Neutral kaon mixing parameterBKfrom unquenched mixed-action lattice QCD

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Abstract
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$\approx${}0.12\,\,fm and a$\approx${}0.09\,\,fm. 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 BKMS\ifmmode\bar\else\textasciimacron\fi{},NDR(2\,\,GeV)=0.527(6)(21), where the first error is statistical and the second is systematic.