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

A new measurement of the Michel parameters and the mean nu /sub tau / helicity in tau lepton decays with the ALEPH detector at LEP is described. The analysed data corresponds to a total integrated luminosity of 155 pb/sup -1/. In the leptonic decays of the tau leptons into e nu /sub e/ nu /sub tau / and mu nu /sub mu / nu /sub tau / the Michel parameters rho /sub e/, xi /sub e/, ( xi delta )/sub e/, nu /sub mu / rho /sub mu /, xi /sub mu / and ( xi delta )/sub mu / are measured. For the determination of the parameters xi /sub pi /, xi /sub rho /, and xi /sub a1/ the final states pi nu /sub tau /, pi pi /sup 0/ nu /sub tau /, pi 2 pi /sup 0/ nu /sub tau / and pi 2 pi /sup +or-/ nu /sub tau / are used. In addition a measurement is performed under the assumptions of e- mu universality and xi /sub pi /= xi /sub rho /= xi /sub a1/, yielding rho /sub l/=0.752+or-0.019, eta /sub l/=0.086+or-0.078, ( xi delta )/sub l/=0.782+or-0.051, xi /sub l/=1.002+or-0.076 and xi /sub had/=0.992+or-0.011. No significant deviation from the standard model assumptions is observed. (23 refs).