Search for $B{}^0_s \rightarrow \ell^{\mp} \tau^{\pm}$ with the Semi-leptonic Tagging Method at Belle

TL;DR

This study searches for lepton-flavor-violating decays of the $B_s^0$ meson into a lepton and a tau using the Belle detector, setting the first upper limit on the $B_s^0 ightarrow e^{ au}$ decay.

Contribution

First search for $B_s^0 ightarrow e^{ au}$ decay and setting new upper limits on these rare processes.

Findings

No evidence for the decays was found.

Upper limits on branching fractions were set at $90\%$ confidence level.

First upper limit established for $B_s^0 \rightarrow e^{\tau}$ decay.

Abstract

We present a search for the lepton-flavor-violating decays $B{}^0_s \rightarrow \ell^{\mp}\tau^{\pm}$, where $\ell = e, \mu$, using the full data sample of $121~\mathrm{fb}^{-1}$ collected at the $\Upsilon(5S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. We use $B{}^0_s \overline{B}{}^0_s$ events in which one $B{}^0_s$ meson is reconstructed in a semileptonic decay mode and the other in the signal mode. We find no evidence for $B{}^0_s \rightarrow \ell^{\mp}\tau^{\pm}$ decays and set upper limits on their branching fractions at $90\%$ confidence level as $\mathcal{B}(B{}^0_s \rightarrow e^{\mp}\tau^{\pm}) < 14 \times 10^{-4}$ and $\mathcal{B}(B{}^0_s \rightarrow \mu^{\mp}\tau^{\pm}) < 7.3 \times 10^{-4}$. Our result represents the first upper limit on the $B{}^0_s \rightarrow e^{\mp}\tau^{\pm}$ decay rate.