What is neutrino oscillation theory?
Neutrino oscillation is the quantum mechanical phenomenon where neutrinos change from one flavor—electron, muon, or tau—to another as they travel through space. This occurs because neutrinos have very small but nonzero masses, and the flavor states are superpositions of mass states. Our Super-Kamiokande experiment provided the first definitive evidence for this effect by observing a deficit of muon neutrinos from cosmic rays in the atmosphere. The data showed that the number of muon neutrinos arriving from below—having traveled through the Earth—was about half that from above, consistent with oscillations. This was a profound result: it meant neutrinos are not massless as the Standard Model of particle physics had assumed. We had to let the data speak, and it told us that the universe is more subtle than we imagined.
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