Excited Baryons in Large N_c QCD Revisited: The Resonance Picture Versus Single-Quark Excitations

Abstract

We analyze excited baryon properties via a 1/N_c expansion from two perspectives: as resonances in meson-nucleon scattering, and as single-quark excitations in the context of a simple quark model. For both types of analysis one can derive novel patterns of degeneracy that emerge as N_c --> \infty, and that are shown to be compatible with one another. This helps justify the single-quark excitation picture and may give some insight into its successes. We also find that in the large N_c limit one of the S_{11} baryons does not couple to the pi-N channel but couples to the eta-N channel. This is empirically observed in the N(1535), which couples very weakly to the pi-N channel and quite strongly to the eta-N channel. The comparatively strong coupling of the N(1650) to the pi-N channel and weak coupling to eta-N channel is also predicted. In the context of the simple quark model picture we reproduce expressions for mixing angles that are accurate up to O(1/N_c) corrections and are in good agreement with mixing angles extracted phenomenologically.