Aspects of near-threshold neutral pion photoproduction off protons

We investigate near-threshold neutral pion photoproduction off protons to f ourth order in heavy-baryon chiral perturbation theory in the light of the new data from MAMI. Ve show that the unitarity cusp at the secondary pi (+) n threshold is in agreement with expectations from the final-state theorem. We also analyze the fourth-order corrections to the P-wave low-energy theo rems and show that potentially large Delta isobar contributions axe cancell ed by sizeable pion loop effects. This solidifies the parameter-free third- order predictions, which arc, in good agreement with the data.