Neutrino mass

Previously thought to be massless, experiments showed that neutrinos have small masses. If so, the three neutrinos – electron-neutrino, muon-neutrino and tauon-neutrino might be capable of oscillating, which would explain why the sun emits only a third the number of neutrinos it ought to.
 Frederick Reines United States 1980 Neutrino mass

Previously thought to be massless, experiments showed that neutrinos have small masses. If so, the three neutrinos – electron-neutrino, muon-neutrino and tauon-neutrino might be capable of oscillating, which would explain why the sun emits only a third the number of neutrinos it ought to.