Neutrino Oscillations: Evidence, Parameters and Open Questions in Neutrino Physics

Abstract

Neutrinos change flavour as they propagate, and this behaviour requires them to carry non-zero mass, a property the original Standard Model did not provide. This review sets out the quantum-mechanical description of flavour mixing, summarises the observations that established it, and assesses what remains unresolved. Atmospheric measurements at Super-Kamiokande, the neutral-current measurement of the solar flux at the Sudbury Neutrino Observatory, and the reactor antineutrino deficit recorded by KamLAND together fixed the two dominant mass-squared splittings and their mixing angles. Short-baseline reactor experiments later measured the third mixing angle with percent-level precision, which opened the search for leptonic charge-parity violation now pursued by long-baseline accelerator beams. Global analyses constrain five of the six oscillation parameters to a few percent, while the sign of the larger splitting, the octant of the atmospheric angle, and the value of the charge-parity phase remain uncertain. Oscillation data fix only mass differences, so the absolute scale is bounded instead by tritium endpoint spectroscopy and by cosmological structure. Whether the neutrino is its own antiparticle is being tested through searches for neutrinoless double beta decay. Experiments now under construction should settle the mass ordering and provide a first measurement of the phase.