Measurement of the absolute branching fractions for purely leptonic Ds+ decays

We report new measurements of the branching fraction B(Ds+→ℓ+ν), where is either μ+ or τ+(→π+ν¯τ), based on 6.32 fb-1 of electron-positron annihilation data collected by the BESIII experiment at six center-of-mass energy points between 4.178 and 4.226 GeV. Simultaneously floating the Ds+→μ+νμ and Ds+→τ+ντ components yields B(Ds+→τ+ντ)=(5.21±0.25±0.17)×10-2, B(Ds+→μ+νμ)=(5.35±0.13±0.16)×10-3, and the ratio of decay widths R=Γ(Ds+→τ+ντ)Γ(Ds+→μ+νμ)=9.73-0.58+0.61±0.36, where the first uncertainties are statistical and the second systematic. No evidence of CP asymmetry is observed in the decay rates Ds±→μ±νμ and Ds±→τ±ντ: ACP(μ±ν)=(-1.2±2.5±1.0)% and ACP(τ±ν)=(+2.9±4.8±1.0)%. Constraining our measurement to the Standard Model expectation of lepton universality (R=9.75), we find the more precise results B(Ds+→τ+ντ)=(5.22±0.10±0.14)×10-2 and ACP(τ±ντ)=(-0.1±1.9±1.0)%. Combining our results with inputs external to our analysis, we determine the c→s¯ quark mixing matrix element, Ds+ decay constant, and ratio of the decay constants to be |Vcs|=0.973±0.009±0.014, fDs+=249.9±2.4±3.5 MeV, and fDs+/fD+=1.232±0.035, respectively.