Observation of the charmonium decay ${\eta }_c\to \gamma \gamma$ in $J\mathrm{/}\psi \to \gamma {\eta }_c$

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 $J\mathrm{/}\psi \to \gamma {\eta }_c$ is observed. We determine the product branching fraction $\mathcal{B}(J\mathrm{/}\psi \to \gamma {\eta }_c)\times \mathcal{B}({\eta }_c\to \gamma \gamma )=(5.23\pm {0.26}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}}\pm {0.30}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}})\times {10}^{-6}$. This result is consistent with the LQCD calculation $(5.34\pm 0.16)\times {10}^{-6}$ from HPQCD in 2023. By using the world-average values of $\mathcal{B}(J\mathrm{/}\psi \to \gamma {\eta }_c)$ and the total decay width of ${\eta }_c$, the partial decay width $\mathrm{\Gamma }({\eta }_c\to \gamma \gamma )$ is determined to be $(11.30\pm {0.56}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}}\pm {0.66}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}}\pm {1.14}_{\mathrm{r}\mathrm{e}\mathrm{f}\mathrm{.}})\mathrm{k}\mathrm{e}\mathrm{V}$, which deviates from the corresponding world-average value by $3.4\sigma$.