Charged radiative pion capture on the nucleon in heavy baryon chiral   perturbation theory
  The differential cross sections and s-wave and p-wave multipoles for $\pi^- p \to \gamma n$ and $\pi^+ n \to \gamma p$ have been calculated through $O(p^3)$ in heavy baryon chiral perturbation theory (HBChPT). Fits to existing data allow several of the low energy constants to be determined. Generally results of the calculation compare well with dispersion relation predictions. 
Harold W. Fearing, Thomas R. Hemmert, Randy Lewis and Christine   Unkmeir