neutral particle was emitted as well, sharing the available energy.
Enrico Fermi later named this particle the neutrino, meaning “little neutral one,” and incorporated it into his 1934 theory of beta decay. The neutrino was not directly detected until 1956, when Clyde Cowan and Frederick Reines observed antineutrinos from a nuclear reactor.
Neutrinos are leptons and interact primarily through the weak nuclear force and gravity, making them exceptionally difficult to detect. They are not the same as neutrons: neutrons are composite particles found in atomic nuclei, whereas neutrinos are elementary particles. Three known neutrino flavors are associated with the electron, muon, and tau.