Two-nucleon one-loop corrections to pion double charge exchange within heavy baryon chiral perturbation theory

One-loop corrections at the two-nucleon level to pion double charge exchange scattering off a nuclear target at threshold are calculated within the framework of heavy baryon chiral perturbation theory. An estimate for the (two-nucleon) one-loop correction is obtained in the static limit and using an impulse approximation. We find a small (1.6%) increase relative to the leading order tree...