Search for charged lepton flavor violating decays of $\Upsilon(1S)$

TL;DR

This paper reports a search for charged lepton flavor violating decays of the $
S$ meson using a large data sample, setting upper limits on such rare decay processes with no significant signals found.

Contribution

First search for charged lepton flavor violation in $
S$ decays using $
S$ produced in $
S$ transitions, establishing upper limits on branching fractions.

Findings

No significant signals observed.

Upper limits set on branching fractions.

Provides constraints on new physics models.

Abstract

We present a search for the charged lepton-flavor-violating decays $\Upsilon(1S)\rightarrow\ell\ell^\prime$ and radiative charged lepton-flavour-violating decays $\Upsilon(1S)\rightarrow\gamma\ell\ell^\prime$ [$\ell,\ell^\prime = e, \mu, \tau$] using the 158 million $\Upsilon(2S)$ sample collected by the Belle detector at the KEKB collider. This search uses $\Upsilon(1S)$ mesons produced in $\pi^+\pi^-\Upsilon(1S)$ transitions. We do not find any significant signal, so we provide upper limits on the branching fractions at the 90% confidence level.