Measurement of the absolute branching fractions for D-s(-) -> l(-) (nu)over-bar(l) and extraction of the decay constant f(Ds)

The absolute branching fractions for the decays D^-_s → l^- ν_ l (l = e,µ, or τ) are measured using a data sample corresponding to an integrated luminosity of 521 fb^(-1) collected at center-of-mass energies near 10.58 GeV with the BABAR detector at the PEP-II e^+e^- collider at SLAC. The number of D^-_s mesons is determined by reconstructing the recoiling system DKXγ in events of the type e^+e^- → DKXD^(*-)_s , where D^(*-) → D^-_s γ and X represents additional pions from fragmentation. The D^-_s → l^- events are detected by full or partial reconstruction of the recoiling system DKXγl. The branching fraction measurements are combined to determine the D^-_s decay constant f_(ds) =(258.6 ± 6:4 ± 7.5) MeV, where the first uncertainty is statistical and the second is systematic.