Observation of the charmonium decay $\eta_c\to\gamma\gamma$ in $J/\psi\to\gamma\eta_c$

TL;DR

This paper reports the first observation of the decay $oldsymbol{ ext{eta}_c o ext{gamma} ext{gamma}}$ in $oldsymbol{J/oldsymbol{ extpsi} o ext{gamma} ext{eta}_c}$ using a large dataset, measuring its branching fraction and decay width with high precision.

Contribution

It provides the first experimental measurement of the $ ext{eta}_c o ext{gamma} ext{gamma}$ decay in $J/ extpsi$ decays, and compares results with lattice QCD predictions.

Findings

Measured product branching fraction: $(5.23 imes 10^{-6})$

Determined $ ext{eta}_c o ext{gamma} ext{gamma}$ decay width: $(11.30 ext{ keV})$

Result deviates from world-average by 3.4 sigma.

Abstract

Using $(2712.4\pm14.3)\times10^{6}$ $\psi(3686)$ events collected with the BESIII detector at the BEPCII collider, the decay $\eta_c\to\gamma\gamma$ in $J/\psi\to\gamma\eta_c$ is observed. We determine the product branching fraction $\mathcal{B}(J/\psi\to\gamma\eta_c)\times\mathcal{B}(\eta_c\to\gamma\gamma)=(5.23\pm0.26_{\rm{stat.}}\pm0.30_{\rm{syst.}})\times10^{-6}$. This result is consistent with the LQCD calculation $(5.34\pm0.16)\times10^{-6}$ from HPQCD in 2023. By using the world-average values of $\mathcal{B}(J/\psi\to\gamma\eta_c)$ and the total decay width of $\eta_c$, the partial decay width $\Gamma(\eta_c\to\gamma\gamma)$ is determined to be $(11.30\pm0.56_{\rm{stat.}}\pm0.66_{\rm{syst.}}\pm1.14_{\rm{ref.}})~\rm{keV}$, which deviates from the corresponding world-average value by $3.4\sigma$.