Search for the lepton flavor violating decay J/ψ → eμ.

We present a search for the lepton flavor violating decay J/ψ → e^±μ^∓ using 8.998 × 10⁹J/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 ℬ (J / ψ → e ± μ ∓) < 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. [ABSTRACT FROM AUTHOR]