M ay 2 00 3 Semi - relativistic meson exchange currents in ( e , e ′ ) and ( e , e ′ p ) reactions

Electron-induced one-nucleon knock-out observables are computed for moderate to high momentum transfer making use of semi-relativistic expressions for the one-body and two-body mesonexchange current matrix elements. Emphasis is placed on the semi-relativistic form of the ∆-isobar exchange current and several prescriptions for the dynamical-equivalent form of the ∆-propagator are analyzed. To this end, the inclusive transverse response function, evaluated within the context of the semi-relativistic approach and using different prescriptions for the ∆-propagator, is compared with the fully relativistic calculation performed within the scheme of the relativistic Fermi gas model. It is found that the best approximation corresponds to using the traditional static ∆propagator. These semi-relativistic approaches, which contain important aspects of relativity, are implemented in a distorted wave analysis of quasielastic (e, e′p) reactions. Final state interactions are incorporated through a phenomenological optical potential model and relativistic kinematics is assumed when calculating the energy of the ejected nucleon. The results indicate that meson exchange currents may modify substantially the TL asymmetry for high missing momentum. PACS: 25.30.Fj, 14.20.Gk, 24.10.Jv, 24.10.Eq, 21.60.Cs