Kaon B parameter from improved staggered fermions in N(f)=2+1 QCD.

We present a calculation of the kaon B-parameter, BK, using lattice QCD. We use improved staggered fermions, with the physical complement of Nf = 2 + 1 light flavors. To control discretization errors, we use four different lattice spacings ranging down to a � 0.045 fm. The chiral and continuum extrapolations are done using SU(2) staggered chiral perturbation theory. Our final result is ˆ BK = 0.727 ± 0.004(stat) ± 0.037(sys), where the dominant systematic error is from our use of