Precision Measurement of the Exclusive $B^0 \to D^{*-} \ell^+ \nu$ Branching Fraction

We report on a precision measurement of the exclusive $B^0 \to D^{*-} \ell^+ \nu$ branching fraction using data collected with the {\it B{\footnotesize A}B{\footnotesize AR}} detector at the PEP-II asymmetric-energy $e^+e^-$ storage rings at SLAC running on the $\Upsilon(4S)$ resonance. Using a data sample of approximately 470 million $B\bar{B}$ pair events we isolate those with the prompt lepton and slow pions from the $D^{*-} \to D^0 \pi_{slow}$ cascade decay. A partial reconstruction technique is used in which the $D^*$ four- momentum is inferred from the slow pion, $\pi_{slow}$. This allows for a much higher statistical precision on this important branching fraction as well as an overall systematic error competitive with full reconstruction techniques.