Observation of a Narrow Charmoniumlike State in Exclusive $B^{\pm }\to K^{\pm }{\pi }^{+}{\pi }^{-}J/\psi$ Decays

We report the observation of a narrow charmoniumlike state produced in the exclusive decay process $B^{\pm }\to K^{\pm }{\pi }^{+}{\pi }^{-}J/\psi$. This state, which decays into ${\pi }^{+}{\pi }^{-}J/\psi$, has a mass of $3872.0\pm 0.6\mathrm{(}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{)}\pm 0.5\mathrm{(}\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{)}\mathrm{M}\mathrm{e}\mathrm{V}$, a value that is very near the $M_{D^0}+M_{D^{*0}}$ mass threshold. The results are based on an analysis of 152M $B$-$\overline{B}$ events collected at the ${\rm Y}(4S)$ resonance in the Belle detector at the KEKB collider. The signal has a statistical significance that is in excess of $10\sigma$.

Figure 1

Distribution of $M({\pi }^{+}{\pi }^{-}{\ell }^{+}{\ell }^{-})-M({\ell }^{+}{\ell }^{-})$ for selected events in the $\Delta E$-$M_{\mathrm{b}\mathrm{c}}$ signal region for (a) Belle data and (b) generic $B$-$\overline{B}$ MC events.

Figure 2

Signal-band projections of (a) $M_{\mathrm{b}\mathrm{c}}$, (b) $M_{{\pi }^{+}{\pi }^{-}J/\psi }$, and (c) $\Delta E$ for the $X(3872)\to {\pi }^{+}{\pi }^{-}J/\psi$ signal region with the results of the unbinned fit superimposed.

Figure 3

$M({\pi }^{+}{\pi }^{-})$ distribution for events in the (a) $M({\pi }^{+}{\pi }^{-}J/\psi )=3872\mathrm{M}\mathrm{e}\mathrm{V}$ signal region, and (b) the ${\psi }^{\prime }$ region. The shaded histograms are sideband data normalized to the signal-box area. Note the different horizontal scales.

Figure 4

Signal-band projections of (a) $M_{\mathrm{b}\mathrm{c}}$ and (b) $M_{\gamma {\chi }_{c1}}$ for the ${\psi }^{\prime }$ region with the results of the unbinned fit superimposed. (c),(d) The corresponding results for the $M=3872\mathrm{M}\mathrm{e}\mathrm{V}$ region.