Search for Charged Lepton Flavor Violation in $J/\psi$ decays at BESIII

Abstract

The observation of any charged lepton flavor violation (CLFV) process would be a clear signal of new physics beyond the Standard Model. Various decay modes, including lepton ($\mu ,\tau$) decays, pseudoscalar meson ($K,\pi$) decays, vector meson ($\varphi ,J/\psi ,{\rm Y}$) decays, and Higgs decays, have been explored to detect CLFV. Focusing on the search for CLFV at the $\tau$-charm region, the results of the search for $J/\psi \to e\tau /e\mu$ using the 10 billion $J/\psi$ events collected by the BESIII experiment are presented. The upper limits (ULs) at the 90$\%$ confidence level are $\mathcal{B}(J/\psi \to e^{\pm }{\tau }^{\mp })<7.5\times 10^{-8}$ and $\mathcal{B}(J/\psi \to e^{\pm }{\mu }^{\mp })<4.5\times 10^{-9}$, respectively. Improving the previously published limits by two orders of magnitudes, the results are the most stringent CLFV searches in heavy quarkonium system.