Determination of an upper limit for the mass of the τ-neutrino at LEP

An upper limit for theν τ mass is determined through the kinematic reconstruction of the decayτ→5π±ν τ in the OPAL detector at LEP. The limit is obtained using a new method based on the comparison of the two-dimensional distribution of energy and invariant mass of the five-pion system with expectations from different neutrino mass hypotheses. From a sample of five events surviving the selection criteria we obtain an upper limit of 74 MeV at 95% confidence level. It is the first measurement at LEP energies, where the larger average multiplicity of\(e^ + e^ - \to q\bar q\) events makes the suppression of this background more robust compared to lower energies.