Measurement of the Michel parameters in leptonic tau decays

Abstract

Abstract.

The Michel parameters of the leptonic \(\tau\) decays are measured using the OPAL detector at LEP. The parameters \(\rho_\ell\), \(\xi_\ell\), \((\xi\delta)_\ell\) (with \(\ell ={\rm e}, \mu\)) and \(\eta_\mu\) are extracted from the energy spectra of the charged decay leptons and from their energy–energy correlations. A new method involving a global likelihood fit of Monte Carlo generated events with complete detector simulation and background treatment has been applied to the data recorded at center-of-mass energies close to \(\sqrt{s} =m_{\rm Z^0}\) corresponding to an integrated luminosity of 155 pb\(^{-1}\). If e-\(\mu\) universality is assumed and inferring the \(\tau\) polarization from neutral current data, the measured Michel parameters are: where the value of \(\eta\) has been constrained using the published OPAL measurements of the leptonic branching ratios and the \(\tau\) lifetime. Limits on non-standard coupling constants and on the masses of new gauge bosons are obtained. The results are in agreement with the V–A prediction of the Standard Model.