Observation of the Charmonium Decay $ {\eta }_{c}\to \gamma \gamma $ in $ J/\psi \to \gamma {\eta }_{c}$

Using $ (2712.4\pm 14.3)\times {10}^{6}\text{}\text{}\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 0.2{6}_{\mathrm{stat}}\pm 0.3{0}_{\mathrm{syst}})\times {10}^{-6}$. This result is consistent with the lattice QCD calculation $ (5.34\pm 0.16)\times {10}^{-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 $ \mathrm{\Gamma }({\eta }_{c}\to \gamma \gamma )$ is determined to be $ (11.30\pm 0.5{6}_{\mathrm{stat}}\pm 0.6{6}_{\mathrm{syst}}\pm 1.1{4}_{\mathrm{ref}})\text{}\text{}\mathrm{keV}$, which deviates from the corresponding world-average value by $ 3.4\sigma $.