Neutrino

Neutrinos are elementary particles with no electric charge. They are leptons, like electrons, but unlike charged leptons they do not respond to electromagnetic forces; in the Standard Model they interact through the weak force, and they are also affected by gravity. Their weak interactions make collisions with ordinary matter uncommon, so most neutrinos travel through the Earth without being stopped. A detector therefore records the secondary particles produced when a neutrino does interact, rather than seeing the neutrino directly.

Experiments have established three active neutrino flavors: electron, muon and tau. Each is associated with the correspondingly named charged lepton, and each has an antineutrino partner. Flavor describes how a neutrino participates in weak interactions; it is distinct from the particle’s mass state.

A neutrino produced or detected with a definite flavor is a quantum combination of neutrino mass states. As it propagates, those components evolve differently, so the probability of detecting a particular flavor changes. This phenomenon, called neutrino oscillation, allows an electron neutrino, for example, to later be detected as a muon or tau neutrino.

Observations of atmospheric neutrinos at Super-Kamiokande and solar neutrinos at the Sudbury Neutrino Observatory established flavor change. Oscillations require nonzero differences between neutrino mass states and therefore show that neutrinos are not massless. They constrain mass-squared differences and mixing, but do not by themselves give the absolute mass of each state. The result revealed a limitation of the simplest version of the Standard Model, which treated neutrinos as massless.

Neutrinos are created in a range of processes. Nuclear reactions in the Sun produce solar neutrinos; radioactive decays and nuclear reactors produce others; and interactions between cosmic rays and Earth’s atmosphere generate atmospheric neutrinos. Supernovae and other energetic astrophysical processes also release neutrinos.

Because the particles interact so weakly, neutrino experiments use large detectors and carefully suppress backgrounds. Some experiments use water or heavy water; IceCube instruments about a cubic kilometre of clear Antarctic ice with optical sensors. When a neutrino interacts, charged particles produced in the reaction can emit light that detectors register. The rarity of such events is a challenge, but it also lets neutrinos escape dense regions and carry information from places that light may not readily reveal.

Wolfgang Pauli proposed the neutrino in 1930 to account for energy and momentum that appeared to be missing in beta decay. Enrico Fermi later developed the theory of beta decay and popularized the particle’s name. Clyde Cowan and Frederick Reines detected reactor antineutrinos in 1956, confirming that neutrinos could be observed experimentally.

The discovery of oscillation in the late twentieth century established that neutrinos have mass. In 2015, the Nobel Prize in Physics recognized Takaaki Kajita and Arthur B. McDonald for key contributions to experiments demonstrating this change of flavor. Today, neutrino detectors support both particle-physics research and neutrino astronomy, which studies high-energy processes through the particles they emit.