Lepton flavor violating tau and B decays and heavy neutrinos

We study lepton flavor violating (LFV) tau and B decays in models with heavy neutrinos to constrain the mixing matrix parameters U_{tau N}. We find that the best current constraints when the heavy neutrinos are purely left-handed come from LFV radiative tau decay modes. To obtain competitive constraints in LFV B decay it is necessary to probe b -> X_{s} tau^{pm} e^{mp} at the 10^{-7} level. When the heavy neutrinos have both left and right-handed couplings, the mixing parameters can be constrained by studying LFV B decay modes and LFV tau decay into three charged leptons. We find that the branching ratios B(tau^{pm} -> l_1^{pm} l_2^{pm} l_3^{mp}), B(B_{s} -> tau^{pm} e^{mp}) and B(b -> X_{s} l_1^{pm} l_2^{mp}) need to be probed at the 10^{-8} level in order to constrain the mixing parameters beyond what is known from unitarity.