Study of the Dalitz decay $J/\psi \to e^+e^- \eta$

Abstract

We study the electromagnetic Dalitz decay J/ψ→e+e−η and search for di-electron decays of a dark gauge boson (γ′) in J/ψ→γ′η with the two η decay modes η→γγand η→π+π−π0 using (1310.6±7.0)×106 J/ψ events collected with the BESIII detector. The branching fraction of J/ψ→e+e−η is measured to be (1.43±0.04(stat)±0.06(syst))×10−5, with a precision that is improved by a factor of 1.5 over the previous BESIII measurement. The corresponding di-electron invariant mass dependent modulus square of the transition form factor is explored for the first time, and the pole mass is determined to be Λ=2.84±0.11(stat)±0.08(syst) GeV/c2. We find no evidence of γ′ production and set 90% confidence level upper limits on the product branching fraction B(J/ψ→γ′η)×B(γ′→e+e−) as well as the kinetic mixing strength between the Standard Model photon and γ′ in the mass range of 0.01≤mγ′≤2.4 GeV/c2.