Search for lepton-number- and baryon-number-violating tau decays at Belle

Abstract

We search for lepton-number- and baryon-number-violating decays ${\ensuremath{\tau}}^{\ensuremath{-}}\ensuremath{\rightarrow}\overline{p}{e}^{+}{e}^{\ensuremath{-}}$, $p{e}^{\ensuremath{-}}{e}^{\ensuremath{-}}$, $\overline{p}{e}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$, $\overline{p}{e}^{\ensuremath{-}}{\ensuremath{\mu}}^{+}$, $\overline{p}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$, and $p{\ensuremath{\mu}}^{\ensuremath{-}}{\ensuremath{\mu}}^{\ensuremath{-}}$ using $921\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of data, equivalent to $(841\ifmmode\pm\else\textpm\fi{}12)\ifmmode\times\else\texttimes\fi{}{10}^{6}$ ${\ensuremath{\tau}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}}$ events, recorded with the Belle detector at the KEKB asymmetric-energy ${e}^{+}{e}^{\ensuremath{-}}$ collider. In the absence of a signal, 90% confidence-level upper limits are set on the branching fractions of these decays in the range $(1.8\ensuremath{-}4.0)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$. We set the world's first limits on the first four channels and improve the existing limits by an order of magnitude for the last two channels.