Search for the charged lepton flavor violating decay $\eta \to e^{\pm }{\mu }^{\mp }$

Abstract

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 $J/\psi \to \gamma {\eta }^{\prime }$ with ${\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}\eta$. 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.

10 pages, 3 figures