Is The Heavy Fermion Formalism Applicable to Meson Production Process?
  We consider tree diagram contributions to neutral pion production in pp ->pp pi0 using chiral perturbation theory in the relativistic and extremely non-relativistic limits. In marked difference with the results from heavy fermion formalism, the impulse and s-wave rescattering terms in the relativistic limit have equal signs and therefore add constructively, giving rise to a substantial contribution to the cross section. We argue that the power series expansion of the nucleon propagator is on the border of its convergence circle. Consequently, a finite order heavy fermion formalism does not predict nucleon pole terms correctly and therefore can not be applied to meson production. 
E.Gedalin, A. Moalem and L.Razdolskaya