Chiral perturbation theory for K+->pi(+)pi(0) decay in the continuum and on the lattice

In this paper we use one-loop chiral perturbation theory in order to compare lattice computations of the K+ --> pi(+) pi(0) decay amplitude with the experimental value. This makes it possible to investigate three systematic effects that plague lattice computations: quenching, finite-volume effects, and the fact that lattice computations have been done at unphysical values of the quark masses and pion external momenta (only this latter effect shows up at the tree level). We apply our results to the most recent lattice computation and find that all three effects are substantial. We conclude that one-loop corrections in chiral perturbation theory help in explaining the discrepancy between lattice results and the real-world value. We also reexamine B-K, which is closely related to the K+ --> pi(+) pi(0) decay amplitude by chiral symmetry.