First Observation of the Decay ${\mathit{B}}^0\to {\mathit{D}}^{*+}{\mathit{D}}^{*-}$

We have observed four fully reconstructed $B^0\to D^{*+}{D}^{*-}$ candidates in $5.8\times {10}^6$ $\Upsilon \left(4S\right)\to B\overline{B}$ decays recorded with the CLEO detector. The background is estimated to be $0.31\pm 0.10$ events. The probability that the background could produce four or more signal candidates with the observed distribution among $D^{*+}$ and $D^{*-}$ decay modes is $1.1\times {10}^{-4\mathrm{}}$. The measured decay rate, $B(B^0\to D^{*+}{D}^{*-})=\left[{6.2}_{-2.9\mathrm{}}^{+4.0\mathrm{}}\right(\mathrm{stat})\pm 1.0(\mathrm{syst}\left)\right]\times {10}^{-4\mathrm{}}$, is large enough for this decay mode to be of interest for the measurement of a time-dependent CP asymmetry.