Isovector excitation of stretched states in nuclei: effects of meson exchange currents, unbound wave functions, and knockout exchange amplitudes on the extraction of particle–hole strengths

Abstract

It is well known that the strength for excitations of [Formula: see text] high spin, stretched states observed via inelastic scattering, is generally much smaller than that predicted by spherical shell-model calculations. In addition, results obtained from electromagnetic and hadronic studies have discrepancies at the 20% level. For us to gain a better understanding of reduced magnetic strength in electron scattering and hopefully close the gap between experiment and theory, calculations of the electron-scattering form factors have been performed including the effects due to meson exchange currents in the transition amplitude and the effects due to unbound wave functions for the valence nucleon. The effect of the meson exchange-current contributions is to uniformly enhance the form factors near the first maximum, resulting in a 16 to 20% further reduction of the stretched particle–hole strength. The effect due to the radial wave functions deduced from Woods–Saxon potentials in which the nucleon is not bound is to reduce the form factors, thereby resulting in an increase in the spectroscopic strength. As regards the comparison of results obtained with electromagnetic and hadronic probes, the implied sensitivity to higher order current and spin–current transition densities associated with the nonlocality due to the tensor knockout exchange amplitudes in nucleon–nucleus scattering is considered explicitly. It is found that the simplest correspondence between electron and nucleon–nucleus scattering is preserved for isovector excitations but not for isoscalar excitations under the usual assumptions for the tensor interaction. It is clear that precise comparisons between experiment and theory (or between probes) cannot be made unless these and related effects are consistently included.