Measurement of the mass and full width of theηcmeson

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Abstract

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

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