Measurement of the absolute branching fractions for D_{s}^{-}→ℓ^{-}ν[over ¯]_{ℓ} and extraction of the decay constant f_{D_{s}}

The absolute branching fractions for the decays Ds-→ℓ-ν̅ 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 Ds- mesons is determined by reconstructing the recoiling system DKXγ in events of the type e+e-→DKXDs*-, where Ds*-→Ds-γ and X represents additional pions from fragmentation. The Ds-→ℓ-νℓ events are detected by full or partial reconstruction of the recoiling system DKXγℓ. The branching fraction measurements are combined to determine the Ds- decay constant fDs=(258.6±6.4±7.5)  MeV, where the first uncertainty is statistical and the second is systematic.