Measurement of B ( J / ψ → η ′ e + e − ) and search for a dark photon

Using a data sample of (1310.6 ± 7.0) × 106 J / ψ decay events collected with the BESIII detector at BEPCII, we study the electromagnetic Dalitz decay J / ψ → η′e+e− with two dominant η′ 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.