Description
Using $(2712.4 \pm 14.3) \times 10^{6}$ $\psi(3686)$ 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 $\mathcal{B}(J/\psi \to \gamma \eta_c) \times \mathcal{B}(\eta_c \to \gamma\gamma) = (5.23 \pm 0.26_{\text{stat.}} \pm 0.30_{\text{syst.}}) \times 10^{-6}$. This result is consistent with recent LQCD calculations, but it differs from the fitted values provided in PDG.
Author
Zhijun Li
(Sun Yat-sen University)
