Not to be confused with the τ⁺ of the τ–θ puzzle, which is now identified as kaon.

Tauon
Composition:	Elementary particle
Statistical behavior:	Fermion
Group:	Lepton
Generation:	Third
Interaction:	Gravity, Electromagnetic, Weak
Symbol(s):	τ−
Antiparticle:	Antitauon (τ+)
Discovered:	Martin Lewis Perl et al. (1975)[1][2]
Mass:	1,776.84±0.17 MeV/c2
Electric charge:	−1 e
Color charge:	None
Spin:	1⁄2

The tauon (from the Greek letter tau (τ) used to represent it), also called the tau, tau lepton, or tau particle, is an elementary particle similar to the electron, with negative electric charge and a spin of ¹⁄₂. Together with the electron, the muon, and the three neutrinos, it is classified as a lepton. Like all elementary particles, the tauon has a corresponding antiparticle of opposite charge but equal mass and spin: the antitauon (also called the a positive tauon). Tauons are denoted by τ⁻ and antitauons by τ⁺.

Tauons have a lifetime of 2.9×10⁻¹³ s and a mass of 1,777 MeV/c² (compared to 105.7 MeV/c² for muons and 0.511 MeV/c² for electrons). Since their interactions are very similar to those of the electron, a tauon can be thought of as a much heavier version of the electron. Due to their greater mass, tauons do not emit as much bremsstrahlung radiation; consequently they are highly penetrating, much more so than electrons. However, at standard energies, their decay length is too small for bremsstrahlung to be noticeable.

As with the case of the other charged leptons, the tauon has an associated tauon neutrino. Tauon neutrinos are denoted by ν_τ.

Contents 1 History 2 Tauon decay 3 See also 4 References 5 External links

[edit] History

The tauon was detected in a series of experiments between 1974 and 1977 by Martin Lewis Perl with his colleagues at the SLAC-LBL group.^[2] 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 tauon directly, but rather discovered anomalous events:

"We have discovered 64 events of the form

e⁺ + e⁻ → e^± + μ^∓ + at least 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⁻ → τ⁺ + τ⁻ → e^± + μ^∓ + 4ν

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-Hamburg, and with the Direct Electron Counter (DELCO) at SPEAR, subsequently established the mass and spin of the tau.

The symbol τ was derived from the Greek τριτον (triton, meaning "third" in English), since it was the third charged lepton discovered.^[3]

Martin Perl shared the 1995 Nobel Prize in Physics with Frederick Reines. The latter was awarded his share of the prize for experimental discovery of the neutrino.

[edit] Tauon decay

The tauon 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 tauon, the hadronic decay is through the weak interaction.

Since the tauonic lepton number is conserved in weak decays, a tauon neutrino is created when a tauon decays to a muon or electron.

The branching ratio of the common purely leptonic tauon decays are:

• 17.85% for decay into a tauon neutrino, electron and electron antineutrino;
• 17.36% for decay into a tauon neutrino, muon and muon antineutrino.

[edit] See also

• Tauonium
• Tauon spin spectroscopy
• Tauon-catalyzed fusion
• List of particles

[edit] References

• C. Amsler et al. (2008). "Review of Particle Physics". Physics Letters B 667: 1. doi:10.1016/j.physletb.2008.07.018. http://pdg.lbl.gov/. 

[edit] External links

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

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