Neutrino is an electrically neutral elementary lepton that interacts primarily through the weak interaction and gravity. Three established neutrino flavour types are associated with the electron, muon and tau charged leptons. Neutrinos have extremely small nonzero masses, as demonstrated by neutrino oscillation experiments, although the absolute masses and their ordering remain important subjects of experimental investigation. Neutrinos have spin one half and are therefore fermions. Their extremely weak interactions allow enormous numbers of neutrinos to pass through ordinary matter with very small probabilities of interaction.
Neutrinos were proposed in the early twentieth century to account for apparent missing energy and momentum in beta decay. Wolfgang Pauli proposed a neutral light particle to preserve conservation laws, and Enrico Fermi incorporated the idea into the theory of beta decay. The particle was eventually detected experimentally in the 1950s through reactions involving antineutrinos produced by nuclear reactors. The history of neutrinos illustrates how conservation principles can motivate predictions of previously unknown particles and how extraordinarily weak interactions can make direct detection difficult.
Neutrino flavour refers to the relationship between a neutrino state and the charged lepton produced with it in a weak interaction. The three known flavour states are electron neutrino, muon neutrino and tau neutrino. Quantum mechanically, flavour states are mixtures of neutrino mass eigenstates. As neutrinos propagate, the different mass components accumulate different phases, allowing the probability of detecting one flavour to change into another. This phenomenon is called neutrino oscillation and demonstrates that neutrinos have nonzero mass.
Neutrino oscillations are described using mixing angles, mass squared differences and propagation conditions. The oscillation probability depends on neutrino energy, travel distance and the differences between the squared masses of the participating mass eigenstates. In matter, coherent interactions with electrons can modify neutrino propagation and alter flavour conversion probabilities. This matter effect is particularly relevant for neutrinos travelling through dense environments such as the Sun or Earth and provides important information about neutrino properties.
Neutrinos are produced in numerous natural and artificial processes. The Sun produces vast numbers through nuclear fusion reactions, and radioactive beta decay produces electron antineutrinos. Cosmic ray interactions in the atmosphere create neutrinos and antineutrinos, while supernovae can release enormous bursts of neutrinos during stellar collapse. Nuclear reactors and particle accelerators can provide controlled sources. Because neutrinos interact so weakly, they can escape dense astrophysical environments and provide information that electromagnetic radiation cannot always carry directly from the same regions.
Neutrino detection requires large detectors because the interaction probability is extremely small. Detectors typically use large quantities of material and sensitive instrumentation to observe secondary charged particles or photons produced when a neutrino interacts. Different detection methods exploit different interaction channels and energy ranges. Background suppression is crucial because cosmic rays, radioactive processes and other interactions can imitate or obscure neutrino events. Large underground detectors are often used to reduce backgrounds from cosmic ray induced particles.
Neutrinos are important in particle physics and astrophysics because they provide evidence for physics beyond the minimal original formulation of the Standard Model. The observation of neutrino mass requires modification of the simplest massless neutrino description. Open questions include the absolute neutrino mass scale, the ordering of the mass eigenstates, the possible nature of neutrinos as distinct particles from their antiparticles, and the values of several mixing parameters. These questions are investigated through oscillation experiments, astrophysical observations and other specialised measurements.
Neutrino should not be confused with neutron. Despite similar names, they are fundamentally different particles. Neutron is a massive composite baryon containing quarks and gluons, whereas neutrino is an elementary lepton with no colour charge and extremely weak interactions. Neutrinos also should not be described as simply particles with zero mass because oscillation observations establish nonzero mass. Their tiny mass, neutral charge and weak interaction make them unusually penetrating particles and explain both their scientific value and the considerable experimental difficulty associated with detecting them.
