Observation of the Electromagnetic Dalitz Transition $h_c \rightarrow e^+e^-\eta_c$

TL;DR

This paper reports the first observation of the electromagnetic Dalitz transition $h_c o e^+e^- eta_c$ using BESIII data, measuring its branching ratio relative to $h_c o gamma eta_c$ with high significance.

Contribution

It provides the first experimental evidence of the electromagnetic Dalitz transition in charmonium states and quantifies its branching ratio relative to the radiative decay.

Findings

First observation of $h_c o e^+e^- eta_c$ with 5.4 sigma significance.

Measured the ratio of branching fractions to be approximately 0.59%.

Provides data for testing quantum chromodynamics models of charmonium transitions.

Abstract

Using $(27.12\pm 0.14)\times10^8$ $\psi(3686)$ decays and data samples of $e^+e^-$ collisions with $\sqrt{s}$ from 4.130 to 4.780~GeV collected with the BESIII detector, we report the first observation of the electromagnetic Dalitz transition $h_c\to e^+e^-\eta_c$ with a statistical significance of $5.4\sigma$. We measure the ratio of the branching fractions $\frac{\mathcal{B}(h_c\rightarrow e^+e^-\eta_c)}{\mathcal{B}(h_c\rightarrow \gamma \eta_c)}$ separately for the $h_c$ samples produced via $\psi(3686)\to\pi^0h_c$ and $e^+e^-\to\pi^+\pi^-h_c$. The average ratio is determined to be $(0.59\pm0.10(\text{stat.})\pm0.04(\text{syst.}))\%$, where the uncertainty includes both statistical and systematic components.