Measurement of the Ds+ →+ν branching fractions and the decay constant f Ds+

Using 482 pb-1 of e+e- collision data collected at a center-of-mass energy of s=4.009 GeV with the BESIII detector, we measure the branching fractions of the decays Ds+→μ+νμ and Ds+→τ+ντ. By constraining the ratio of decay rates of Ds+ to τ+ντ and to μ+νμ to the Standard Model prediction, the branching fractions are determined to be B(Ds+→μ+νμ)=(0.495±0.067±0.026)% and B(Ds+→τ+ντ)=(4.83±0.65±0.26)%. Using these branching fractions, we obtain a value for the decay constant fDs+ of (241.0±16.3±6.5) MeV, where the first error is statistical and the second systematic.