Measurement of the leptonic branching ratios of the τ lepton

The leptonic branching ratios of the tau lepton have been determined from data collected by the OPAL detector in 1991 and 1992. From a sample of 27196 e+e−→τ+τ− candidates we find 7322$$\tau \to e\nu \bar \nu$$ and 7941$$\tau \to \mu \nu \bar \nu$$ candidates. Using efficiency and background estimates determined from a study of Monte Carlo events and control samples of data, the branching ratios$$B(\tau \to e\nu \bar \nu )$$=(18.14±0.20±0.28)% and$$B(\tau \to \mu \nu \bar \nu )$$=(17.48±0.18±0.23)% have been obtained. These new results have been combined with the published results for the 1990 OPAL data to yield the following branching ratios for data taken between 1990 and 1992:$$\begin{gathered} B(\tau \to e\nu \bar \nu ) = (18.04 \pm 0.33)\% , \hfill \\ B(\tau \to \mu \nu \bar \nu ) = (17.36 \pm 0.27)\% . \hfill \\ \end{gathered} $$ These leptonic branching ratios are used with other properties of the muon and tau-lepton to test the universality of charged current leptonic couplings in these decays. The ratio$$R_\tau = B(\tau \to hadrons + \nu _\tau )/B(\tau \to e\bar \nu _e \nu _\tau )$$ is calculated using our measured values of the leptonic branching fractions of the tau and tau lifetime from which a value of αs(Q2=Mτ2) is extracted. The value of αs(Q2=MZ2) is obtained byQ2 evolution and agrees with the value from the Z0 line shape analysis.