0 v 1 2 7 N ov 1 99 7 1 Light and strange baryons , two - baryon systems and the chiral symmetry

Beyond the scale of spontaneous breaking of chiral symmetry light and strange baryons should be considered as systems of three constituent quarks with confining interaction and a chiral interaction that is mediated by Goldstone bosons between the constituent quarks. The flavor-spin structure and sign of the short-range part of the Goldstone boson exchange interaction reduces the SU(6) F S symmetry down to SU(3) F × SU(2) S , induces hyperfine splittings and provides correct ordering of the lowest states with positive and negative parity. A unified description of light and strange baryon spectra calculated in a semirelativistic framework is presented. It is demonstrated that the same short-range part of the Goldstone boson exchange between the constituent quarks induces a strong short-range repulsion in NN system when the latter is treated as 6Q system. Similar to the NN system there should be a short-range repulsion in other NY and Y Y two-baryon systems. We also find that the compact 6Q system with the " H-particle " quantum numbers lies a few hundreds MeV above the ΛΛ threshold. It then suggests that the deeply bound H-particle should not exist. 1. Spontaneous Breaking of Chiral Symmetry and its Implications At low temperatures and densities the SU(3) L × SU(3) R chiral symmetry of QCD Lagrangian is spontaneously broken down to SU(3) V by the QCD vacuum (in the large N c limit it would be U(3) L × U(3) R → U(3) V). A direct evidence for the spontaneously broken chiral symmetry is a nonzero value of the quark condensates for the light flavors < |¯ qq| >≈ −(240 − 250MeV) 3 , which represent the order parameter. That this is indeed so, we know from three sources: current algebra, QCD sum rules, and lattice gauge calculations. There are two important generic consequences of the spontaneous breaking of chiral symmetry (SBCS). The first one is an appearance of the octet of pseudoscalar mesons of low mass, π, K, η, which represent the associated approximate Goldstone bosons (in the large N c limit the flavor singlet state η ′ should be added). The second one is that valence (practically massless) quarks acquire a dynamical mass, which have been called historically constituent quarks. Indeed, the nonzero value of the quark condensate itself implies at the formal level that there should be rather big dynamical mass, which could be in general a moment-dependent quantity [1]. Thus …