A neutrino is an elementary particle with no electric charge and an exceptionally small mass. Neutrinos interact with matter through the weak interaction and gravity, making them difficult to detect despite their abundance. Three known varieties, called flavors, are associated with the electron, muon, and tau. Their ability to change flavor during propagation provides evidence that neutrinos have mass, requiring an extension of the minimal Standard Model of particle physics. (neutrinos.fnal.gov)
Properties and classification
Neutrinos belong to the lepton family, alongside the electron and its heavier relatives. They are spin-½ fermions. Unlike electrically charged particles, they do not undergo ordinary electromagnetic interactions; unlike quarks, they do not participate in the strong interaction. Their weak interactions occur through charged-current and neutral-current processes. A charged-current interaction can produce the charged lepton associated with the neutrino’s flavor, whereas a neutral-current interaction leaves a neutrino in the final state. (pdg.lbl.gov)
The three flavors are written νₑ, νμ, and ντ. Flavor describes how a neutrino is produced or detected through weak interactions, rather than identifying a unique mass. Antineutrinos are the corresponding antiparticles and differ in their weak-interaction behavior. Whether neutrinos and antineutrinos are fundamentally distinct particles remains an experimental question. Most neutrinos encountered near Earth pass through large quantities of matter without interacting, although the interaction probability increases with energy and very energetic neutrinos can be absorbed within Earth. (neutrinos.fnal.gov)
Prediction and discovery
Wolfgang Pauli proposed a neutral particle in 1930 to account for apparently missing energy and momentum in beta decay. The proposal preserved conservation laws by allowing an unseen particle to carry away part of the decay’s energy. Enrico Fermi subsequently incorporated the particle into his theory of beta decay and popularized the name neutrino, an Italian diminutive meaning “little neutral one.” (neutrinos.fnal.gov)
In 1956, Clyde Cowan and Frederick Reines established the detection of electron antineutrinos from a nuclear reactor. Their experiment identified inverse beta decay: an antineutrino interacts with a proton, producing a positron and a neutron. Positron annihilation followed by neutron capture supplied a distinctive coincidence signal. The muon neutrino was identified in 1962, demonstrating that neutrinos were not a single interchangeable species. Direct observation of tau-neutrino interactions was announced by Fermilab’s DONUT experiment in 2000. (neutrinos.fnal.gov)
Sources
Neutrinos arise in many nuclear and particle processes. In the Sun, nuclear fusion reactions produce electron neutrinos. Because neutrinos escape the solar interior comparatively readily, their detection provides information about reactions in the core, rather than merely about the visible surface. Early measurements found fewer solar electron neutrinos than predicted, establishing the long-standing solar neutrino problem. (neutrinos.fnal.gov)
Other natural sources include interactions of cosmic radiation with Earth’s atmosphere and radioactive decay inside the planet. Electron antineutrinos from uranium and thorium decay chains, commonly called geoneutrinos, provide information about radioactive materials within Earth. In reactors, beta decays of nuclear-fission products generate intense electron-antineutrino fluxes. (nobelprize.org)
Particle accelerators produce controlled neutrino beams by directing protons onto a target. The resulting short-lived particles, including pions, decay into neutrinos and other particles. These beams allow experiments to compare neutrino behavior over known distances. Astronomical sources also generate neutrinos: the detection of a burst from supernova SN 1987A provided direct evidence from stellar core collapse, while IceCube has detected high-energy neutrinos of astrophysical origin. (neutrinos.fnal.gov)
Oscillation and mass
Neutrino oscillation occurs because flavor states are quantum superpositions of three mass states, conventionally labeled ν₁, ν₂, and ν₃. The components evolve differently during propagation, changing the probability that a neutrino will be detected in a particular flavor. The relationship between flavor and mass states is described by the Pontecorvo–Maki–Nakagawa–Sakata mixing matrix. Oscillation probabilities depend on mixing parameters, differences between squared masses, neutrino energy, travel distance, and interactions with surrounding matter. (neutrinos.fnal.gov)
Super-Kamiokande reported compelling atmospheric-neutrino evidence in 1998. Results from the Sudbury Neutrino Observatory in 2001 showed that solar electron neutrinos changed into other active flavors, resolving the solar deficit. These experimental contributions were recognized by the 2015 Nobel Prize in Physics, awarded to Takaaki Kajita and Arthur B. McDonald. (neutrinos.fnal.gov)
Oscillations establish mass differences, not the absolute mass scale. Complementary measurements examine beta-decay spectra. A KATRIN result published in 2025 set an upper limit of 0.45 eV/c², at 90 percent confidence, on the effective electron-antineutrino mass. This is a bound on a weighted combination of masses, not a measurement of each individual mass. (arxiv.org)
Detection and unresolved questions
Neutrino detectors register the products of rare interactions rather than observing neutrinos directly. Large target volumes and background suppression are therefore essential. Water and ice detectors measure Cherenkov radiation emitted by secondary charged particles moving faster than light travels in that medium—not faster than light in vacuum. IceCube reconstructs incoming directions and energies from the timing and spatial distribution of these signals. (neutrinos.fnal.gov)
Major research questions concern the origin of neutrino masses and whether neutrinos are their own antiparticles. Searches for neutrinoless double-beta decay test the latter possibility. Experiments also investigate possible differences between neutrino and antineutrino oscillations. Hypothetical sterile neutrinos, which lack ordinary weak interactions, would represent additional states beyond the three established active flavors; their existence has not been definitively demonstrated. (pdg.lbl.gov)