Off-shell and medium effects on the electromagnetic nucleon current

Abstract

Ambiguities in the nuclear electromagnetic current operator within the nuclear medium are used to examine the significance of the off-shell form of the current operator for the case of40Ca. The standard form of the electromagnetic current operator and two alternate forms, one of which is the popular de Forest prescription, are used to study current operator ambiguities in the context of the quasi-elastic (e, e′) and (e, e′p) reactions in the presence of realistic final-state interactions. Nucleons may themselves be modified in the nuclear medium and the associated modification of the current operator, which has been modelled through the use of density-dependent form factors, is studied here in the (e, e′p) reaction. For the quasi-elastic inclusive reaction we find large current-operator-dependent effects in both the longitudinal and transverse response. For the transverse response these effects are clearly linked to the nature of the final-state interactions and are traced to virtual pair processes present in the relativistic case. In particular, the de Forest current yields a greatly enhanced transverse response and a large high-energy tail in the relativistic case. Current operator ambiguities do not seriously affect Coulomb Sum Rule saturation trends and produce less dramatic effects for the exclusive (e, e′p) reaction. Our results suggest that present knowledge of the nucleon electromagnetic current operator is insufficient to allow firm predictions of the electronuclear reactions to be made to the precision required to resolve other physical issues.