ONE-LOOP CORRECTIONS TO THRESHOLD GAMMA-P-]P-PI-0 PRODUCTION IN THE LINEAR SIGMA-MODEL

One-loop corrections to threshold neutral pion photoproduction from protons are calculated in the linear sigma model. The total Born plus one-loop result is similar to that found using chiral perturbation theory (CHPT), and expansion of the threshold amplitude in M(pi)/M reproduces the CHPT result to order (M(pi)/M)2. It is also shown that expansion of the amplitude in the external kinematical variables reproduces the ''old'' low-energy theorem. An in-depth study is made, in this model, of the validity and accuracy of expanding the amplitude in the external kinematical variables.