Measurement of the Relative Branching Fractions for B- -> D/D*/D**(D(*)pi) l- anti-nu with a Large Sample of Tagged B Mesons

We present a study of B semileptonic decays into charm final states based on 211.7 fb^{-1} of data collected at the \Upsilon(4S) resonance with the babar detector at the pep2 e^+e^- storage ring. Using a novel technique based on the simultaneous fit of a set of variables reconstructed on the recoil of a B tagged in an hadronic decay mode, we measure the relative branching fractions \Gamma(B^- \to D^0\ell^- \bar{\nu}_{\ell}) / \Gamma(B^- \to D X \ell^- \bar{\nu}_{\ell}) = 0.210 \pm 0.017 (stat.) \pm 0.021 (syst.), \Gamma(B^- \to D^{*0}\ell^- \bar{\nu}_{\ell}) / \Gamma(B^- \to D X \ell^- \bar{\nu}_{\ell}) = 0.611 \pm 0.022 (stat.) \pm 0.027 (syst.) and \Gamma(B^- \to D^{**0}(D^{(*)}\pi)\ell^- \bar{\nu}_{\ell}) / \Gamma(B^- \to D X \ell^- \bar{\nu}_{\ell}) = 0.173 \pm 0.017 (stat.) \pm 0.021 (syst.) .