A next-to-next-to-leading-order pp -> pp pi(0) transition operator in chiral perturbation theory

We discuss the results of a systematic calculation of the next-to-next-to-l eading-order amplitude for the pp --> pp pi(0) S-wave production at the thr eshold in heavy-baryon chiral perturbation theory. We find six new diagrams , two of which can be viewed as vertex corrections, of 15 - 20 %, to the pi -exchange graph, whereas the rest are much larger, one exceeding 900 %. We discuss the reasons for this enhancement, as well as the steps necessary to be taken before the final comparison with experiment.