The τ lepton

The τ (tau) is an elementary particle that can be thought of as a much heavier cousin of the electron, with a spin of ½. It belongs to the family of leptons, along with the electron, the muon, and the three neutrinos. Despite the origin of the word lepton (meaning fine, small, thin) the τ is very massive at 1776.82 MeV/c², which is nearly 3500 times the mass of the electron and around twice the mass of the proton.

The discovery of the τ

The τ was detected in a series of experiments between 1974 and 1977 by Martin Lewis Perl and his colleagues at the SLAC-LBL group. 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.

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.