1/Nccorrections to theSU(4)c

Abstract

In the large N c 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 = (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/N c corrections to this picture, pointing out a four-fold ambiguity in the correspondence of the observed baryons with the large N c states. For each of the four possible assignments, we fit the coefficients of the quark operators contributing to the mass spectrum to O(N −1 c ). We comment on the implications of our results for the constituent quark model description of these states. *

Compatibility of quark and resonant picture excited baryon multiplets in the1/Ncexpansion of QCD

“…These constraints imply that various resonances with distinct I and J quantum numbers become degenerate as N c becomes large. On the other hand, treatments using a large N c quark picture exhibit precisely the same degeneracy patterns [1,6,7], and thus capture at least some nontrivial QCD dynamics.…”

Decays of nonstrange negative parity baryons in the1/Ncexpansion

“…This has been found to be the case for the masses of the negative parity baryons, where it was shown [6,8] that effective mass operators of spin-orbit type produce such a breaking. In the large N c limit a different scheme becomes rigorously valid [13,14]. In the real world with N c = 3 there is, however, plenty of evidence that the dominating spinflavor breaking in masses are the hyperfine effects that are order 1/N c , while the spin-orbit effects are substantially smaller [8,9].…”

“…In the sector of ground state baryons (identified for N c = 3 with the spin 1/2 octet and spin 3/2 decuplet in the case of three flavors), that symmetry gives rise to several important relations that hold at different orders in the 1/N c expansion [3,4,5]. The domain of excited baryons (baryon resonances) has also been explored in the framework of the 1/N c expansion [6,7,8,9,10,11,12,13,14] with very promising results. The analyses carried out so far have been constrained to states that belong to a definite spin-flavor and orbital multiplet, i.e.…”

Complete analysis of baryon magnetic moments in the1/Ncexpansion

“…[9]. It has been used in the 1/N c expansion several times: for the masses of the 56 [10], for charge radii and quadrupole moments [11,12,13], for the masses and couplings of the orbitally-excited baryon multiplet 70 [14,15,16,17,18], and even for the ∆ → Nγ couplings closely related to the ∆N transition magnetic moments [19]. This paper is organized as follows: In Sec.…”