Search for the lepton flavor violating decay $J/\psi\to e\mu$

TL;DR

This study searches for lepton flavor violation in $J/$ decays using a large dataset, finding no signal and setting the most stringent upper limit to date, thus constraining new physics models.

Contribution

It provides the first stringent upper limit on $J/\psi \to e\mu$ decay, significantly improving previous bounds and advancing searches for lepton flavor violation in heavy quarkonium.

Findings

No evidence for $J/\psi \to e\mu$ decay was observed.

An upper limit on the branching fraction was set at $4.5 \times 10^{-9}$.

This result improves previous limits by a factor of over 30.

Abstract

We present a search for the lepton flavor violating decay $J/\psi\to e^{\pm}\mu^{\mp}$ using $8.998\times 10^{9} J/\psi$ events collected with the BESIII detector at the BEPCII $e^+e^-$ storage ring. No excess of signal above background is observed; we therefore set an upper limit on the branching fraction of $\mathcal{B}(J/\psi\to e^{\pm}\mu^{\mp})<4.5\times10^{-9}$ at the $90\%$ confidence level. Improving the previous best result by a factor of more than 30, this measurement places the most stringent limit to date on lepton flavor violation in the heavy quarkonium sector.