Measurement of the mass and full width of the ${\eta }_c$ meson

In a sample of 7.8 million $J/\psi$ decays collected in the Beijing Spectrometer, the process $J/\overrightarrow{\psi }\gamma {\eta }_c$ is observed for five different ${\eta }_c$ decay channels: $K^{+}{K}^{-}{\pi }^{+}{\pi }^{-},$ ${\pi }^{+}{\pi }^{-}{\pi }^{+}{\pi }^{-},$ $K^{\pm }{K}_S^0{\pi }^{\mp }$ (with $K_S^0\to {\pi }^{+}{\pi }^{-}),$ $\phi \phi$ (with $\overrightarrow{\phi }{K}^{+}{K}^{-})$ and $K^{+}{K}^{-}{\pi }^0.$ From these signals, we determine the mass of ${\eta }_c$ to be $2976.6\pm 2.9\pm 1.3$ MeV. Combining this result with a previously reported result from a similar study using $\psi \mathrm{}\left(2S\right)\mathrm{}\to \gamma {\eta }_c$ detected in the same spectrometer gives $m_{{\eta }_c}=2976.3\mathrm{}\pm 2.3\pm 1.2$ MeV. For the combined samples, we obtain ${\Gamma }_{{\eta }_c}=11.0\mathrm{}\pm 8.1\pm 4.1$ MeV.