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

Abstract

Using a data sample of (1310.6±7.0)×106J/ψ decay events collected with the BESIII detector at BEPCII, we study the electromagnetic Dalitz decay J/ψ→η′e+e− with two dominant $\eta' $ decay modes, η′→γπ+π− and η′→π+π−η. The branching fraction is determined to be B(J/ψ→η′e+e−)=(6.59±0.07±0.17)×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→η′γ′,γ′→e+e−. Excluding the ω and ϕ mass regions, no significant signal is observed in the mass range from 0.1 to 2.1 GeV/c2. We set upper limits at the 90% confidence level on B(J/ψ→η′γ′)×B(γ′→e+e−), B(J/ψ→η′γ′ and the mixing strength as a function of dark photon mass. This is among the first searches for dark photons in charmonium decays.