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

TL;DR

This study searches for rare tau decays violating lepton and baryon number, setting new upper limits on their branching fractions, with the most stringent limits to date for six decay modes.

Contribution

First search for these specific tau decay modes, establishing the world's first limits and significantly improving previous constraints.

Findings

No signal observed in any decay mode.

Set upper limits on branching fractions between 1.8 and 4.0 x 10^{-8}.

Improved limits by an order of magnitude for two channels.

Abstract

We search for lepton-number- and baryon-number-violating decays $\tau^{-}\to\overline{p}e^{+}e^{-}$, $pe^{-}e^{-}$, $\overline{p}e^{+}\mu^{-}$, $\overline{p}e^{-}\mu^{+}$, $\overline{p}\mu^{+}\mu^{-}$, and $p\mu^{-}\mu^{-}$ using 921 fb$^{-1}$ of data, equivalent to $(841\pm12)\times 10^6$ $\tau^{+}\tau^{-}$ events, recorded with the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. In the absence of a signal, $90\%$ confidence-level upper limits are set on the branching fractions of these decays in the range $(1.8$-$4.0)\times 10^{-8}$. 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.