Measurement of the branching fraction of $\Upsilon(4S) \to B^0 \bar B^0$ at {\sc BaBar}

Based on a data sample of 476 million $B\bar B$ pairs collected at the $\Upsilon(4S)$ resonance with the {\sc BaBar} detector at the PEP-II asymmetric-energy $B$-Factory at the SLAC National Accelerator Laboratory, we conduct a direct, model-independent measurement of the branching fraction of the $\Upsilon(4S)$ decay to a $B^0 \bar B^0$ pair. The $B$ mesons are partially reconstructed in the decay $\bar B^0 \to D^{*+} \ell^- \bar\nu_\ell$. This measurement is important for normalizing many $B$-decay branching fractions.