Chiral perturbation theory for K+ to pi+ pi0 decay in the continuum and on the lattice

We use one-loop chiral perturbation theory (ChPT) to compare lattice results for the K+ to pi+ pi0 decay amplitude with the experimental value. Three systematic effects: quenching, finite-volume effects, and the use of unphysical values of quark masses and pion external momenta can be investigated. We find that the corrections help in explaining the discrepancy between lattice and experimental results. We also discuss the relation to B_K.