Observation of theηc(2S)in ExclusiveB→KKSK−π+Decays

We report the observation of a narrow peak in the $K_S{K}^{-}{\pi }^{+}$ invariant mass distribution in a sample of exclusive $B\to K{K}_S{K}^{-}{\pi }^{+}$ decays collected with the Belle detector at the KEKB asymmetric $e^{+}{e}^{-}$ collider. The measured mass of the peak is $M=3654\pm 6(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 8(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})\mathrm{M}\mathrm{e}\mathrm{V}/c^2$, and we place a 90% confidence level upper limit on the width of $\Gamma <55\mathrm{M}\mathrm{e}\mathrm{V}/c^2$. The properties agree with heavy-quark potential model expectations for the ${\eta }_c(2S)$ meson, the $n=2$ singlet $S$ charmonium state.