Measurement of the Michel parameters in leptonic $\tau$ decays using the OPAL detector at LEP

A measurement of the Michel parameters in tau decays is presented which involves a novel method to fit the energy spectra and energy- energy correlations of the charged decay leptons from tau -pair events produced in e/sup +/e/sup -/ collisions close to square root s =m/sub Z0/. The parameters rho /sub l/, xi /sub l/, ( xi delta )/sub l/ (with l=e, mu ) and nu /sub mu / have been extracted from a global likelihood fit of Monte Carlo generated events to the data set recorded with the OPAL detector in the years 1990-1995. If e- mu universality is assumed and inferring the tau polarization from neutral current data, the measured Michel parameters are rho 0.781+or-0.028+or-0.018, xi =0.98+or-0.22+or-0.10, xi delta =0.65+or-0.14+or-0.07 and eta =0.027+or-0.055+or-0.005, where the value of eta has been constrained using the published OPAL measurements of the leptonic branching ratios and the tau lifetime. Limits on the masses of new intermediate bosons and on non-standard couplings are obtained. It is explained how such limits can be derived from an investigation of the Michel parameter space. (16 refs).