Excited baryons in large N_c QCD reexamined: The resonance picture versus single-quark excitations

We analyze excited baryon properties via a 1/N_c expansion from two perspectives: as resonances in mesonnucleon 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→∞, 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 S₁₁ baryons does not couple to the π-N channel but couples to the η-N channel. This is empirically observed in the N(1535), which couples very weakly to the π-N channel and quite strongly to the η-N channel. The comparatively strong coupling of the N(1650) to the π-N channel and weak coupling to η;-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.