Measurement of the absolute branching fractions for D−s→ℓ−¯νℓ and extraction of the decay constant fDs

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 D K X γ in events of the type e + e − → D K X D * − s , where D * − s → D − s γ and X represents additional pions from fragmentation. The D − s → ℓ − ν ℓ events are detected by full or partial reconstruction of the recoiling system D K X γ ℓ . The branching fraction measurements are combined to determine the D − s decay constant f D s = ( 258.6 ± 6.4 ± 7.5 ) MeV , where the first uncertainty is statistical and the second is systematic