Search for the charged lepton flavor violating decay $\eta \to e^{\pm}\mu^{\mp}$
Based on $(10.087 \pm 0.044) \times 10^{9}~J/\psi$ events collected at the center-of-mass energy $\sqrt{s} = 3.097$~GeV with the BESIII detector, we search for the charged lepton flavor violating decay $\eta \to e^{\pm}\mu^{\mp}$ through the process $J/\psi \to \gamma \eta'$ with $\eta' \to \pi^{+} \pi^{-} \eta$. No signal is observed, and an upper limit on the branching fraction is determined to be $\mathcal{B}(\eta \to e^{\pm}\mu^{\mp}) < 6.8 \times 10^{-7}$ at the 90\% confidence level. This result improves the previous best limit by one order of magnitude.
hep-ex↗