Origins of the baryon spectrum

Abstract

I begin with a key problem of light and strange baryon spectroscopy which suggests a clue for our understanding of underlying dynamics. Then I discuss spontaneous breaking of chiral symmetry in QCD, which implies that at low momenta there must be quasiparticles - constituent quarks with dynamical mass, which should be coupled to other quasiparticles - Goldstone bosons. Then it is natural to assume that in the low-energy regime the underlying dynamics in baryons is due to Goldstone boson exchange (GBE) between constituent quarks. Using as a prototype of the microscopical quark-gluon degrees of freedom the instanton-induced 't Hooft interaction I show why the GBE is so important. When the 't Hooft interaction is iterated in the qq t-channel it inevitably leads to a pole which corresponds to GBE. This is a typical antiscreening behavior: the interaction is represented by a bare vertex at large momenta, but it blows up at small momenta in the channel with GBE quantum numbers, explaining thus a distinguished role of the latter interaction in the low-energy regime. I show how the explicitly flavour-dependent short-range part of the GBE interaction between quarks, perhaps in combination with the vector-meson exchange interaction, solves a key problem of baryon spectroscopy and present spectra obtained in a simple analytical calculation as well as in exact semirelativistic three-body approach.