Tau lepton

The tau lepton (often called the tau or occasionally the tauon) is a negatively charged elementary particle with a lifetime of 3×10^−13 seconds and a high mass of 1777 MeV (compared to 939 MeV for protons and 0.511 MeV for electrons). It has an associated antiparticle (the anti-tau) and neutrinos (the tau neutrino and tau antineutrino).

Classification

The tau lepton belongs to the 3rd generation of leptons. It is the third generation counterpart of the electron (1st generation) and the muon (2nd generation). Like the electron and muon, the tau lepton appears to be pointlike; no structure has been detected, and if there is any, it would have to be on a scale of less than 10^−18 meters. Also, like the electron and muon, the tau has a spin of 1/2. The tau lepton and its antiparticle carry the same electric charges as the electron and positron, respectively.

Decay

The tau is the only lepton that can decay into hadrons - the other leptons do not have the necessary mass. Like the other decay modes of the tau lepton, the hadronic decay is through the weak interaction.

Since tau-like lepton number is conserved (only approximately, due to neutrino oscillations), a tau neutrino is created when a tau lepton decays to a muon or electron.

The branching ratio for the decay of a tau into an electron and neutrinos is about 18%, and similar for decay into a muon and neutrinos. The branching ratio for hadronic decay is about 64%.