Observation of the Dalitz Decay $\eta' \to \gamma e^+e^-$

TL;DR

This paper reports the first observation of the Dalitz decay of the eta' meson into a photon and an electron-positron pair, measuring its branching fraction and transition form factor using a large data sample from BESIII.

Contribution

First experimental measurement of the decay $ ext{eta'} o ext{gamma} e^+ e^-$, including branching fraction and transition form factor analysis, confirming Vector Meson Dominance model predictions.

Findings

Measured branching fraction ${ m B}( ext{eta'} o ext{gamma} e^+ e^-)$ = (4.69 ± 0.20 ± 0.23)×10^{-4}

Determined the ratio $rac{ ext{Gamma}( ext{eta'} o ext{gamma} e^+ e^-)}{ ext{Gamma}( ext{eta'} o ext{gamma} ext{gamma})}$ = (2.13 ± 0.09 ± 0.07)×10^{-2}

Transition form factor consistent with Vector Meson Dominance model

Abstract

We report the first observation of the Dalitz decay $\eta' \to \gamma e^+e^-$, based on a data sample of 1.31 billion $J/\psi$ events collected with the BESIII detector. The $\eta'$ mesons are produced via the $J/\psi \to \gamma \eta'$ decay process. The ratio $\Gamma(\eta' \to \gamma e^+ e^-)/\Gamma(\eta'\to\gamma\gamma)$ is measured to be $(2.13\pm0.09(\text{stat.})\pm0.07(\text{sys.}))\times10^{-2}$. This corresponds to a branching fraction ${\cal B}(\eta' \to \gamma e^+e^-)= (4.69 \pm0.20(\text{stat.})\pm0.23(\text{sys.}))\times10^{-4}$. The transition form factor is extracted and different expressions are compared to the measured dependence on the $e^+e^-$ invariant mass. The results are consistent with the prediction of the Vector Meson Dominance model.