Leptonic-decay--constant ratio fK^+/f\pi^+ from lattice QCD with physical light quarks
A. Bazavov, C. Bernard, C. DeTar, J. Foley, W. Freeman, Steven Gottlieb, U. M. Heller, J. E. Hetrick, J. Kim, J. Laiho, L. Levkova, M. Lightman, J. Osborn, S. Qiu, R. L. Sugar, D. Toussaint, R. S. Van de Water, and R. Zhou
Accepted
A calculation of the ratio of leptonic decay constants fK+/fp+ makes possible aprecise determination of the ratio of CKM matrix elements |Vus|/|Vud| in the Standard Model, and places a stringent constraint on the scale of new physics that would lead to deviations from unitarity in the first row of the CKM matrix. We compute fK+/fp+ numerically in unquenched lattice QCD using gauge-field ensembles recently generated that include four flavors of dynamical quarks: up, down, strange, and charm. We analyze data at four lattice spacings a 0.06, 0.09, 0.12, and 0.15 fm with simulated pion masses down to the physical value 135 MeV. We obtain fK+/fp+ = 1.1947(26)(37), where the errors are statistical and total systematic, respectively. This is our first physics result from our Nf = 2+1+1 ensembles, and the first calculation of fK+/fp+ from lattice-QCD simulations at the physical point. Our result is the most precise lattice-QCD determination of fK+/fp+, with an error comparable to the current world average. When combined with experimental measurements of the leptonic branching fractions, it leads to a precise determination of |Vus|/|Vud| = 0.2309(9)(4) where the errors are theoretical and experimental, respectively.