Neutrino oscillation

A neutrino can be produced as an electron, muon, or tau neutrino and later be detected as a different flavor. The standard explanation is that flavor states are mixtures of different neutrino mass states, so the quantum phases change as the particle travels.

Early detectors saw fewer electron neutrinos from the Sun than solar models predicted. Neutrino oscillation explained that some solar electron neutrinos arrived at Earth as muon or tau neutrinos instead.

Super-Kamiokande in Japan provided strong evidence from atmospheric neutrinos in the late 1990s. The Sudbury Neutrino Observatory in Canada showed that the total solar neutrino flux matched expectations once all flavors were counted.

Oscillation requires different neutrino mass states to accumulate different quantum phases during travel. In the original Standard Model neutrinos were massless, so oscillation was direct evidence that the model needed extension.