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# Tau lepton

Composition: Tau Lepton Elementary particle Fermion Lepton Third Gravity, Electromagnetic, Weak Antitauon 1776.99±0.29 MeV/c2 −1 e None ½

The tau lepton (often called the tau, tau particle, or occasionally the tauon, symbol $\tau^{-}\ \,$ ) is a negatively charged elementary particle with a lifetime of 2.90×10−13 seconds and a mass of 1777 MeV/c2 (compared to 938 MeV/c2 for protons and 0.511 MeV/c2 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 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:

• 17.84% for decay into a tau neutrino, electron and electron neutrino
• 17.36% for decay into a tau neutrino, muon and muon neutrino

## Discovery

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 [1]. 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

$e^+ + e^- \rightarrow e^{\pm} + \mu^{\mp} + \geq \mbox{ 2 undetected particles}$

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:

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

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 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.