title: Leptonic-decay-constant ratio f(K+)/f(π+) from lattice QCD with physical light quarks.
abstract: A calculation of the ratio of leptonic decay constants f(K+)/f(π+) makes possible a precise determination of the ratio of Cabibbo-Kobayashi-Maskawa (CKM) matrix elements |V(us)|/|V(ud)| 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 f(K+)/f(π+) 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 f(K+)/f(π+) = 1.1947(26)(37), where the errors are statistical and total systematic, respectively. This is our first physics result from our N(f) = 2+1+1 ensembles, and the first calculation of f(K+)/f(π+) from lattice-QCD simulations at the physical point. Our result is the most precise lattice-QCD determination of f(K+)/f(π+), 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 |V(us)|/|V(ud)| = 0.2309(9)(4) where the errors are theoretical and experimental, respectively.
authors.affiliation: Physics Department, Brookhaven National Laboratory, Upton, New York 11973, USA.
authors.forename: A
authors.initials: A
authors.lastname: Bazavov
authors.forename: C
authors.initials: C
authors.lastname: Bernard
authors.forename: C
authors.initials: C
authors.lastname: DeTar
authors.forename: J
authors.initials: J
authors.lastname: Foley
authors.forename: W
authors.initials: W
authors.lastname: Freeman
authors.forename: Steven
authors.initials: S
authors.lastname: Gottlieb
authors.forename: U M
authors.initials: UM
authors.lastname: Heller
authors.forename: J E
authors.initials: JE
authors.lastname: Hetrick
authors.forename: J
authors.initials: J
authors.lastname: Kim
authors.forename: J
authors.initials: J
authors.lastname: Laiho
authors.forename: L
authors.initials: L
authors.lastname: Levkova
authors.forename: M
authors.initials: M
authors.lastname: Lightman
authors.forename: J
authors.initials: J
authors.lastname: Osborn
authors.forename: S
authors.initials: S
authors.lastname: Qiu
authors.forename: R L
authors.initials: RL
authors.lastname: Sugar
authors.forename: D
authors.initials: D
authors.lastname: Toussaint
authors.forename: R S
authors.initials: RS
authors.lastname: Van de Water
authors.forename: R
authors.initials: R
authors.lastname: Zhou