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# Tauon

The tauon (or tau lepton) is a negatively charged elementary particle with a lifetime of 3x10-13 s and a high mass of 1777 MeV (compared to 939 MeV for protons and 0.511 MeV for electrons). It has an associated antiparticle and neutrino. The antiparticle is called the antitauon.

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

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

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

The branching ratio for the decay of a tauon 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%.

## The Discovery of the Tauon

The tauon was detected in 1975 by Martin Perl with his colleagues at the SLAC-LBL group. Their equipment consisted of SLAC's 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 tauon directly, rather they discovered anomalous events:

$e^+ + e^- \rightarrow e^{\pm} + \mu^{\mp} + \mbox{undetected particles}$

There must have been undetected particles because not all energy from the intial collision could be accounted for in the final state. However, they did not detect any other muons or electrons, or any hadrons or photons. It was proposed that this event was the production and subsequent decay of a new particle pair:

$e^+ + e^- \rightarrow \tau^+ + \tau^- \rightarrow e^{\pm} + \mu^{\mp} + \mbox{four neutrinos}$

This was difficult to verify because the energy to produce the τ + τ - 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 tauon.

Martin L. Perl shared the 1995 Nobel Prize for physics with Frederick Reines (the latter was awarded his share of the prize for detecting the neutrino).