On sterile neutrino mixing with \nu_{\tau}
  Matrix element $U_{\tau N}$ of sterile neutrino N mixing with $\nu_{\tau}$ is the least constrained in the literature among the three $U_{\alpha N}$ ($\alpha=e,\mu,\tau$) mixing parameters characterizing the sterile neutrino phenomenology. We study the contribution of massive dominantly sterile neutrinos to purely leptonic $\tau$-decays and semileptonic decays of $\tau$ and K, D mesons. We consider some decays allowed in the Standard Model (SM) as well as Lepton Flavor and Lepton Number Violating (LFV, LNV) decays forbidden in the SM. From the existing experimental data on the branching ratios of these processes we derived new limits on $U_{\tau N}$ more stringent than the ones existing in the literature. These limits are extracted in a model independent way without any ad hoc assumptions on the relative size of the three different sterile neutrino mixing parameters. 
Juan Carlos Helo, Sergey Kovalenko, Ivan Schmidt
hep-ph