Evidence forηc→γγand measurement ofJ/ψ→3γ

The decay of $J/\ensuremath{\psi}$ to three photons is studied using $\ensuremath{\psi}(3686)\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}J/\ensuremath{\psi}$ in a sample of $1.0641\ifmmode\times\else\texttimes\fi{}{10}^{8}$ $\ensuremath{\psi}(3686)$ events collected with the BESIII detector. Evidence of the direct decay of ${\ensuremath{\eta}}_{c}$ to two photons, ${\ensuremath{\eta}}_{c}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}$, is reported, and the product branching fraction is determined to be $\mathcal{B}(J/\ensuremath{\psi}\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\eta}}_{c},\phantom{\rule{0ex}{0ex}}{\ensuremath{\eta}}_{c}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma})=(4.5\ifmmode\pm\else\textpm\fi{}1.2\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$, where the first error is statistical and the second is systematic. The branching fraction for $J/\ensuremath{\psi}\ensuremath{\rightarrow}3\ensuremath{\gamma}$ is measured to be $(11.3\ifmmode\pm\else\textpm\fi{}1.8\ifmmode\pm\else\textpm\fi{}2.0)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ with improved precision.