Upper limit on the ν τ mass from τ → 3 hν τ decays

A new technique has been used to determine a limit on theν τ mass using Z0→τ + τ − events selected from 140 pb−1 of data collected with the OPAL detector at LEP during the period 1990–1994. Using Z0→τ + τ − events in which bothτ-leptons decay to three charged particles, a novel likelihood analysis is applied to the 2-dimensional distribution of missing mass squared and missing energy, from which we obtain a limit ofMν gt <35.3 MeV at 95% confidence level. Combining our new result with OPAL’s previous published measurement fromτ→ 5πν τ decays, we obtain the new 95% confidence level limit ofMν τ <29.9 MeV.