Electromagnetic properties of one- and two-baryon systems in the quark potential model

Abstract

These lectures review calculations of the electromagnetic properties of one- and two-baryon systems using the nonrelativistic quark model. We start with a short discussion of spontaneous chiral symmetry breaking, which is essential in understanding the transition from QCD to the constituent quark model. We then discuss the chiral version of the nonrelativistic quark model which simulates the symmetries and dynamical content of the underlying field theory in terms of gluon, pion, and sigma exchange between constituent quarks. In the second lecture, the electromagnetic properties of the one-baryon system are investigated in this framework. In particular, we focus on the effect of gluon and pion degrees of freedom in the electromagnetic current operators. These must be included in order for the electromagnetic current to be conserved. We also study the effect of scalar exchange currents 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 consistent treatment of gluon-, pion-, and scalar-exchange currents in the quark potential model is the new point of the present work. We show that exchange currents have a large effect on various electromagnetic properties, such as magnetic moments, charge and magnetic radii of the nucleon and Δ(1232). We point out that one cannot extract quantitative information about the quark-quark potential from electromagnetic observables without taking two-body exchange currents into account. In the third lecture, we use the quark potential model to study the electromagnetic properties of the deuteron. We emphasize that a quark-model description leads to short-range modifications of the conventional single-nucleon and two-nucleon currents, due to the quark-exchange currents. These appear as a consequence of the Pauli principle at the quark level.