Measurements of the mass and width of the $\eta_c$ using the decay $\psi(3686)\rightarrow\gamma\eta_c$

Abstract

The mass and width of the lowest-lying $S$-wave spin singlet charmonium state, the $η_c$, are measured using a data sample of $1.06×10^8 ψ(3686)$ decays collected with the BESIII detector at the BEPCII storage ring. We use a model that incorporates full interference between the signal reaction, $ψ(3686)→γη_c$, and a nonresonant radiative background to describe the line shape of the $η_c$ successfully. We measure the $η_c$ mass to be $2984.3±0.6±0.6 \text{MeV}/c^2$ and the total width to be $32.0±1.2±1.0 \text{MeV}$, where the first errors are statistical and the second are systematic.