The 29th International Workshop on Weak Interactions and Neutrinos

We present a search for the lepton flavor violating decay /→ using 8.998×10^9 / events collected with the BESIII detector at the BEPCII + − storage ring. No excess of signal above background is observed; we therefore set an upper limit on the branching fraction of ℬ(/→)< 4.5×10^(−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.