Observation of ${\eta }_c\to \omega \omega$ in $J\mathrm{/}\psi \to \gamma \omega \omega$

Using a sample of $(1310.6\pm 7.0)\times {10}^6$ $J\mathrm{/}\psi$ events recorded with the BESIII detector at the symmetric electron positron collider BEPCII, we report the observation of the decay of the $(1^1{S}_0)$ charmonium state ${\eta }_c$ into a pair of $\omega$ mesons in the process $J\mathrm{/}\psi \to \gamma \omega \omega$. The branching fraction is measured for the first time to be $$\mathcal{B}$(\eta_c\to\omega\omega)= (2.88\pm0.10\pm0.46\pm0.68)\times10^{-3}$, where the first uncertainty is statistical, the second systematic and the third is from the uncertainty of $\mathcal{B}(J\mathrm{/}\psi \to \gamma {\eta }_c)$. The mass and width of the ${\eta }_c$ are determined as $M=(2985.9\pm 0.7\pm 2.1)$MeV/$c^2$ and $\mathrm{\Gamma }=(33.8\pm 1.6\pm 4.1)$MeV.