Application of the relativistic random-phase and distorted wave impulse approximations to quasielastic proton-nucleus scattering

[摘要] ENGLISH ABSTRACT: In this dissertation a fully relativistic model for polarized inclusive quasielastic proton-nucleusscattering is developed. Using a standard relativistic impulse approximation (RIA) treatment ofquasielastic scattering and a two-body SPVAT form of the current operator, it is shown how thebehaviour of projectile and target can be decoupled. Subsequently, different models for projectileand target can be adopted and combined to examine a variety of relativistic effects.The most simplistic model of the target is provided by a mean-field nuclear matter approximationto the relativistic meson-nucleon model, quantum hadrodynamics (QHD). Here relativisticeffects manifest as an effective mass, which is lower than the free mass, of the constituent nucleons.This model is improved upon by including many-body correlations through medium-modificationof meson propagators in the relativistic random-phase approximation (RPA).Since it is generally accepted that the strong nuclear force and the extended range of the nuclearpotential lead to distortion effects on the projectile and ejectile (seen as a modulation of the wavefunctions), our formalism is geared towards the use of relativistic distorted waves (RDWIA). Thedistorted waves are written as partial wave expansions and are solutions to the Dirac equation withpotentials. The inclusion of distortions, however, greatly increases the computational burden andwe show how a number of analytical and numerical techniques can be used to facilitate the processof calculation. It is also shown how the standard relativistic plane wave treatment (RPWIA) can,instead, be easily employed to obtain a baseline for determining the impact of distortions.A calculation is performed for the reaction 40Ca(!p, !p !) at a beam energy of 500 MeV. Here it isfound that the effect of correlations on the RPWIA calculation can be seen as a quenching of thecross section that is expected to become more pronounced at lower energies or for higher densitytargets. A RDWIA calculation shows additional reduction and if target correlations are includedthis effect is enhanced. To our knowledge this is the first calculation that attempts to include boththese effects (RPA and RDWIA) in the context of quasielastic proton-nucleus scattering.

[发布机构] Stellenbosch University