Neutral kaon mixing parameter B_K from unquenched mixed-action lattice QCD

We calculate the neutral kaon mixing parameter B_K in unquenched lattice QCD using asqtad-improved staggered sea quarks and domain-wall valence quarks. We use the “2+1” flavor gauge configurations generated by the MILC Collaboration, and simulate with multiple valence and sea-quark masses at two lattice spacings of a≈0.12  fm and a≈0.09  fm. We match the lattice determination of B_K to the continuum value using the nonperturbative method of Rome-Southampton, and extrapolate B_K to the continuum and physical quark masses using mixed-action chiral perturbation theory. The “mixed-action” method enables us to control all sources of systematic uncertainty and therefore to precisely determine B_K; we find a value of B_K^{MS̅ ,NDR}(2  GeV)=0.527(6)(21), where the first error is statistical and the second is systematic.