THRESHOLD PION-PHOTOPRODUCTION IN CHIRAL PERTURBATION-THEORY

We analyze threshold pion photoproduction in the framework of baryon chiral perturbation theory. In case of neutral pion photoproduction, virtual pions generate infrared singularities which modify the familiar form of the low-energy theorem for the electric dipole amplitude E0+. In particular, we predict E0+pi0p(s(th)) = -(1.33+/-0.09)x10(-3)/M(pi+). We also find a satisfactory description of the recently measured total cross section for gammap --> pi0p in the threshold region. The process gamman --> pi0n is entirely given by the loop and counterterm contributions. It would be interesting to have an experimental determination of the corresponding cross section in the threshold region. In case of the charged channels, the loop corrections are much more tame.