Leptonic decay-constant ratio fₖ₊/fpi+ from lattice QCD with physical light quarks

الأصل · EN

A calculation of the ratio of leptonic decay constants fₖ₊/fπ₊ makes possible a precise 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₊/fπ₊ 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₊/fπ₊ = 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₊/fπ₊ from lattice-QCD simulations at the physical point. Our result is the most precise lattice-QCD determination of fₖ₊/fπ₊, 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.