Search for the Decays ${\mathit{B}}^0\to {\mathit{D}}^{\left(*\right)+}{D}^{\left(*\right)-}$

Using the CLEO-II data set we have searched for the decays $B^0\to D^{(*)+}{D}^{(*)-}$. We observe one candidate signal event for the decay $B^0\to D^{*+}{D}^{*-}$ with an expected background of $0.022\pm 0.011$ events. This yield corresponds to a branching fraction of $B(B^0\to D^{*+}{D}^{*-})=\left[{5.3}_{-3.7\mathrm{}}^{+7.1\mathrm{}}\right(\mathrm{stat})\pm 1.0(\mathrm{syst}\left)\right]\times {10}^{-4\mathrm{}}$ and an upper limit of $B(B^0\to D^{*+}{D}^{*-})<2.2\mathrm{}\times {10}^{-3\mathrm{}}$ at the 90% C.L. We see no significant excess of signal above the expected background level in the other modes, and we calculate the 90% C.L. upper limits on the branching fractions to be $B(B^0\to D^{*\pm }{D}^{\mp })<1.8\mathrm{}\times {10}^{-3\mathrm{}}$ and $B(B^0\to D^{+}{D}^{-})<1.2\mathrm{}\times {10}^{-3\mathrm{}}$.