Search for the lepton flavor violating decay $J/ψ\to eμ$

Abstract

We present a search for the lepton flavor violating decay $J/ψ\to e^{\pm}μ^{\mp}$ using $8.998\times 10^{9} J/ψ$ 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 $\mathcal{B}(J/ψ\to e^{\pm}μ^{\mp})<4.5\times10^{-9}$ 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.