Quark exchange currents in nuclei: Effective operator description

Abstract

In this paper we introduce for the first time effective quark exchange current operators which can be used in nuclear structure calculations. To this end we first solve the equation of motion for a microscopic quark hamiltonian using the quark cluster model. We then construct the electromagnetic current operator on the quark level, i.e. the photon is coupled directly to the quarks. By eliminating the quark (internal) degrees of freedom, we derive an effective electromagnetic current operator on the nucleon level. A part of this effective current corresponds to the conventional impulse and meson exchange currents with vertex factors predicted by the quark model. In addition, this effective current contains new non-local and isospin-dependent terms which are generated by the Pauli principle on the quark level (quark exchange between nucleons). When we evaluate these quark exchange currents, we use harmonic-oscillator wave functions as nuclear wave functions including short-range correlations. We introduce these short-range correlations by solving the Bethe-Goldstone equation with our effective NN potential, which is derived from a microscopic quark hamiltonian. We investigate the role of these additional quark exchange currents in the magnetic moments and the elastic magnetic form factors of several closed-shell ± 1 nuclei, such as 15N, 17O, and 39K.