# Measurement of the Michel parameters in leptonic tau decays Academic Article

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• Overview
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• Research
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• 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.