A measurement of the tau lepton to muon branching ratio

The 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&parl0;t^-→m^- n&d1;mn t&parr0;=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 $gm/ge$ = 1.0005 ± 0.0044 and $gt/ge$ = 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± > 1.28 tan β, in the Minimal Supersymmetric Standard Model.