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

Using a data sample of $(1310.6\pm 7.0)\times {10}^6$ $J\mathrm{/}\psi$ decay events collected with the BESIII detector at BEPCII, we study the electromagnetic Dalitz decay $J\mathrm{/}\psi \to {\eta }^{\mathrm{\prime }}{e}^{+}{e}^{-}$ with two dominant ${\eta }^{\mathrm{\prime }}$ decay modes, ${\eta }^{\mathrm{\prime }}\to \gamma {\pi }^{+}{\pi }^{-}$ and ${\eta }^{\mathrm{\prime }}\to {\pi }^{+}{\pi }^{-}\eta$. The branching fraction is determined to be $$\mathcal{B}$(J/\psi \to \eta’ e^+e^-) = (6.59\pm0.07\pm0.17) \times 10^{-5}$, which improves in precision by a factor of 2 over the previous BESIII measurement. A search for the dark photon (${\gamma }^{\mathrm{\prime }}$) is performed via $J/\psi \to\eta’ \gamma ‘, \gamma’ \to e^{+}e^{-}$\mathrm{.}Excludingthe$\omega$and$\phi$ mass regions, no significant signal is observed in the mass range from 0.1 to 2.1 GeV/$c^2$. We set upper limits at the 90% confidence level on $$\mathcal{B}$(J/\psi \to \eta’ \gamma ‘)\times$\mathcal{B}$(\gamma ’ \to e^+e^-)$,$$\mathcal{B}$(J/\psi \to\eta’ \gamma’$) and the mixing strength as a function of dark photon mass. This is among the first searches for dark photons in charmonium decays.