LIMITS ON NEUTRINO MIXING WITH NEW HEAVY-PARTICLES

We study the effects induced by new neutral fermions below their mass threshold, due to their possible mixing with the standard neutrinos. W e use as experimental constraints the recent results on lepton univers ality, together with, the measurement of the mu decay rate and the upd ated LEP data. In particular, the inclusion in our data set of the mos t recent determinations of the tau branching fractions, mass and lifet ime implies that a previous indication of a non-vanishing mixing for n u(tau) is no longer present. We obtain new stringent limits on the mix ing parameters between nu(e), nu(mu), nu(tau) and heavy neutral states of different weak isospin. If no assumption on the type of neutrinos involved in the mixing is made, we find s(nu)e2 < 0.0071, s(nu)mu2 < 0 .0014 and s(nu)tau2 < 0.033.