Study of $B\to X(3872)K\pi$ at Belle

We report the first observation of $B^0\to X(3872)(K^{+}{\pi }^{-})$ and evidence for $B^{+}\to X(3872)(K^0{\pi }^{+})$. The product of branching fractions for the former decay mode is measured to be ${\cal B}(B^0 \to X(3872) (K^+ \pi^-)) \times {\cal B}(X(3872) \to J/\psi \pi^+ \pi^-) = (7.9 \pm 1.3(\mbox{stat.})\pm 0.4(\mbox{syst.})) \times 10^{-6}$andalsofindthat$B^{0}\to X(3872) K^{*}(892)^{0}$doesnotdominatethe$B^{0}\to X(3872)K^{+}\pi^{-}$decaymodeincontrasttoothercharmoniumstateslike$\psi’$. The product of branching fractions for the latter decay mode is measured to be ${\cal B}(B^+ \to X(3872) (K^0 \pi^+)) \times {\cal B}(X(3872) \to J/\psi \pi^+ \pi^-) = (10.6 \pm 3.0(\mbox{stat.}) \pm 0.9(\mbox{syst.})) \times 10^{-6}$. This study is based on the full and final data sample of 711~fb${}^{-1}$ ($772\times {10}^{6}B\bar{B}$ pairs) collected at the $\mathrm{{\rm Y}}(4S)$ resonance with the Belle detector at the KEKB collider.