  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^{MSbar, NDR}(2 GeV) = 0.527(6)(20), where the first error
is statistical and the second is systematic.
