Towers of positive parity excited baryons and their mixing in the large Nc limit

Abstract

We consider configuration mixing for the nonstrange positive parity excited baryons in the [56′,0+], [56,2+], [70,0+], and [70,2+] quark model SU(6)×O(3) multiplets contained in the N=2 band. Starting from the effective mass operator for these states, we show by an explicit calculation that in the large Nc limit they fall into six towers of degenerate states labeled by K=0,1,1′,2,2′,3. We find that the mixing of the spin-flavor states is much simpler than what is naively expected in the quark model. To leading order in Nc, only states carrying the same K label can mix, which implies that, for the spin-flavor states we started with, configuration mixing can be parameterized by just two constants: μK=1 and μK=2. The obtained mass degeneracies and mixing pattern constitute a signature of the contracted spin-flavor symmetry for baryons in the large Nc limit.