Measurement of the Michel parameters and the $\nu_{\tau}$ helicity in $\tau$ lepton decays

Measurement of the Michel parameters and the neutrino helicity of the tau lepton by ALEPH with the full LEP I data sample. ALEPH Collaboration A new measurement of the Michel parameters and the mean neutrino helicity in the decay of the tau lepton with the detector ALEPH at LEP is described. The analysed data collected during the years from 1991 to 1995 corresponds to a total integrated luminosity of 154pb-1. Using the leptonic decays of the tau leptons into electron and muon the Michel parameters rho_e, xi_e (xi delta)_e, eta_mu, rho_mu, xi_mu and (xi delta)_mu are measured. For the determination of the parameters xi_pion, xi_rho and xi_a1 the final states (pi nu), ( pipi0 nu), (pi2pi0 nu) and (3pi+- nu) are used. In addition a measurement is performed under the assumptions of e-mu universality and xi_pi=xi_rho=xi_a1, yielding rho_l=0.752+/- 0.019, eta_l=0.086+/- 0.078, (xi delta)_l=0.78+/- 0.051, xi_l=1.002+/- 0.076$ and \xi_had=0.992+/- 0.011. No deviation from the Standard Model assumption of the V-A structure of the weak interaction is observed within the errors.