Baryon spectroscopy: Symmetries, symmetry breaking and hadronic loops

Abstract

The problem of hadronic loop effects in baryon spectroscopy is thoroughly discussed. It is argued that such effects very likely constitute the dominant contribution to the observed splitting and mixing pattern of the (56, 0 +) and (70, 1 −) baryon multiplets. In particular, this dominance is demonstrated in the original Isgur-Karl-Koniuk model of baryons, in which hadronic loops are shown to provide an explanation for at least 2 3 of the observed size of splittings, both for the ground-state and excited baryons. The unitarity-induced mixing angles in the (70, 1 −)-multiplet are also shown to be in good agreement with experiment. For the ground-state baryons the formula relating Σ − Λ and Δ − N mass differences—as originally derived by de Rujula, Georgi, and Glashow from the single gluon exchange—is obtained from the hadronic loop effects as well. This (and other) results are derived after taking into account a complete set of symmetry-related hadronic loops. Consideration of such a complete set of symmetry-related processes is shown to be crucial in restoring proper symmetry properties of the calculated spectrum.