Observation of $X(3872)$ in $B to X(3872) Kpi$ decays

We report the first observation of $B^0 to X(3872) (K^{+}pi^{-})$ and evidence for $B^+ to X(3872) (K^{0}pi^{+})$. We measure the product of branching fractions for the former to be ${cal B}(B^0 to X(3872) (K^+ pi^-)) times {cal B}(X(3872) to J/psi pi^+ pi^-) = (7.9 pm 1.3(mbox{stat.})pm 0.4(mbox{syst.})) times 10^{-6}$ and find that $B^{0}to X(3872) K^{*}(892)^{0}$ does not dominate the $B^{0}to X(3872)K^{+}pi^{-}$ decay mode. We also measure ${cal B}(B^+ to X(3872) (K^0 pi^+)) times {cal B}(X(3872) to J/psi pi^+ pi^-) = (10.6 pm 3.0(mbox{stat.}) pm 0.9(mbox{syst.})) times 10^{-6}$. This study is based on the full data sample of 711~fb$^{-1}$ ($772times 10^6 Bbar B$ pairs) collected at the $Upsilon(4S)$ resonance with the Belle detector at the KEKB collider.