Neutrinos

Neutrinos are elementary fermions in the lepton family. They are electrically neutral, have very small rest masses, and interact with ordinary matter only via the weak nuclear force and gravity. Because of their weak interactions, neutrinos can pass through light-years of material with only a small chance of interaction.

Three known flavors correspond to the charged leptons: electron, muon, and tau. Neutrinos are produced and detected in weak interactions as flavor eigenstates, but they propagate as a quantum superposition of mass eigenstates. This leads to neutrino oscillations, where the observed flavor changes with distance and energy. Oscillations imply nonzero masses and mixing, described by the Pontecorvo–Maki–Nakagawa–Sakata (PMNS) matrix.

Neutrinos originate from the sun and other stars, radioactive decay, cosmic-ray interactions in the atmosphere, nuclear reactors, particle accelerators, and stellar collapses such as supernovae. They also permeate the universe as a relic background from the early universe known as the cosmic neutrino background.

Detectors rely on huge targets to capture rare interactions, via Cherenkov radiation in water or ice, scintillation in organic or inorganic media, or radiochemical methods. Notable experiments include Super-Kamiokande, SNO, KamLAND, Borexino, IceCube, and Daya Bay, among others. These experiments have established the existence of neutrino flavor change, measured mixing angles and mass-squared differences, and constrained absolute masses and possible Majorana nature.

Because neutrinos interact so rarely, they provide unique insights into stellar processes, the evolution of the universe, and physics beyond the Standard Model.