Baryon Resonances in the 1/nc Expansion

The 1/Nc expansion of QCD provides a valuable semiquantitative tool to study baryon scattering amplitudes and the short-lived baryon resonances embedded within them. A generalization of methods originally applied in chiral soliton models in the 1980’s provides the key to deriving a rigorous 1/Nc expansion. One obtains model-independent relations among amplitudes that impose mass and width degeneracies among resonances of various quantum numbers. Phenomenological evidence confirms that patterns of resonant decay predicted by 1/Nc agree with data. One may extend the analysis to subleading orders in 1/Nc, where again agreement with data is evident, in both meson-baryon scattering and pion photoproduction.