Relativistic structure of one-meson and one-gluon exchange forces and the lower excitation spectrum of the Nucleon and the Delta

摘要

The lower excitation spectrum of the nucleon and $\Delta$ is calculated in arelativistic chiral quark model. Corrections to the baryon mass spectrum fromthe second order self-energy and exchange diagrams induced by pion and gluonfields are estimated in the field -theoretical framework. Convergent resultsfor the self-energy terms are obtained when including the intermediate quarkand antiquark states with a total momentum up to $j=25/2$. Relativisticone-meson and color-magnetic one-gluon exchange forces are shown to generatespin 0, 1, 2, etc. operators, which couple the lower and the upper componentsof the two interacting valence quarks and yield reasonable matrix elements forthe lower excitation spectrum of the Nucleon and Delta. The only contributionto the ground state nucleon and $\Delta$ comes from the spin 1 operators, whichcorrespond to the exchanged pion or gluon in the l=1 orbit, thus indicating,that the both pion exchange and color-magnetic gluon exchange forces cancontribute to the spin of baryons. Is is shown also that the contribution ofthe color-electric component of the gluon fields to the baryon spectrum isenormously large (more than 500 MeV with a value $\alpha_s=0.65$) and one needsto restrict to very small values of the strong coupling constant or to excludecompletely the gluon-loop corrections to the baryon spectrum. With thisrestriction, the calculated spectrum reproduces the main properties of thedata, however needs further contribution from the two-pion exchange andinstanton induced exchange (for the nucleon sector) forces in consistence withthe realistic NN-interaction models.