The differential cross sections for π−p→γn and π+n→γp are computed up to O(p3) in heavy baryon chiral perturbation
theory (HBChPT). The expressions at O(p) and O(p2) have no free
parameters. There are three unknown parameters at O(p3), low energy
constants of the HBChPT Lagrangian, which are determined by fitting to
experimental data. Two acceptable fits are obtained, which can be separated by
comparing with earlier dispersion relation calculations of the inverse process.
Expressions for the multipoles, with emphasis on the p-wave multipoles, are
obtained and evaluated at threshold. Generally the results obtained from the
best of the two fits are in good agreement with the dispersion relation
predictions.Comment: 24 pages, Latex, using RevTe