Study of Tau lepton decays with ALEPH detector at LEP

Using the data accumulated at LEP in 1989 and 1990 with the ALEPH detector, the inclusive and exclusive branching ratios of the τ lepton have been measured assuming lepton universality in Z0 decays. The inclusive branching fractions for the τ decay into one, three, and five charged particles have been determined to be (85.45±0.97)%, (14.35±0.48)%, and (0.10± 0.05)% respectively, in agreement with the world averages. New undetected decay modes are determined to have a branching fraction of less than 2.1% at 95% CL. The measured branching ratios for quasi-exclusive channels are slightly larger than, but consistent with the world averages, except for the modes τ→3 hadrons+ντ and τ→hadron+2π0ντ, which are significantly larger. These latter branching ratios have been found to be (9.5±0.7)% and (10.2±1.1)%, respectively. The sum of all the measured quasi-exclusive branching ratios is (100.4±1.8)%. A fully exclusive analysis of modes with neutral pions shows no evidence for new photonic decay modes with a branching fraction limit of 3.4% at 95% CL