EXCHANGE CURRENTS IN BARYONS

This paper reviews calculations of the electromagnetic properties of b aryons using the constituent quark model. We start with a short discus sion of spontaneous chiral symmetry breaking, which is essential in un derstanding the transition from QCD to the constituent quark model. We then discuss a chiral version of the constituent quark model, which s imulates the symmetries and dynamical content of the underlying field theory in terms of gluon, pion and sigma exchange between constituent quarks. We show that the electromagnetic current charge and current op erators, usually approximated by one-body operators (impulse approxima tion), must be supplemented by appropriate two-body terms (exchange cu rrents). The latter represent the gluon and pion exchange degrees of f reedom in the electromagnetic current operator. These exchange current s must be included for reasons of completeness and consistency. Most i mportantly, however, they are needed in order for the electromagnetic current to be conserved. We also study the effect of scalar exchange c urrents connected with the confinement and sigma exchange potentials. By including these two-body exchange currents we go beyond the single- quark impulse approximation, which has mainly been used up to now. The inclusion of gluon- pion-, and scalar-exchange currents in the quark potential model is the new point of the present work. We show that for some observables, such as the magnetic moments, charge, and magnetic radii of the proton and charged Delta(1232) states, exchange currents contribute at the level of some 10%. The same holds true for the magne tic moments of the entire baryon octet, with the exception of the Xi(- ) magnetic moment. On the other hand, the neutron charge radius, the q uadrupole moments of the Delta, and the N --> Delta transition quadrup ole moment, are dominated by pion and gluon exchange contributions to the charge density operator. The inclusion of the pion and gluon excha nge currents leads to a neutron charge radius of the correct size and sign. Based on the gluon and pion exchange current diagrams, we derive parameter-free relations between the neutron charge radius, the quadr upole moment of the Delta, and the N --> Delta transition quadrupole m oment. Neglecting configuration mixing, we find that the neutron charg e radius and the N --> Delta transition quadrupole moment are simply r elated as Q(N-->Delta) = r(n)(2)/root 2. The implications of Siegert's theorem for the calculation of the E2 form factor in the N --> Delta transition are studied. Finally, we discuss the axial coupling constan t of the nucleon. We show that the inclusion of axial pair exchange cu rrents does not significantly alter the NRQM prediction.