BK with two flavors of dynamical overlap fermions

We present a two-flavor QCD calculation of B-K on a 16(3)x32 lattice at a similar to 0.12 fm (or equivalently a(-1)=1.67 GeV). Both valence and sea quarks are described by the overlap fermion formulation. The matching factor is calculated nonperturbatively with the so-called RI/MOM scheme. We find that the lattice data are well described by the next-to-leading order (NLO) partially quenched chiral perturbation theory (PQChPT) up to around a half of the strange quark mass (m(s)(phys)/2). The data at quark masses heavier than m(s)(phys)/2 are fitted including a part of next-to-next-to-leading order terms. We obtain B-K((MS) over bar) (2 GeV)=0.537(4)(40), where the first error is statistical and the second is an estimate of systematic uncertainties from finite volume, fixing topology, the matching factor, and the scale setting.