  We compute O(alpha_s) lattice-to-continuum perturbative matching coefficients
for the Delta S=2 flavor changing four-quark operators for the Asqtad improved
staggered action. In conjunction with lattice simulations with three flavors of
light, dynamical quarks, our results yield an unquenched determination of B_K,
the parameter that determines the amount of indirect CP violation in the
neutral kaon system. Its value is an important input for the unitarity triangle
analysis of weak decays.
