Baryon mass splittings in the 1/Nc expansion

The I=0,1,2,3 mass splittings of the spin-1/2 octet and spin-3/2 decuplet baryons are analyzed in the 1/Nc expansion combined with perturbative flavor breaking. We show there is considerable experimental evidence that the baryon masses satisfy the hierarchy predicted by this expansion. Since flavor symmetry-breaking suppression factors alone are not sufficient to describe the observed hierarchy, we conclude that there is firm evidence for the 1/Nc expansion in the baryon masses. Our analysis differs from nonrelativistic SU(6).