First Measurement of the Absolute Branching Fraction of ${\eta }_c\to \gamma \gamma$

We apply a tag-and-probe method to precisely measure the absolute branching fraction of the decay ${\eta }_c\to \gamma \gamma$ with the BESIII experiment at BEPCII. Starting with a large initial sample of $2712.4\pm 14.3$ million $\psi (3686)$ events, a sample of 0.16 million ${\eta }_c$ events are tagged via the golden channel $\psi (3686)\to {\pi }^0{h}_c$, $h_c\to \gamma {\eta }_c$, effectively avoiding interference effects. The absolute branching fraction of ${\eta }_c\to \gamma \gamma$ is measured for the first time to be $\mathcal{B}({\eta }_c\to \gamma \gamma )=(2.45\pm {0.48}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}}\pm {0.09}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}})\times {10}^{-4}$. Using the world average value of the total width of the ${\eta }_c$, the partial decay width of ${\eta }_c\to \gamma \gamma$ is calculated to be $\mathrm{\Gamma }({\eta }_c\to \gamma \gamma )=(7.48\pm {1.48}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}}\pm {0.30}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}})\mathrm{k}\mathrm{e}\mathrm{V}$.