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 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

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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

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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%.

Discovery

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The tau lepton was detected through 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 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, rather they discovered anomalous events:

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

There must have been undetected particles because not all energy from the initial 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:

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

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

See also

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• List of particles

External links

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• Nobel Prize in Physics 1995
• Perl's logbook showing tau lepton discovery
• A Tale of Three Papers gives the covers of the three original papers announcing the discovery.

References

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• M. L. Perl et al, "Evidence for Anomalous Lepton Production in e⁺-e^- Annihilation" Phys. Rev. Lett., 35, 1489 (1975)

Leptons

Tau | Tauon | Tau-Lepton | Tau (partícula) | Tauon | 타우온 | טאו (חלקיק) | Taonas | Tau-lepton | Tauon | Taon | Tau | Tauón