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Using a sample of $(1310.6\pm7.0)\times10^6$ $J/\psi$ events recorded withthe BESIII detector at the symmetric electron positron collider BEPCII, wereport the observation of the decay of the $(1^1 S_0)$ charmonium state$\eta_c$ into a pair of $\omega$ mesons in the process$J/\psi\to\gamma\omega\omega$. The branching fraction is measured for the firsttime to be $\mathcal{B}(\eta_c\to\omega\omega)=(2.88\pm0.10\pm0.46\pm0.68)\times10^{-3}$, where the first uncertainty isstatistical, the second systematic and the third is from the uncertainty of$\mathcal{B}(J/\psi\to\gamma\eta_c)$. The mass and width of the $\eta_c$ aredetermined as $M=(2985.9\pm0.7\pm2.1)\,$MeV/$c^2$ and$\Gamma=(33.8\pm1.6\pm4.1)\,$MeV.

Observation of $\eta_c\to\omega\omega$ in $J/\psi\to\gamma\omega\omega$