Large Nc, constituent quarks, and N, Δ charge radii

The authors show how one may define baryon constituent quarks in a rigorous manner, given physical assumptions that hold in the large-Nc limit of QCD. This constituent picture gives rise to an operator expansion that has been used to study large-Nc baryon observables; here they apply it to the case of charge radii of the N and Δ states. For example, one finds the relation rp2 - rΔ+2 = rn2 -rΔ02 to be broken only by three-body, O(1/Nc2) effects for any Nc