Observation of the decay ${\eta }_c\to \gamma \gamma$ at BESIII

In the decays of ${\eta }_c\to \gamma \gamma$ and $J\mathrm{/}\psi \to \gamma {\eta }_c$, there are discrepancies between the theoretical calculations and the PDG experimental values, referred to as the charmonium QCD puzzle. We observe the 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, and 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 measurement is consistent with the two most recent lattice QCD calculations, providing significant input toward resolving the charmonium QCD puzzle.