Pseudoscalar decay constants, light-quark masses, and B_K from mixed-action lattice QCD

We present updated results for the leptonic decay constants f_π and f_K, the light u, d, and s-quark masses, and the neutral kaon mixing parameter B_K from mixed-action lattice simulations with staggered sea quarks and domain-wall valence quarks. We use the publicly-available 2+1 flavor MILC asqtad-improved staggered gauge configurations with multiple light sea-quark masses and three lattice spacings, and compute the kaon mixing matrix element with several partially-quenched valence-quark masses. We then extrapolate to the physical light-quark masses and the continuum using partially-quenched chiral perturbation theory formulated for mixed-action lattice simulations. For B_K we match the lattice four-fermion operator to the continuum using the nonperturbative method of Rome-Southampton. Our new results benefit from two significant improvements over our published work: (1) we have added a third lattice spacing of a≈0.06 fm to better control the continuum extrapolation, and (2) we have implemented a new lattice renormalization scheme (the RI/SMOM_γµ scheme developed by Sturm et al.) that suppresses chiral-symmetry breaking and other infrared effects and, in practice, also shrinks the size of the 1-loop perturbative coefficient needed to match to the continuum MS scheme. When combined with the use of volume-averaged momentum sources and twisted-boundary conditions, this significantly reduces the systematic uncertainty in the renormalization factor Z_BK