Measurement of $\mathcal{B}(J/\ddot{I}ˆ\to \hat{I}{\cdot }^{\prime }{e}^{+}{e}^{-}$) and search for a dark photon

Abstract

Using a data sample of $(1310.6\pm 7.0)\times 10^6$ $J/\ddot{I}ˆ$ decay events collected with the BESIII detector at BEPCII, we study the electromagnetic Dalitz decay $J/\ddot{I}ˆ\to \hat{I}{\cdot }^{\prime }{e}^{+}{e}^{-}$ with two dominant $\hat{I}{\cdot }^{\prime }$ decay modes, $\hat{I}{\cdot }^{\prime }\to {\hat{I}}^3\ddot{I}{€}^{+}\ddot{I}{€}^{-}$ and $\hat{I}{\cdot }^{\prime }\to \ddot{I}{€}^{+}\ddot{I}{€}^{-}\hat{I}\cdot$. The branching fraction is determined to be $\mathcal{B}(J/\ddot{I}ˆ\to \hat{I}{\cdot }^{\prime }{e}^{+}{e}^{-})=(6.59\pm 0.07\pm 0.17)\times 10^{-5}$, which improves in precision by a factor of 2 over the previous BESIII measurement. A search for the dark photon ($γ'$) is performed via $J/ψ\toη' γ', γ' \to e^{+}e^{-}$. Excluding the $\ddot{I}‰$ and $\ddot{I}•$ 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 $\mathcal{B}(J/ψ\to η' γ')\times\mathcal{B}(γ' \to e^+e^-)$, $\mathcal{B}(J/ψ\toη' γ'$) and the mixing strength as a function of dark photon mass. This is among the first searches for dark photons in charmonium decays.