Measurement of the $B^0\to D^{\ast-} D_s^{\ast+}$ and $D_s^+\to\phi\pi^+$ Branching Fractions

We present measurements of the branching fractions $\BR(B^0\to D^{\ast-} D_s^{\ast+})$ and $\BR(D_s^+\to\phi\pi^+)$, based on $123\times10^6$ Y(4S)->BBbar decays collected by the BaBar detector at the PEP-II asymmetric-energy $e^+e^-$ B factory. A partial reconstruction technique is used to measure $\BR(B^0\to D^{\ast-} D_s^{\ast+})$ and the decay chain is fully reconstructed to measure the branching fraction product $\BR(B^0\to D^{\ast-} D_s^{\ast+})\times\BR(D_s^+\to\phi\pi^+)$. Comparing these two measurements provides a model-independent determination of the \Dsphipi\ branching fraction. We obtain $\BR(B^0\to D^{\ast-} D_s^{\ast+}) = (1.88 \pm 0.09 \pm 0.17)%$ and $\BR(D_s^+\to\phi\pi^+) = (4.81 \pm 0.52 \pm 0.38)%$, where the first uncertainties are statistical and the second systematic.