We report on a search for ${\overline{B}}^0\to D^{(*)0}{\overline{K}}^{(*)0}$ decays based on $85\times {10}^6$ $B\overline{B}$ events collected with the Belle detector at KEKB. The ${\overline{B}}^0\to D^0{\overline{K}}^0$ and ${\overline{B}}^0\to D^0{\overline{K}}^{*0}$ decays have been observed for the first time with the branching fractions $\mathcal{B}({\overline{B}}^0\to D^0{\overline{K}}^0)=({5.0}_{-1.2}^{+1.3}\pm 0.6)\times {10}^{-5}$ and $\mathcal{B}({\overline{B}}^0\to D^0{\overline{K}}^{*0})=({4.8}_{-1.0}^{+1.1}\pm 0.5)\times {10}^{-5}$. No significant signal has been found for the ${\overline{B}}^0\to D^{*0}{\overline{K}}^{(*)0}$ and ${\overline{B}}^0\to {\overline{D}}^{(*)0}{\overline{K}}^{*0}$ decay modes, and upper limits at 90% C.L. are presented.

• Received 29 December 2002

DOI:https://doi.org/10.1103/PhysRevLett.90.141802