What phenomenon earned the 2015 Nobel Prize in Physics for Takaaki Kajita and Arthur B. McDonald?

Neutrino oscillations earned the 2015 Nobel Prize in Physics for Takaaki Kajita and Arthur B. McDonald.

Neutrino oscillation is the phenomenon in which neutrinos change from one flavor to another as they travel. The three known flavors are electron, muon, and tau neutrinos. Oscillation requires neutrinos to have nonzero masses, overturning the older assumption that they were massless.

Kajita’s Super-Kamiokande observations showed that atmospheric muon neutrinos were disappearing in a way consistent with flavor change. McDonald’s Sudbury Neutrino Observatory showed that solar neutrinos had not vanished; many had changed into other flavors before detection.

The result solved the long-standing solar-neutrino problem and demonstrated physics beyond the simplest Standard Model. Neutrinos are electrically neutral and interact very weakly, making them difficult to detect. Oscillation probabilities depend on distance, energy, and the differences between neutrino mass states.