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

Abstract

A measurement of the Michel parameters in τ decays is presented [1] which involves a novel method to fit the energy spectra and energy-energy correlations of the charged decay leptons from τ-pair events produced in e +e − collisions close to √ s = m Z 0 . The parameters ϱ l , Ξ l , ( Ξδ) l (with l = e, μ) and η μ 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-μ universality is assumed and inferring the τ polarization from neutral current data, the measured Michel parameters are ϱ = 0.781 ± 0.028 ± 0.018, Ξ = 0.98 ± 0.22 ± 0.10, Ξ δ = 0.65 ± 0.14 ± 0.07 and η = 0.027 ± 0.055 ± 0.005, where the value of η has been constrained using the published OPAL measurements of the leptonic branching ratios and the τ 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.