Measurements of the Mass and Full-Width of the $η_c$ Meson

Abstract

In a sample of 58 million $J/ψ$ events collected with the BES II detector, the process J/$ψ\toγη_c$ is observed in five different decay channels: $γK^+K^-π^+π^-$, $γπ^+π^-π^+π^-$, $γK^\pm K^0_S π^\mp$ (with $K^0_S\toπ^+π^-$), $γϕϕ$ (with $ϕ\to K^+K^-$) and $γp\bar{p}$. From a combined fit of all five channels, we determine the mass and full-width of $η_c$ to be $m_{η_c}=2977.5\pm1.0 ({stat.})\pm1.2 ({syst.})$ MeV/$c^2$ and $Γ_{η_c} = 17.0\pm3.7 ({stat.})\pm7.4 ({syst.})$ MeV/$c^2$.