Study of the $X(3872)$ and $Y(4260)$ in $B^0\to J/\psi {\pi }^{+}{\pi }^{-}{K}^0$ and $B^{-}\to J/\psi {\pi }^{+}{\pi }^{-}{K}^{-}$ decays

We present results of a search for the $X(3872)$ in $B^0\to X(3872)K_S^0,X(3872)\to J/\psi {\pi }^{+}{\pi }^{-}$, improved measurements of $B^{-}\to X(3872)K^{-}$, and a study of the $J/\psi {\pi }^{+}{\pi }^{-}$ mass region above the $X(3872)$. We use $232\times {10}^6$ $B\overline{B}$ pairs collected at the ${\rm Y}(4S)$ resonance with the BABAR detector at the PEP-II $e^{+}{e}^{-}$ asymmetric-energy storage rings. The results include the 90% confidence interval $1.34\times {10}^{-6}<\mathcal{B}(B^0\to X(3872)K^0,X\to J/\psi {\pi }^{+}{\pi }^{-})<10.3\times {10}^{-6}$ and the branching fraction $\mathcal{B}(B^{-}\to X(3872)K^{-},X\to J/\psi {\pi }^{+}{\pi }^{-})=(10.1\pm 2.5\pm 1.0)\times {10}^{-6}$. We observe a $(2.7\pm 1.3\pm 0.2)\mathrm{MeV}/c^2$ mass difference of the $X(3872)$ produced in the two decay modes. Furthermore, we search for the $Y(4260)$ in $B$ decays and set the $95\%$ C.L. upper limit $\mathcal{B}(B^{-}\to Y(4260)K^{-},Y(4260)\to J/\psi {\pi }^{+}{\pi }^{-})<2.9\times {10}^{-5}$.

Figure 1

Signal region projections of $m(J/\psi {\pi }^{+}{\pi }^{-})$ and $m_{\mathrm{ES}}$ for $B^{-}\to X(3872)K^{-}$ (a,c) and $B^0\to X(3872)K_S^0$ (b,d). The dashed line represents the combinatorial background PDF, the dotted line represents the sum of the combinatorial and peaking background PDF, and the solid line the sum of all background plus the signal PDF. The shaded area shows events in the $m_{\mathrm{ES}}$ sideband region $|{\mathrm{m}}_{\mathrm{ES}}-5260\mathrm{MeV}/c^2|<6\mathrm{MeV}/c^2$.

Figure 2

Projection of the two-dimensional fit for the mass region above $4\mathrm{GeV}/c^2$ for events within the $m_{\mathrm{ES}}$ signal region in $B^{-}\to J/\psi {\pi }^{+}{\pi }^{-}{K}^{-}$. The lines represent the same event types as in Fig. 1.