Observation of the decay $J/\psi \to e^+ e^- \eta(1405)$ with $\eta(1405) \to \pi^0 f_0(980)$

TL;DR

This paper reports the first observation of the decay $J/\psi o e^+ e^- \eta(1405)$, measuring its branching fraction and transition form factor, thus providing insights into the structure of the $\eta(1405)$ meson.

Contribution

The study provides the first measurement of the electromagnetic Dalitz decay of $J/\psi$ to $\eta(1405)$ and the transition form factor, enhancing understanding of the meson's internal structure.

Findings

Observation of $J/\psi o e^+ e^- \\eta(1405)$ with 9.6 sigma significance

Branching fraction measured as $(2.02 \\pm 0.24 \\text{(stat.)} \\pm 0.09 \\text{(syst.)}) imes 10^{-7}$

Transition form factor of $J/\psi o e^+ e^- \\eta(1405)$ presented for the first time

Abstract

Using a data sample of $(10087\pm44)\times 10^6$ $J/\psi$ events collected by the BESIII detector in 2009, 2012, 2018 and 2019, the electromagnetic Dalitz process $J/\psi \to e^+ e^- \eta(1405)$ is observed via the decay $\eta(1405) \to \pi^0 f_0(980)$, $f_0(980) \to \pi^+ \pi^-$, with a significance of about $9.6\sigma$. The branching fraction of this decay is measured to be ${\mathcal B}(J/\psi \to e^+ e^- \pi^0 \eta(1405) \to e^+ e^- \pi^0 f_0(980) \to e^+ e^- \pi^0 \pi^+ \pi^-)=(2.02\pm0.24(\rm{stat.})\pm0.09(\rm{syst.}))\times 10^{-7}$. The branching-fraction ratio ${\mathcal B}(J/\psi \to e^+ e^- \eta(1405))$/${\mathcal B}(J/\psi \to \gamma \eta(1405))$ is determined to be $(1.35\pm0.19(\rm{stat.})\pm0.06(\rm{syst.}))\times10^{-2}$. Furthermore, an $e^+e^-$ invariant-mass dependent transition form factor of $J/\psi \to e^+ e^-\eta(1405)$ is presented for the first time. The obtained result provides input for different theoretical models, and is valuable for the improved understanding the intrinsic structure of the $\eta(1405)$ meson.