B_K from Two-flavor Dynamical Domain Wall Fermions

We report preliminary results from an ongoing calculation of $B_K$ for $N_f=2$ dynamical QCD with domain wall fermions. Simulations have been done with three dynamical quark masses on $16^3 \times 32$ volumes with $L_s = 12$, where the lattice spacing is $a^{-1} = 1.81(6)$ GeV. Using measurements on $\sim 70$ lattices for each dynamical mass and extrapolating $\mdyn=\mval$ to the kaon point, we find $B_K^{\rm \bar{MS}}(\mu=2{\rm GeV}) = 0.503(20)$.