Search for the decay $B^0\to D{K}^{\ast 0}$ followed by $D\to K^{-}{\pi }^{+}$

We report a study of the decay $B^0\to D{K}^{+}{\pi }^{-}$ followed by $D\to K^{-}{\pi }^{+}$, where $D$ indicates $D^0$ or ${\bar{D}}^0$. We reconstruct the $D{K}^{+}{\pi }^{-}$ state in a phase space corresponding to $D K^{*}(892)^0$\mathrm{.}TheCP-violatingangle$\phi_3$ affects its decay rate via the interference between $b\to u$ and $b\to c$ transitions. The result is obtained from a 711 ${\mathrm{f}\mathrm{b}}^{-1}$ data sample that contains 772 $\times {10}^{6}B\bar{B}$ pairs collected at the $\mathrm{{\rm Y}}(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. We measure the ratio ${\cal R}_{DK^{*0}} \equiv \Gamma(B^0\rightarrow [K^-\pi^+]DK^+\pi^-)/\Gamma(B^0\rightarrow [K^+\pi^-]DK^+\pi^-)$tobe$(4.1 ^{+ 5.6 + 2.8}{- 5.0 - 1.8}) \times 10^{-2}$,andsetanupperlimitof${\cal R}{DK^{*0}} < 0.16$ at the 95% confidence level.