Neutrinos are elementary particles that carry no electric charge and have very small masses. They belong to the lepton family in the Standard Model of particle physics. Because they do not feel the electromagnetic force or the strong nuclear force, neutrinos can pass through large amounts of matter with little chance of interacting.
Three types of neutrino
The Standard Model contains three neutrino flavors: the electron neutrino, muon neutrino, and tau neutrino. Each is paired with a charged lepton—the electron, muon, or tau. Neutrinos participate in the weak interaction, the force involved in processes such as beta decay.
Where neutrinos come from
Neutrinos are produced in many energetic or nuclear processes. The Sun emits vast numbers as fusion reactions convert hydrogen into helium. Nuclear reactors produce antineutrinos. Cosmic-ray collisions in Earth’s atmosphere create neutrinos, and violent astrophysical events such as supernovae can release enormous bursts. Particle accelerators can create controlled neutrino beams for experiments.
Why are neutrinos so hard to detect?
A neutrino can cross matter without leaving a measurable trace because its probability of interacting is extremely small. Experiments compensate by using very large detector volumes, intense neutrino sources, and shielding from other particles. Some detectors are placed deep underground, under ice, or underwater to reduce background signals.
Researchers do not usually “see” the neutrino itself. They detect the particles and light produced in the rare event when a neutrino interacts with matter in or near the detector.
Neutrino oscillation changed particle physics
Experiments showed that neutrinos can change flavor as they travel, a phenomenon called neutrino oscillation. Oscillation requires differences among neutrino mass states, establishing that neutrinos have nonzero mass. That result is important because the simplest form of the Standard Model originally treated neutrinos as massless.
What scientists still want to know
Open questions include the absolute scale of neutrino masses, the ordering of those masses, details of how neutrinos and antineutrinos behave, and whether neutrinos are their own antiparticles. Answers could connect particle physics with questions about the evolution of the universe.
Neutrinos are leptons, a different family from quarks, which combine to form particles such as protons and neutrons.
