Measurement of $B\left(J/\psi \to \eta \prime {e}^{+}{e}^{-}\right)$ and search for a dark photon

Using a data sample of $\left(1310.6±7.0\right)×{10}^{6}\text{}\text{}J/\psi $ decay events collected with the BESIII detector at BEPCII, we study the electromagnetic Dalitz decay $J/\psi \to {\eta }^{\prime }{e}^{+}{e}^{-}$ with two dominant ${\eta }^{\prime }$ decay modes, ${\eta }^{\prime }\to \gamma {\pi }^{+}{\pi }^{-}$ and ${\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}\eta $. The branching fraction is determined to be $B\left(J/\psi \to {\eta }^{\prime }{e}^{+}{e}^{-}\right)=\phantom{\rule{0ex}{0ex}}\left(6.59±0.07±0.17\right)×{10}^{-5}$, which improves in precision by a factor of 2 over the previous BESIII measurement. A search for the dark photon (${\gamma }^{\prime }$) is performed via $J/\psi \to {\eta }^{\prime }{\gamma }^{\prime }$, ${\gamma }^{\prime }\to {e}^{+}{e}^{-}$. Excluding the $\omega $ and $\varphi $ mass regions, no significant signal is observed in the mass range from 0.1 to $2.1\text{}\text{}\mathrm{GeV}/{c}^{2}$. We set upper limits at the 90% confidence level on $B\left(J/\psi \to {\eta }^{\prime }{\gamma }^{\prime }\right)×B\left({\gamma }^{\prime }\to {e}^{+}{e}^{-}\right)$, $B\left(J/\psi \to {\eta }^{\prime }{\gamma }^{\prime }$) and the mixing strength as a function of dark photon mass. This is among the first searches for dark photons in charmonium decays.