A measurement of the τ−→μ−ν̄μντ branching ratio

Abstract The τ − →μ − ν μ ν τ branching ratio has been measured using data collected from 1990 to 1995 by the OPAL detector at the LEP collider. The resulting value of B (τ − →μ − ν μ ν τ )=0.1734±0.0009( stat )±0.0006( syst ) has been used in conjunction with other OPAL measurements to test lepton universality, yielding the coupling constant ratios g μ / g e =1.0005±0.0044 and g τ / g e =1.0031±0.0048, in good agreement with the Standard Model prediction of unity. A value for the Michel parameter η =0.004±0.037 has also been determined and used to find a limit for the mass of the charged Higgs boson, m H ± >1.28tan β , in the Minimal Supersymmetric Standard Model.