Study of $B \to X(3872) K π$ at Belle

We report the first observation of $B^0 \to X(3872) (K^{+}π^{-})$ and evidence for $B^+ \to X(3872) (K^{0}π^{+})$. The product of branching fractions for the former decay mode is measured to be ${\cal B}(B^0 \to X(3872) (K^+ π^-)) \times {\cal B}(X(3872) \to J/ψπ^+ π^-) = (7.9 \pm 1.3(\mbox{stat.})\pm 0.4(\mbox{syst.})) \times 10^{-6}$ and also find that $B^{0}\to X(3872) K^{*}(892)^{0}$ does not dominate the $B^{0}\to X(3872)K^{+}π^{-}$ decay mode in contrast to other charmonium states like $ψ'$. The product of branching fractions for the latter decay mode is measured to be ${\cal B}(B^+ \to X(3872) (K^0 π^+)) \times {\cal B}(X(3872) \to J/ψπ^+ π^-) = (10.6 \pm 3.0(\mbox{stat.}) \pm 0.9(\mbox{syst.})) \times 10^{-6}$. 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 $Υ(4S)$ resonance with the Belle detector at the KEKB collider.

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