Observation of ${\eta }_{c}\to \omega \omega $ in $J/\psi \to \gamma \omega \omega $

Using a sample of $\left(1310.6±7.0\right)×{10}^{6}$ $J/\psi $ events recorded with the Beijing Spectrometer III detector at the Beijing Electron Positron Collider II, we report the observation of the decay of the $\left({1}^{1}{S}_{0}\right)$ charmonium state ${\eta }_{c}$ into a pair of $\omega $ mesons in the process $J/\psi \to \gamma \omega \omega $. The branching fraction is measured for the first time to be $B\left({\eta }_{c}\to \omega \omega \right)=\left(2.88±0.10±0.46±0.68\right)×{10}^{-3}$, where the first uncertainty is statistical, the second systematic, and the third is from the uncertainty of $B\left(J/\psi \to \gamma {\eta }_{c}\right)$. The mass and width of the ${\eta }_{c}$ are determined as $M=\left(2985.9±0.7±2.1\right)\text{}\text{}\mathrm{MeV}/{c}^{2}$ and $\Gamma =\left(33.8±1.6±4.1\right)\text{}\text{}\mathrm{MeV}$.