A precise measurement of the τ lepton lifetime

Abstract Three different techniques are used to measure the mean decay length of the τ lepton with a high precision vertex detector in a sample of 11 800 τ pairs coming from Z decays, collected in1991 by ALEPH at LEP. Events in which both τ's decay into one charged track are analyzed using two largely independent methods. Displaced vertices in three-prong decays yield another independent measurement. The derived lifetime is 295.5 ± 5.9 ± 3.1 fs, using m τ = 1777.1 ± 0.5 MeV/ c 2 . Including previous (1989–1990) ALEPH measurements, the combined τ lifetime is 294.7 ± 5.4 ± 3.0 fs.