Observation of $ψ(3686) \to η' e^+ e^-$

Abstract

Using a data sample of $448.1×10^6 ψ(3686)$ events collected with the BESIII detector at the BEPCII collider, we report the first observation of the electromagnetic Dalitz decay $ψ(3686) \to η' e^+ e^-$, with significances of 7.0σ and 6.3σ when reconstructing the η′ meson via its decay modes $\eta'\to\gamma \pi^+ \pi^-$ and $\eta'\to\pi^+\pi^-\eta$ (η→γγ), 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.