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

Abstract

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 ${\cal B}\left( {J/\psi \to {e^ \pm }{\mu ^ \mp }} \right) < \,4.5 \times {10^{ - 9}}$ 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.