A precise measurement of the tau lepton lifetime

The tau lepton lifetime has been measured using three different methods with the DELPHI detector. Two measurements of one-prong decays are combined, accounting for correlations, giving a result of tau(tau) = 291.8 +/- 3.3 (stat.) +/- 2.0(sys.) fs while the decay length distribution of three-prong decays gives the result tau(tau) = 286.7 +/- 4.9 (stat.) +/- 3.3 (sys.) fs. Combining the results presented here with previous DELPHI measurements, we get tau(tau) = 291.4 +/- 3.0 fs and find that the ratio of the coupling constant for tau decay relative to that for muon decay is 0.990 +/- 0.009, compatible with lepton universality.