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

Abstract

Using a data sample of $(1310.6\pm 7.0)\times 10^6$ $J/\psi$ 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 $\mathcal{B}(J/\psi \to {\eta }^{\prime }{e}^{+}{e}^{-})=(6.59\pm 0.07\pm 0.17)\times 10^{-5}$, which improves in precision by a factor of 2 over the previous BESIII measurement. A search for the dark photon (${\gamma }^{\prime }$) 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 GeV/$c^2$. We set upper limits at the 90\% confidence level on $\mathcal{B}(J/\psi \to {\eta }^{\prime }{\gamma }^{\prime })\times \mathcal{B}({\gamma }^{\prime }\to e^{+}{e}^{-})$, $\mathcal{B}(J/\psi \to {\eta }^{\prime }{\gamma }^{\prime }$) and the mixing strength as a function of dark photon mass. This is among the first searches for dark photons in charmonium decays.