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

An upper limit for theντ mass is determined through the kinematic reconstruction of the decayτ→5π±ντ 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.