Overview
The tau neutrino is the neutral lepton associated with the tau charged lepton. It is an active neutrino flavor in the Standard Model and, like other neutrinos, it carries no net electric charge. The tau neutrino is commonly referred to as the tauon neutrino and belongs to the third generation of leptons; see general information on an elementary particle and the tau connection at tau.
Properties and interactions
Tau neutrinos interact only through the weak nuclear force (and gravity at an extremely small level). In charged-current interactions a tau neutrino can produce a tau lepton if the available energy exceeds the production threshold; neutral-current interactions proceed via Z-boson exchange. As with other flavors, the tau neutrino state is a superposition of mass eigenstates, which leads to flavor change or neutrino oscillation. Its electrically neutral nature is summarized under no net electric charge.
Production, oscillations and detection
Sources of tau neutrinos include decays of hadrons that produce taus, neutrino oscillations from other flavors in flight, and high-energy astrophysical processes. Detecting tau neutrinos is challenging because a charged-current interaction produces a short-lived tau lepton that decays quickly, often within submillimeter to meter scales depending on energy. Experiments identify tau appearance by characteristic decay topologies such as short "kink" tracks, displaced vertices, or multi-prong decay signatures. Direct observations and appearance measurements have been reported by accelerator and emulsion experiments and by long-baseline oscillation searches.
Astrophysical importance and experiments
High-energy neutrino telescopes aim to detect tau neutrinos from cosmic sources because flavor ratios measured at Earth can test production mechanisms and propagation effects. Observing tau neutrinos at very high energies helps discriminate source models and probe neutrino mixing over astronomical distances. Laboratory and long-baseline experiments study tau appearance to constrain the elements of the lepton mixing (PMNS) matrix and to search for possible CP violation in the lepton sector.
Context and open questions
- The tau neutrino is part of the third generation of leptons, distinct from electron and muon neutrinos by its charged partner.
- Key open issues in neutrino physics that involve tau neutrinos include the absolute neutrino mass scale, the ordering of mass eigenstates, and whether additional sterile neutrino species or nonstandard interactions exist.
- Ongoing and planned experiments continue to refine appearance measurements, cross sections, and the role of tau neutrinos in astrophysical fluxes; for background on particle categories see elementary particle summaries and charge properties at no net electric charge.