Tauon

Tauon, more commonly called the tau lepton, is a fundamental particle of the Standard Model. It is a third-generation charged lepton with electric charge -1, spin 1/2, and no color charge, meaning it does not participate in strong interactions. The tau is represented by the symbol τ and is distinct from the lighter electron and muon, though it participates in the same electroweak family.

The tau is much heavier than the electron and muon, with a mass of about 1776.8 MeV/c^2. It is unstable and decays via the weak interaction with a mean lifetime of roughly 2.9 × 10^-13 seconds. Because of this short lifetime, taus are produced only in high-energy processes and are observed through their decay products.

Tau decays populate both leptonic and hadronic final states. About 17–18% of tau decays proceed leptonically to an electron plus an electron-neutrino and a tau-neutrino, and a similar fraction to a muon plus a muon-neutrino and a tau-neutrino. The remaining decays are hadronic, producing one or more hadrons (often pions) plus a tau neutrino, and these account for the larger share of decays.

Historically, the tau was discovered in 1975 by Martin Perl and collaborators at the SLAC National Accelerator Laboratory. The term tauon appears in older literature as an alternative name for the tau lepton; in modern usage, tau lepton or τ is standard.

In experimental physics, the tau lepton is important for tests of lepton universality, measurements of weak interactions, and studies of hadronic tau decays that illuminate QCD dynamics at low energies.