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Baryon Masses and Axial Couplings in the Combined 1/Nc and Chiral Expansions

The effective theory for baryons with combined 1/Nc and chiral expansions is analyzed for non-strange baryons. Results for baryon masses and axial couplings are obtained in the small scale expansion, to be coined as the ξ-expansion, in which the 1/Nc and the low energy power countings are linked according to 1/Nc=O(ξ)=O(p). Masses and axial couplings are analyzed to O(ξ^3) and O(ξ^2) respectively, which correspond to next-to-next to leading order evaluations, and require one-loop contributions in the effective theory. The role of the spin-flavor approximate symmetry in baryons, consequence of the large Nc limit, is manifested in the physical world with Nc=3 in a significant way, as the analysis of its breaking in the masses and the axial couplings shows. Applications to the recent lattice QCD results on baryon masses and the nucleon's axial coupling are presented. It is shown that those results are naturally described within the effective theory at the order considered in the ξ-expansion.