A Measurement of the tau leptonic branching fractions

A sample of 25000 \Z\rightarrow\tt events collected by the DELPHI experiment at LEP in 1991 and 1992 is used to measure the leptonic branching fractions of the \tau lepton. The results are B(\TEL) = (17.51 \pm 0.39)\% and B(\tau\rightarrow \mu\nu\bar{\nu}) = (17.02 \pm 0.31)\%. The ratio of the muon and electron couplings to the weak charged current is measured to be g_{\mu}/g_{e} = 1.000 \pm 0.013, satisfying e-\mu universality. The average leptonic branching fraction corrected to the value for a massless lepton, assuming e-\mu universality, is found to be B(\tau\rightarrow l \nu\bar{\nu}) = (17.50 \pm 0.25)\%