Measurement of $f_{D_s}$ in $D_s \rightarrow \mu \nu$ decays

We present a measurement of the branching fraction $BR(D_s \rightarrow \mu \nu)$ using the complete data-set (521 ${\rm fb}^{-1}$) collected by the {\sl B{\scriptsize A}B{\scriptsize AR}} detector at the PEP-II asymmetric-energy $B$ factory at SLAC. The $D_s$ yield is determined by a fit to the mass recoiling against reconstructed $D K X \gamma$ candidates in events of the type $e^+e^- \rightarrow D K X D_s^* $, where $D_s^* \rightarrow D_s \gamma$ and $X$ represents additional pions from fragmentation. The neutrino yield is determined using the mass recoiling against reconstructed $D K X \gamma \mu$ candidates. This full-reconstruction method provides high resolution in the neutrino mass and thus good background separation.