1/Nc corrections to the SU(4)c symmetry for L=1 baryons: The Three Towers

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In the large Nc limit, the mass spectrum of the L=1 orbitally excited baryons N* has a very simple structure, with states degenerate in pairs of spins J=(1/2,3/2), (3/2,5/2), corresponding to irreducible representations (towers) of the contracted SU(4)c symmetry group. The mixing angles are completely determined in this limit. Using a mass operator approach, we study 1/Nc corrections to this picture, pointing out a four-fold ambiguity in the correspondence of the observed baryons with the large Nc states. For each of the four possible assignments, we fit the coefficients of the quark operators contributing to the mass spectrum to O(Nc⁻¹). We comment on the implications of our results for the constituent quark model description of these states.