A Measurement of the Mass and Full-Width of the $η_c$ Meson

Abstract

In a sample of 7.8 million $J/ψ$ decays collected in the Beijing Spectrometer, the process J/$ψ\toγη_c$ is observed for five different $η_c$ decay channels: $K^+K^-π^+π^-$, $π^+π^-π^+π^-$, $K^\pm K^0_S π^\mp$ (with $K^0_S\toπ^+π^-$), $ϕϕ$ (with $ϕ\to K^+K^-$) and $K^+K^-π^0$. From these signals, we determine the mass of $η_c$ to be $2976.6\pm2.9\pm1.3$ MeV. Combining this result with a previously reported result from a similar study using $ψ(2S)\toγη_c$ detected in the same spectrometer gives $m_{η_c} = 2976.3\pm2.3\pm1.2$ MeV. For the combined samples, we obtain $Γ_{η_c} = 11.0\pm 8.1\pm 4.1$ MeV.