Lepton number violation in tau lepton decays

Recent studies of novel four-body lepton number violating decays of $\tau$ leptons and neutral $B$ mesons are summarized and updated. These decays are assumed to be enhanced by the exchange of resonant Majorana neutrinos. It is shown that the ${\tau }^{-}\to {\pi }^{+}{l}^{-}{l}^{-}{\nu }_{\tau }$ decay channels, with $l=e$ or $\mu$, can provide stronger constraints on the mixing vs. mass parameter space of resonant Majorana neutrinos than analogous three-body decays of charged $B$ mesons.