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A calculation of the ratio of leptonic decay constants f_{K^+}/f_{\pi^+}makes possible a precise determination of the ratio of CKM matrix elements|V_{us}|/|V_{ud}| in the Standard Model, and places a stringent constraint onthe scale of new physics that would lead to deviations from unitarity in thefirst row of the CKM matrix. We compute f_{K^+}/f_{\pi^+} numerically inunquenched lattice QCD using gauge-field ensembles recently generated thatinclude four flavors of dynamical quarks: up, down, strange, and charm. Weanalyze data at four lattice spacings a ~ 0.06, 0.09, 0.12, and 0.15 fm withsimulated pion masses down to the physical value 135 MeV. We obtainf_{K^+}/f_{\pi^+} = 1.1947(26)(37), where the errors are statistical and totalsystematic, respectively. This is our first physics result from our N_f = 2+1+1ensembles, and the first calculation of f_{K^+}/f_{\pi^+} from lattice-QCDsimulations at the physical point. Our result is the most precise lattice-QCDdetermination of f_{K^+}/f_{\pi^+}, with an error comparable to the currentworld average. When combined with experimental measurements of the leptonicbranching fractions, it leads to a precise determination of |V_{us}|/|V_{ud}| =0.2309(9)(4) where the errors are theoretical and experimental, respectively.

Leptonic decay-constant ratio f_{K^+}/f_{pi^+} from lattice QCD with physical light quarks