The tau lepton (often called the tau, tau particle, or occasionally the tauon; symbol ) is a negatively charged elementary particle with a lifetime of and a mass of (compared to for protons and for electrons). It has an associated antiparticle (the anti-tau) and neutrinos (the tau neutrino and tau antineutrino).
Since tau-like lepton number is conserved in weak decays, a tau neutrino is created when a tau lepton decays to a muon or electron.
The branching ratio of the common tau decays are:
The tau lepton was detected in a series of experiments between 1974 and 1977 by Martin Lewis Perl with his colleagues at the SLAC-LBL group Their equipment consisted of SLAC's then-new e+-e− colliding ring, called SPEAR, and the LBL magnetic detector. They could detect and distinguish between leptons, hadrons and photons. They did not detect the tau lepton directly, but rather discovered anomalous events:
"We have discovered 64 events of the form
for which we have no conventional explanation."
The need for at least 2 undetected particles was shown by the inability to conserve energy and momentum with only one. However, no other muons, electrons, photons, or hadrons were detected. It was proposed that this event was the production and subsequent decay of a new particle pair:
This was difficult to verify, because the energy to produce the pair is similar to the threshold for D meson production. Work done at DESY-Heidelberg, and with the Direct Electron Counter (DELCO) at SPEAR, subsequently established the mass and spin of the tau.
Martin Perl shared the 1995 Nobel Prize for physics with Frederick Reines. The latter was awarded his share of the prize for detecting the neutrino.