Baryon-Baryon Interaction in the Quark Cluster Model

The quark cluster model approach to the baryon-baryon interaction is reviewed and recent application to the charge symmetry breaking in nuclear force is discussed. According to quantum chromodynamics (QCD), hadrons are bound states of quarks and gluons, and their interactions are governed by the non-Abelian gauge interaction. Their structures are potentially very complex as QCD is strongly interacting in the hadronic energy region. Indeed, we have many reasons to believe that the QCD vacuum (ground state) has nontrivial structure, such as chiral symmetry breaking, nontrivial topology. On the other hand, the low energy excitations of mesons and baryons show regular spectrum, which indicates their structures are rather simple. In terms of the Fock space decomposition, dominant component of a meson or a baryon seems to be simply q ¯ q or qqq. Chiral symmetry breaking is a key to resolve this seemingly contradictory situation. That is, spontaneous chiral symmetry breaking induces a gap in the quark spectrum, so that the effective quark, called constituent quark, acquires a mass of order 300 MeV. As this mass gap reduces the mixing of higher Fock components, or mixing of extra q ¯ q, the leading Fock component becomes dominant. This is the basis of the quark model, in which hadrons can be treated as few-body quantum systems and various techniques developed for few-body quantum systems may be applied. One fruitful application of the quark model is the quark cluster model (QCM) approach to the baryon-baryon interactions. The QCM, originally developed for the nucleon-nucleon interaction, successfully explained the origin of the strong short-range repulsion 1). Later it was applied to other baryons, such as the ∆ resonance, and the hyperons, Λ, Σ, and Ξ. Short range parts of such baryon-baryon interactions, which had not been well studied experimentally, have been predicted. For instance, the H dibaryon 2) is predicted to be a bound state of ΛΛ in QCM, 3) which motivated extensive dibaryon searches. Realistic baryon-baryon interactions have been generated in using a hybrid picture , i.e., superposition of the short-range quark exchange interaction and the meson exchange potential at longer distances 4). Such hybrid models of nuclear force are very successful and furthermore they have enough predictive power on the other baryon-baryon interactions. Recent development of hypernuclear experiments have confirmed some of the characteristic features of the QCM based Λ − N interactions.