SEMILEPTONIC LEPTON-NUMBER/FLAVOUR-VIOLATING tau DECAYS IN MAJORANA NEUTRINO MODELS

Motivated by the recent investigation of neutrinoless $\tau$-lepton decays by the CLEO collaboration, we perform a systematic analysis of such decays in a possible new-physics scenario with heavy Dirac/Majorana neutrinos, including heavy-neutrino nondecoupling effects, finite quark masses, and quark as well as meson mixings. We find that $\tau$-lepton decays into an electron or muon and a pseudoscalar or vector meson can have branching ratios close to the experimental sensitivity. Numerical estimates show that the predominant decay modes of this kind are $\tau^-\to e^-\phi$, $\tau^-\to e^-\rho^0$, and $\tau^-\to e^-\pi^0$, with branching ratios of order $10^{-6}$.