What phenomenon solved the solar neutrino problem by showing neutrinos change flavor?

What phenomenon solved the solar neutrino problem by showing neutrinos change flavor? Neutrino oscillation solved it.

The solar neutrino problem began when detectors found far fewer electron neutrinos arriving from the Sun than the standard solar model predicted. The deficit was first measured in the late 1960s and persisted through several experiments, raising questions about either the Sun's internal physics or the detectors themselves.

Neutrino oscillation provided the explanation: neutrinos have mass and can change between electron, muon, and tau flavors as they travel. Solar detectors were especially sensitive to electron neutrinos, so many neutrinos had not vanished; they had transformed into flavors those detectors could not efficiently see.

The Sudbury Neutrino Observatory in Canada supplied decisive evidence in the early 2000s by measuring solar neutrinos both by flavor and in total. Its results showed that the overall neutrino flux matched solar-model predictions, while the electron-neutrino portion was reduced. The related matter-enhanced MSW effect is important inside the Sun, but neutrino oscillation is the standard quiz answer.