Correlation effects in the relativistic impulse approximation treatment of proton-nucleus elastic scattering

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Abstract
Target nucleon correlation contributions to intermediate energy proton-nucleus elastic scattering observables are explored within the context of a semirelativistic multiple scattering model. A general, nonlocal expression for this second-order relativistic optical potential model is derived and discussed. Use is made of the local, projectile-target nucleon interaction given by the relativistic impulse approximation together with a shell model target nucleus wave function represented in a suitable, four-component form. For this initial investigation a simplified model is then assumed in which only the scalar and time-like vector components of the projectile-nucleon interaction are retained and in which the lower components of the target wave function are omitted. Intermediate propagation of positive and negative energy projectile states are included, as are important nonlocalities arising from the Dirac propagator for the projectile. Numerical estimates are provided for several cases of interest. Individual terms in the correlation contribution to the optical potential are fairly large; significant cancellations occur, however, resulting in relatively small changes in the calculated proton-nucleus elastic scattering observables. The differential cross section predictions are increased in magnitude at forward angles, although to a lesser degree than in nonrelativistic models of correlation effects. Analyzing powers and spin rotation functions, which are generally described quite well by the first-order relativistic impulse approximation model, are not significantly affected by these corrections, except at large scattering angles. These results indicate that the semirelativistic impulse approximation model considered here is stable with respect to higher-order multiple scattering contributions, at least to the level of two-body target nucleon correlations. The erroneous target mass dependence of the first-order relativistic impulse approximation model is not corrected by these effects, however.