A measurement of the tau lifetime

The tau leptonlifetime is measured using four different methods with the DELPHIdetector. Three measurements using one prong decays are combined, accounting for correlations, resulting in ττ=298 ±7 ( stat.)±4 (syst.) fs while the decay length distribution of three prong decays gives ππ=298±13 ( stat)±(syst.) fs. The combined result is ττ=298±7 fs. The ratio of the Fermi coupling constant from tau decay relative to that from muon decay is found to be 0.985±0.013, compatible with leptonuniversality.