tau lepton

The tau lepton is the third charged lepton (in order of increasing mass), with an electric charge of -1, a spin of ½, and a mass of 1.784 GeV. It is unstable with a lifetime of 3 × 10-13 sec. The tau lepton was first observed in 1975 in experiments carried out at SLAC (Stanford Linear Accelerator), having already been predicted in theory.