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

Abstract

We study the electromagnetic Dalitz decay $J/\psi \to e^+e^- \eta$ and search for di-electron decays of a dark gauge boson $(\gamma')$ in $J/\psi \to \gamma' \eta$ with the two $\eta$ decay modes $\eta \rightarrow \gamma \gamma$and $\eta \rightarrow \pi^+\pi^-\pi^0$ using $(1310.6±7.0)×10^6$ $J/\psi$ events collected with the BESIII detector. The branching fraction of $J/\psi \to e^+e^- \eta$ is measured to be $(1.43 \pm 0.04 ({\rm stat}) \pm 0.06 ({\rm syst}))\times 10^{-5}$, 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 $\Lambda = 2.84 \pm 0.11({\rm stat}) \pm 0.08({\rm syst})$ GeV/$c^2$. We find no evidence of $\gamma'$ production and set 90% confidence level upper limits on the product branching fraction $\mathcal{B}(J/\psi \to \gamma' \eta)\times \mathcal{B}(\gamma' \to e^+e^-)$ as well as the kinetic mixing strength between the Standard Model photon and $\gamma'$ in the mass range of $0.01 \le m_{\gamma'} \le 2.4$ GeV/$c^2$.