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

TL;DR

This paper presents updated measurements of the Dalitz decay $J/ar{psi} o e^+e^- ar{eta}$, including the transition form factor pole mass, branching fraction, and constraints on a light dark gauge boson, using a large $J/ar{psi}$ dataset.

Contribution

The study provides the first corrected measurement of the transition form factor pole mass and updated decay parameters for $J/ar{psi}$ Dalitz decay, improving previous results and searching for dark sector signals.

Findings

Transition form factor pole mass $\Lambda$ measured as 2.56 GeV/$c^2$

Branching fraction of $J/ar{psi} o e^+e^- ar{eta}$ is $(1.42 imes 10^{-5})$

Constraints on dark gauge boson $\gamma'$ coupling are set.

Abstract

We report corrected results on a study of the electromagnetic Dalitz decay $J/\psi \to e^+e^- \eta$ and search for di-electron decays of a light 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 $(1310.6\pm 7.0)\times10^6$ $J/\psi$ events collected with the BESIII detector. In particular we update the measurement of the pole mass of the transition form factor (TFF) of $J/\psi \to e^+e^- \eta$ to be $\Lambda = 2.56 \pm 0.04({\rm stat}) \pm 0.03({\rm syst})$ GeV/$c^2$. The branching fraction of $J/\psi \to e^+e^-\eta$ and the kinetic-mixing strength between the Standard Model photon and $\gamma'$ are also updated after taking the corrected $\Lambda$ value into account. The measured branching fraction of $J/\psi \to e^+e^- \eta$ is updated to be $(1.42 \pm 0.04({\rm stat}) \pm 0.07 ({\rm syst}))\times 10^{-5}$, where the first and second uncertainties are statistical and systematic, respectively.