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

BESIII Collaboration, M. Ablikim, M. N. Achasov +662

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 p…