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

## TL;DR

This paper reports the first observation of the decay of the $ ext{eta}_c$ meson into two omega mesons in $J/\psi$ radiative decays, measuring its branching fraction, mass, and width with high precision.

## Contribution

It provides the first measurement of the branching fraction, mass, and width of $ ext{eta}_c o ext{omega} ext{omega}$ decay in $J/\psi$ decays, using a large BESIII data sample.

## Key findings

- Branching fraction $ ext{B}( ext{eta}_c o ext{omega} ext{omega})$ measured as $(2.88 extpm0.10 extpm0.46 extpm0.68) imes10^{-3}$.
- Mass of $ ext{eta}_c$ determined as $2985.9 extpm0.7 extpm2.1$ MeV/$c^2$.
- Width of $ ext{eta}_c$ measured as $33.8 extpm1.6 extpm4.1$ MeV.

## Abstract

Using a sample of $(1310.6\pm7.0)\times10^6$ $J/\psi$ 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 $J/\psi\to\gamma\omega\omega$. 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 $\mathcal{B}(J/\psi\to\gamma\eta_c)$. The mass and width of the $\eta_c$ are determined as $M=(2985.9\pm0.7\pm2.1)\,$MeV/$c^2$ and $\Gamma=(33.8\pm1.6\pm4.1)\,$MeV.