Two-pion-exchange potential and the π<i>N</i>amplitude

Abstract

We discuss the two-pion-exchange (TPE) potential which emerges from a box diagram with one nucleon (the spectator) restricted to its mass shell, and the other nucleon line replaced by a subtracted, covariant \ensuremath{\pi}N scattering amplitude which includes \ensuremath{\Delta}, Roper, and ${\mathit{D}}_{13}$ isobars, as well as contact terms and off-shell (nonpole) dressed nucleon terms. The \ensuremath{\pi}N amplitude satisfies chiral symmetry constraints and fits \ensuremath{\pi}N data below \ensuremath{\sim} 700 MeV pion energy. We find that this TPE potential can be well approximated by the exchange of an effective sigma and delta meson, with parameters close to the ones used in one-boson-exchange models that fit NN data below the pion production threshold. \textcopyright{} 1996 The American Physical Society.