We report the first observation of the decay $B^0\to D^{\pm }{D}^{*\mp }$ with the Belle detector at the KEKB $e^{+}{e}^{-}$ Collider operated at the ${\rm Y}(4S)$ resonance. The sum of branching fractions $\mathcal{B}(B^0\to D^{+}{D}^{*-})+\mathcal{B}(B^0\to D^{-}{D}^{*+})$ is measured to be $(1.17\pm {0.26}_{-0.25}^{+0.22})\times {10}^{-3}$ using the full reconstruction method where both charmed mesons from $B^0$ decays are reconstructed. A consistent value [$(1.48\pm {0.38}_{-0.31}^{+0.28})\times {10}^{-3}$] is obtained using a partial reconstruction technique that uses only the slow pion from the $D^{*-}\to {\overline{D}}^0{\pi }^{-}$ decay and a fully reconstructed $D^{+}$ to reconstruct the $B^0$.

• Received 6 June 2002

DOI:https://doi.org/10.1103/PhysRevLett.89.122001