Evidence for $\eta_{c} \rightarrow \gamma\gamma$ and Measurement of $J/\psi\rightarrow 3\gamma$

Abstract

The decay of $J/\psi$ to three photons is studied using $\psi(3686)\to\pi^+\pi^-J/\psi$ in a sample of $1.0641\times10^8$ $\psi(3686)$ events collected with the BESIII detector. Evidence of the direct decay of $\eta_c$ to two photons, $\eta_c\to\gamma\gamma$, is reported, and the product branching fraction is determined to be $\mathcal{B}(J/\psi\to\gamma\eta_c,\eta_c\to \gamma\gamma)=(4.5\pm1.2\pm0.6)\times10^{-6}$, where the first error is statistical and the second is systematic. The branching fraction for $J/\psi\to3\gamma$ is measured to be $(11.3\pm1.8\pm2.0)\times 10^{-6}$ with improved precision.