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

We calculate the neutral kaon mixing parameter BK 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 ≈0.12fm and a ≈0.09fm. We match the lattice determination of BK to the continuum value using the nonperturbative method of Rome-Southampton, and extrapolate BK 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 BK; we find a value of BKMS̄,NDR(2GeV)=0.527(6)(21) , where the first error is statistical and the second is systematic.