neutrino oscillation
Neutrino oscillation is the astonishing fact that a neutrino can change its flavor as it travels. A neutrino born as an electron neutrino may, after flying some distance, reveal itself as a muon neutrino, and later perhaps swing back. It is as if a coin you tossed kept smoothly cycling between heads and tails in mid-air, the odds shifting with how far it had flown. This is the phenomenon that solved the solar neutrino problem.
The reason it happens is subtle and deeply quantum. The flavor a neutrino is born with is not a single, definite version of the particle but a blend of three underlying states that have slightly different masses. Because those mass states travel at very slightly different rates, the blend gets continuously rescrambled along the way, and the mixture that arrives can favor a different flavor than the one that left. The chance of finding each flavor rises and falls like a wave as the neutrino moves — hence 'oscillation' — with the rhythm set by the energy of the neutrino and the distance traveled.
Oscillation is profound because it can only happen if neutrinos have mass. A truly massless particle moves at the speed of light, its internal clock frozen, and would never change. So the very observation of oscillation — first nailed down for atmospheric neutrinos by Super-Kamiokande in 1998 and for solar neutrinos by SNO around 2002 — was the first hard proof that neutrinos are not massless, overturning a core assumption of the original Standard Model. It earned the 2015 Nobel Prize and remains one of the few confirmed cracks in that theory.
Super-Kamiokande compared muon neutrinos made by cosmic rays in the sky directly overhead with those made on the far side of the Earth. The ones that had travelled thousands of kilometres through the planet arrived depleted — they had had time to oscillate away into other flavors.
Neutrinos that travelled farther had oscillated more — the classic signature.
Oscillation reveals only the differences between neutrino masses, not the masses themselves; it proves at least two neutrinos are massive but cannot tell you how heavy any single one is.