Improved determination of $B_K$ with staggered quarks

Abstract

We present results for the kaon mixing parameter $B_K$ obtained using improved staggered fermions on a much enlarged set of MILC asqtad lattices. Compared to our previous publication, which was based largely on a single ensemble at each of the three lattice spacings $a\approx 0.09$fm, $0.06$fm and $0.045$fm, we have added seven new fine and four new superfine ensembles, with a range of values of the light and strange sea-quark masses. We have also increased the number of measurements on one of the original ensembles. This allows us to do controlled extrapolations in the light and strange sea-quark masses, which we do simultaneously with the continuum extrapolation. This reduces the extrapolation error and improves the reliability of our error estimates. Our final result is ${\hat{B}}_K=0.7379\pm 0.0047(\text{stat})\pm 0.0365(\text{sys})$.

Comments: 6 pages, 5 tables, 2 figures, updated references