Kaon B-parameter in mixed action chiral perturbation theory

We calculate the kaon B-parameter, B-K, in chiral perturbation theory for a partially quenched, mixed-action theory with Ginsparg-Wilson valence quarks and staggered sea quarks. We find that the resulting expression is similar to that in the continuum, and in fact has only two additional unknown parameters. At 1-loop order, taste-symmetry violations in the staggered sea sector only contribute to flavor-disconnected diagrams by generating an O(a(2)) shift to the masses of taste-singlet sea-sea mesons. Lattice discretization errors also give rise to an analytic term which shifts the tree-level value of B-K by an amount of O(a(2)). This term, however, is not strictly due to taste breaking, and is therefore also present in the expression for B-K for pure Ginsparg-Wilson lattice fermions. We also present a numerical study of the mixed B-K expression in order to demonstrate that both discretization errors and finite volume effects are small and under control on the MILC improved staggered lattices.