Two-pion-exchange contributions to the pp→ppπ0 reaction

Our previous study of the near-threshold pp→ppπ0 reaction based on a hybrid nuclear effective field theory is further elaborated by examining the momentum dependence of the relevant transition operators. We show that the two-pion-exchange diagrams give much larger contributions than the one-pion-exchange diagram, even though the former is of higher order in the Weinberg counting scheme. The relation between our results and an alternative counting scheme, the momentum counting scheme, is also discussed.