Search for the charged lepton flavour violating decay ${\eta }^{\prime }\to e\mu$

Abstract

Based on $(8998\pm 40)\times 10^6$ $J/\psi$ events collected in $e^{+}{e}^{-}$ collisions at $\sqrt{s}=3.097$ GeV with the BESIII detector, we present a search for the charged lepton flavour violating decay ${\eta }^{\prime }\to e\mu$ with $J/\psi \to \gamma {\eta }^{\prime }$. No significant signal is observed, and an upper limit on its decay branching fraction is set to be $6.3\times 10^{-7}$ at the 90% confidence level, improving the previous best result by nearly three orders of magnitude.

20 pages, 5 figures, 3 tables