Universal short-range repulsion in the baryon system originating from the confinement. Approach in string-junction model

Aim of this study is to show a way to unifiedly understand the origin of repulsive core of baryon-baryon interaction and the origin of universal repulsion of three-baryon interaction whose importance has recently become noticeable. 'Universal' means its effect independent of flavor and spin of baryons. As shown in recent lattice QCD calculations, the confinement mechanism in the single baryon is realized in the Y-shaped structure of the strings embodying the squeezed color flux-tubes that consists of a junction at the central position and three oriented strings linking the quarks with the junction. Here we adopt a string-junction model suitable for description of such baryonic structure, in order to find out substantial features that the confinement mechanism persists in multi-baryon systems when baryons closely approach to each other. First we clarify an originating mechanism of the repulsive core of baryon-baryon interaction, and next by extending this approach to the three-baryon system, we derive a universal short-range three-body repulsion. Key concept in the study lies in the recognition that such short-range repulsion appears as latent effect implying the energy necessary for full overlap of baryons, that is, the energy for creation of the junction and anti-junction pair which leads to specific exotic baryon states of gluonic excitation. The short-range interaction derived in such a way is universal, because the effect comes from the color degrees of freedom. This three-body repulsion prevents the equation of state from the dramatic softening due to hypero-mixing in neutron-star matter, and thus we can disolve the difficulty that otherwise theory contradicts the observations on the masses of neutron stars. (author)