The 13th International Workshop on e+e- collisions from Phi to Psi

We present a search for the lepton flavor violating decay $J/\psi \to e \mu$ 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 $J/\psi \to e \mu < 4.5\times 10^{-9}$ 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.