Search for charged-lepton flavor violation in $\Upsilon(2S) \to \ell^\mp\tau^\pm$ ($\ell=e,\mu$) decays at Belle

TL;DR

This study searches for charged-lepton flavor violation in $S$ decays to electron or muon and tau, using Belle data, but finds no evidence and sets the most stringent upper limits to date.

Contribution

First search for charged-lepton flavor violation in $S$ decays to $ au$ and $e/$ with the most stringent upper limits established.

Findings

No evidence for charged-lepton flavor violation observed.

Upper limits on branching fractions are $Ytomutau < 0.23 imes 10^{-6}$ and $Ytoetau < 1.12 imes 10^{-6}$.

Results improve constraints on new physics models.

Abstract

We report a search for the charged-lepton flavor violation in $\Upsilon(2S) \to \ell^\mp\tau^\pm$ ($\ell=e,\mu$) decays using a $25~\fbi$ $\Upsilon(2S)$ sample collected by the Belle detector at the KEKB $e^{+}$$e^-$ asymmetric-energy collider. We find no evidence for a signal and set upper limits on the branching fractions ($\mathcal{B}$) at 90\% confidence level. We obtain the most stringent upper limits: $\mathcal{B}(\Ytomutau) < 0.23 \times 10^{-6}$ and $\mathcal{B}(\Ytoetau) < 1.12 \times 10^{-6}$.