Observation of the ${\eta }_c(2S)$ in Exclusive $B\to K{K}_S{K}^{-}{\pi }^{+}$ 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.

Figure 1

The $M_{\mathrm{b}\mathrm{c}}$ projections for $40\mathrm{M}\mathrm{e}\mathrm{V}/c^2$ bins of $M_{K_{S}K\pi }$, with central values ranging from 2840 (a) to $3800\mathrm{M}\mathrm{e}\mathrm{V}/c^2$ (y). Only events with $|\Delta E|<40\mathrm{M}\mathrm{e}\mathrm{V}$ are included; the charged and neutral $B$ decay modes are combined.

Figure 2

The (a) $M_{\mathrm{b}\mathrm{c}}$ and (b) $\Delta E$ projections for the $M_{K_{S}K\pi }=3640\mathrm{M}\mathrm{e}\mathrm{V}/c^2$ mass bin. The curves are the results of the simultaneous fit described in the text.

Figure 3

The distribution of signal events from the simultaneous fits to $M_{\mathrm{b}\mathrm{c}}$ and $\Delta E$ for each $K_{S}K\pi$ mass bin. The curve is the result of the fit described in the text.