Measurement of $\mathcal{B}(J/\psi \to \eta' e^+ e^- $) and search for dark photon

Using a data sample of $(1310.6pm7.0)times10^{6}$ $J/psi$ decay events collected with the BESIII detector at BEPCII, we study the electromagnetic Dalitz decay $J/psi etau0027 e^+e^-$ with two dominant $etau0027$ decay modes, $etau0027 gamma pi^+ pi^-$ and $etau0027 pi^+pi^-eta$. The branching fraction is determined to be $mathcal{B}(J/psi etau0027 e^+e^-) = (6.59pm0.07pm0.17) times 10^{-5}$, which improves in precision by a factor of 2 over the previous BESIII measurement. A search for the dark photon ($gamma u0027$) is performed via $J/psi toetau0027 gamma u0027, gammau0027 e^{+}e^{-}$. Excluding the $omega$ and $phi$ mass regions, no significant signal is observed in the mass range from 0.1 to 2.1 GeV/$c^{2}$. We set upper limits at the 90% confidence level on $mathcal{B}(J/psi etau0027 gamma u0027)timesmathcal{B}(gamma u0027 e^+e^-)$, $mathcal{B}(J/psi toetau0027 gammau0027$) and the mixing strength as a function of dark photon mass. This is among the first searches for dark photons in charmonium decays.