Tau lepton mass

Martin Perl's group found the tau at SLAC between 1974 and 1977 in events producing an electron and a muon with missing energy, establishing a third generation of matter before anyone had seen the bottom or top quark. At 1776.86 MeV/c² it outweighs the proton, which is why it is the only lepton heavy enough to decay into hadrons — about 65 % of taus end as pions rather than as an electron or muon.

Its mean life is 2.9 × 10⁻¹³ s, so even at relativistic speeds a tau travels well under a millimetre before decaying, and detectors identify it by a displaced vertex rather than a track. Note that the tau's mass is known to only about 7 parts per 100 000 — feeble by CODATA standards but limited by threshold-scan statistics at electron-positron colliders, not by any technique that could be pushed much further.