Measurement of the Pseudoscalar Decay Constant fDs Using Charm-Tagged Events in e+e- Collisions at s=10.58 GeV

Using 230.2  fb-1 of e+e- annihilation data collected with the BABAR detector at and near the peak of the Υ(4S) resonance, 489±55 events containing the pure leptonic decay Ds+→μ+νμ have been isolated in charm-tagged events. The ratio of partial widths Γ(Ds+→μ+νμ)/Γ(Ds+→ϕπ+) is measured to be 0.143±0.018±0.006 allowing a determination of the pseudoscalar decay constant fDs=(283±17±7±14)  MeV. The errors are statistical, systematic, and from the Ds+→ϕπ+ branching ratio, respectively.