A Measurement of the Mass and Full-Width of the ${\eta }_c$ Meson

Abstract

In a sample of 7.8 million $J/\psi$ decays collected in the Beijing Spectrometer, the process J/$\psi \to \gamma {\eta }_c$ 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 $K_S^0\to {\pi }^{+}{\pi }^{-}$), $\varphi \varphi$ (with $\varphi \to K^{+}{K}^{-}$) 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 $\psi (2S)\to \gamma {\eta }_c$ detected in the same spectrometer gives $m_{{\eta }_c}=2976.3\pm 2.3\pm 1.2$ MeV. For the combined samples, we obtain ${\Gamma }_{{\eta }_c}=11.0\pm 8.1\pm 4.1$ MeV.

4 pages, 3 figures and 1 table