The Kaon B-Parameter in Staggered Chiral Perturbation Theory
  We calculate the kaon B-parameter, B_K, to next-to-leading order in staggered chiral perturbation theory. We extend the usual power counting to include the effects of using perturbative (rather than non-perturbative) matching factors. Taste breaking enters through the O(a^2) terms in the effective action, through mixing with higher-dimension operators, and through the truncation of matching factors. These effects cause mixing with several additional operators, complicating the chiral and continuum extrapolations. We summarize the results here; all details can be found in hep-lat/0507012. 
Ruth S. Van de Water and Stephen R. Sharpe