2-PION-EXCHANGE POTENTIAL AND THE PI-N AMPLITUDE

We discuss the two-pion-exchange (TPE) potential which emerges from a box diagram with one nucleon (the spectator) restricted to its mass sh ell, and the other nucleon line replaced by a subtracted, covariant pi N scattering amplitude which includes Delta, Roper, and D-13 isobars, as well as contact terms and off-shell (nonpole) dressed nucleon term s. The pi N amplitude satisfies chiral symmetry constraints and fits p i N data below similar to 700 MeV pion energy. We find that this TPE p otential can be well approximated by the exchange of an effective sigm a and delta meson, with parameters close to the ones used in one-boson -exchange models that fit NN data below the pion production threshold.