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

We present a search for the lepton flavor violating decay $J\mathrm{/}\psi \to e^{\pm }{\mu }^{\mp }$ using $8.998\times {10}^{9}J\mathrm{/}\psi$ 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 $\mathcal{B}(J\mathrm{/}\psi \to e^{\pm }{\mu }^{\mp })<4.5\times {10}^{-9}$ at the $90\mathrm{\%}$ 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.