The tau (τ), also called the tau lepton, tau particle, or tauon, is an
and the antitau by
Tau leptons have a lifetime of 2.9×10−13 s and a
Because of their short lifetime, the range of the tau is mainly set by their decay length, which is too small for bremsstrahlung to be noticeable. Their penetrating power appears only at ultra-high velocity and energy (above
As with the case of the other charged leptons, the tau has an associated
The tau was anticipated in a 1971 paper by
colliding ring, called
We have discovered 64 events of the form
+ at least two undetected particles
for which we have no conventional explanation.
The need for at least two 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
The symbol τ was derived from the Greek τρίτον (triton, meaning "third" in English), since it was the third charged lepton discovered.