Measurements of the mass and full-width of the ηc meson

Abstract In a sample of 58 million J / ψ events collected with the BES II detector, the process J / ψ → γη c is observed in five different decay channels: γK + K − π + π − , γπ + π − π + π − , γK ± K 0 S π ∓ (with K 0 S → π + π − ), γφφ (with φ → K + K − ) and γp p . From a combined fit of all five channels, we determine the mass and full-width of the η c to be m η c =2977.5±1.0 ( stat. )±1.2 ( syst. )  MeV /c 2 and Γ η c =17.0±3.7 ( stat. )±7.4 ( syst. )  MeV /c 2 .