Measurement of the Michel parameters in leptonic $\tau$ decays with the OPAL detector at LEP

An upper limit for the tau -neutrino mass has been determined from the decay tau to 5 pi /sup +or-/ nu /sub tau / using data collected with the OPAL detector from 1991 to 1995 in e/sup +/e/sup -/ collisions at square root (s) approximately=M/sub Z0/. A limit of 43.2 MeV at 95Ø.000554330L is obtained using a two-dimensional method in the 5 pi invariant mass and energy distribution from 22 selected events. Combining this result with OPAL's previously published measurement using tau /sup +/ tau /sup -/ to 3h/sup +or-/v/sub tau /+3h/sup -or+/ nu tau decays, a new combined limit of m/sub nu tau / <27.6 MeV (95 CL) is obtained.