Constraints on the tau neutrino mass and mixing from precise measurements of tau decay rates

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, tau(-)-->e(-)<(nu)over bar>(e) nu(tau), tau(-)-->mu(-)<(nu)over bar>(mu)nu(t)au, tau-->pi(-)nu(tau), and tau-->K(-)nu(tau). We present predictions for the tau decay widths, allowing for a nonzero tau neutrino mass m(nu tau) and for mixing with a neutrino of mass m(nu L)>M(Z)/2, which is parametrized using a Cabibbo-like mixing angle theta(L). By comparison of these theoretical predictions with the experimental measurements, we obtain the following bounds at the 90% confidence level: m(nu tau)<42 MeV and sin(2) theta(L)<0.014.