The $\eta_c(3654)$ and hyperfine splitting in charmonium

Abstract

The hyperfine splitting for the 2S charmonium state is calculated and the predicted number is $\Delta_{\rm HF}(2S) = 57 \pm 8$ MeV, being by derivation the lower bound of this splitting. It results in $M(\eta_c(2S))= 3630 \pm 8$ MeV, which is smaller by two standard deviations than found in the Belle experiment \cite{ref.01}, but close to the $\eta_c(2S)$ mass observed by the same group in the experiment $e^+e^- \to J/\psi \eta_c$ \cite{ref.06} where $M(\eta_c(2S)) = 3622 \pm 12$ MeV was found.