Masses of orbitally excited baryons in large Nc QCD

We present the first phenomenological study of the masses of orbitally excited baryons in large Nc QCD. Restricting here to the nonstrange sector of the l=1 baryons, the 1/Nc expansion is used to order and select a basis of effective operators that spans the nine observables (seven masses and two mixing angles). Fits are performed using subsets of the complete set of nine operators, including corrections up to O(1/Nc) where leading order is Nc1. This study shows that the 1/Nc expansion provides an excellent framework for analyzing the mass spectrum, and uncovers a new hierarchy of operator contributions.