Neutrino Oscillations

Neutrinos have mass

First published: Y. Fukuda et al. (Super-Kamiokande), "Evidence for Oscillation of Atmospheric Neutrinos", *Phys. Rev. Lett.* 81 (1998): 1562–1567; Q. R. Ahmad et al. (SNO), "Direct Evidence for Neutrino Flavor Transformation from Neutral-Current Interactions in the Sudbury Neutrino Observatory", *Phys. Rev. Lett.* 89 (2002): 011301.

Neutrinos born as one flavour arrive at Earth as another. Neutrinos must have mass — a long-standing solar puzzle resolved.

Solar models predicted a certain flux of electron neutrinos from the Sun, but for decades detectors observed only about a third of that flux ("solar neutrino problem"). Two experiments resolved the puzzle. Super-Kamiokande (1998) observed that atmospheric muon neutrinos disappear in transit from creation to detection, with a dependence on path length consistent with oscillation. SNO (2001–2002) used heavy water to detect both electron-specific and flavour-blind reactions, showing that the total solar neutrino flux matched solar-model predictions — the missing electron neutrinos had simply *changed flavour* en route. Flavour oscillation requires that neutrinos have non-zero mass (their mass eigenstates differ from their flavour eigenstates), forcing the Standard Model to be extended. The discovery was recognised with the 2015 Nobel Prize.