Observation of the hindered electromagnetic Dalitz decay $\psi(3686) \to e^+ e^- \eta_c$

TL;DR

This paper reports the first observation and measurement of the hindered electromagnetic Dalitz decay of $oldsymbol{\psi(3686) o e^+ e^- oldsymbol{\eta_c}}$, providing new insights into the electromagnetic properties of this transition.

Contribution

It presents the first measurement of the electromagnetic Dalitz decay $oldsymbol{\psi(3686) o e^+ e^- oldsymbol{\eta_c}}$, confirming theoretical predictions and enhancing understanding of charmonium electromagnetic transitions.

Findings

Branching fraction measured as (3.77 ± 0.40_stat ± 0.18_syst)×10^{-5}

Decay observed with a significance of 7.9σ

Results agree with vector meson dominance model predictions

Abstract

Using a data sample of $(448.1 \pm 2.9)\times10^6 ~\psi(3686)$ decays collected at an $e^+ e^-$ center-of-mass energy of $3.686~\rm{GeV}$ by the BESIII detector at BEPCII, we report an observation of the hindered electromagnetic Dalitz decay $\psi(3686) \to e^+ e^- \eta_c$ with a significance of $7.9\sigma$. The branching fraction is determined to be $\mathcal{B} (\psi(3686) \to e^+ e^- \eta_c) = (3.77 \pm 0.40_{\rm stat.} \pm 0.18_{\rm syst.})\times 10^{-5}$, agreeing well with the prediction of the vector meson dominance model. This is the first measurement of the electromagnetic Dalitz transition between the $\psi(3686)$ and the $\eta_c$, which provides new insight into the electromagnetic properties of this decay, and offers new opportunities to measure the absolute branching fractions of $\eta_c$ decays.