Observation of $\psi(3686) \to \eta' e^+ e^- $

TL;DR

This paper reports the first observation of the electromagnetic Dalitz decay of the $a9(3686)$ charmonium state into $7eta' e^+ e^-$, measuring its branching fraction with high significance using BESIII data.

Contribution

First observation of the decay $a9(3686) o 7eta' e^+ e^-$ with measured branching fraction, expanding understanding of electromagnetic decays of charmonium states.

Findings

Branching fraction measured as (1.90 b1 0.25 b1 0.11) d7 10^{-6}.

Significance of observation exceeds 6 sigma.

First measurement of this decay mode.

Abstract

Using a data sample of $448.1 \times 10^6$ $\psi(3686)$ events collected with the BESIII detector at the BEPCII collider, we report the first observation of the electromagnetic Dalitz decay $\psi(3686) \to \eta' e^+ e^-$, with significances of 7.0$\sigma$ and 6.3$\sigma$ when reconstructing the $\eta'$ meson via its decay modes $\eta'\to\gamma \pi^+ \pi^-$ and $\eta'\to\pi^+\pi^-\eta$ ($\eta \to \gamma\gamma$), respectively. The weighted average branching fraction is determined to be $\mathcal{B}(\psi(3686) \to \eta' e^+ e^-)= (1.90 \pm 0.25 \pm 0.11) \times 10^{-6}$, where the first uncertainty is statistical and the second systematic.