We have derived constraints on the tau neutrino mass and fourth generation mixing from an analysis of the partial widths of tau lepton decays, in particular: We present predictions for the tau decay widths, allowing for a non-zero tau neutrino mass, m ντ , and for mixing with a neutrino of mass m ν L > M Z /2, which is parametrised using a Cabibbo-like mixing angle, θ L . By comparison of these theoretical predictions with the experimental measurements, we obtain the following bounds at the 90%

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... confidence level: m ντ < 42 MeV and sin 2 θ L < 0.014. Abstract We have derived constraints on the tau neutrino mass and fourth generation mixing from an analysis of the partial widths of tau lepton decays, in particular: ! e e , ! , ! , and ! K . We present predictions for the tau decay widths, allowing for a non-zero tau neutrino mass, m , and for mixing with a neutrino of mass m L > M Z =2, which is parametrised using a Cabibbo-like mixing angle, L . By comparison of these theoretical predictions with the experimental measurements, we obtain the following bounds at the 90% con dence level: m < 42 MeV and sin 2 L < 0:014.