Quasielastic Electron Scattering from Nuclei: Random-Phase vs. Ring Approximations

We investigate the extent to which the nuclear transverse response to electron scattering in the quasielastic region, evaluated in the random-phase approximation can be described by ring approximation calculations. Different effective interactions based on a standard model of the type g'+V_pi+V_rho are employed. For each momentum transfer, we have obtained the value of g'_0 permitting the ring response to match the position of the peak and/or the non-energy weighted sum rule provided by the random-phase approach has been obtained. It is found that, in general, it is not possible to reproduce both magnitudes simultaneously for a given g'_0 value.Comment: 7 pages, 4 Postscript figures, to appear in Physical Review

OFF-SHELL EFFECTS OF NUCLEON-NUCLEON POTENTIAL MODELS ON PROTON-NUCLEUS ELASTIC SCATTERING OBSERVABLES

Les observables de spin et la section efficace de diffusion élastique p+40Ca sont calculées dans un modèle de convolution complète utilisant les matrices t de l'interaction NN libre basées sur les potentiels de Paris et de Bonn. Les deux familles de calculs sont comparées entre elles et aux données à 200 MeV ainsi qu'au modèle t.rho plus conventionnel. Les calculs en convolution complète reproduisent beaucoup mieux les données et montrent qu'à cette énergie un bon traitement des effets hors couche est plus important que le choix entre les deux potentiels NN considérés.Spin observables and cross sections are calculated for p+40Ca elastic scattering in a full-folding model using nucleon-nucleon (NN) free t-matrices based on the Paris and Bonn (OBE) potentials. The two sets of calculated observables are compared with each other, with conventional tρ results and with data at 200 MeV. The differences between full-folding and tρ results are striking, with the full-folding results being in much better agreement with the data. At this energy the choice between the two NN potentials considered is found to be less critical than a proper treatment of off-shell effects as prescribed by the full-folding model

Phase shifts and in-medium cross sections for dressed nucleons in nuclear matter

The dressing of nucleons as embodied in single-particle spectral functions is incorporated in the description of nucleon-nucleon scattering in nuclear matter at a density corresponding to k F51.36 fm 21 . In order to clarify the new features associated with the complete off-shell behavior of the single-particle motion, results involving mean-field particles are also presented with special emphasis on the behavior of the phase shifts when bound pair states occur. Both the 1 S0 and 3 S1- 3 D1 channels exhibit this feature at the considered density for mean-field particles at zero temperature. An important tool to assess the effect of the dressing of the particles is the two-particle density of states. A sizable reduction with respect to the mean-field density of states is obtained. At 2e F this reduction corresponds to z kF 2 , where z kF is the strength of the quasiparticle pole at k F , and it can therefore be as large as 0.5. This reduction has significant consequences for the strength of pairing correlations both in the 3 S1- 3 D1 channel where it leads to a dramatic decrease of the attraction at the Fermi energy and for the 1 S0 channel which no longer shows a pairing signal. Phase shifts and cross sections for dressed particles are determined based on expressions which fold the effective interaction with the dressed but noninteracting two-particle spectral function. This folding procedure yields similar results to an ‘‘on-shell’’ prescription reminiscent of the result for free or mean-field particles, except for cross sections deep in the Fermi sea. Comparison of phase shifts and cross sections to the case of mean-field particles indicates that smaller phase shifts in an absolute sense and considerable reductions of the in-medium cross sections for dressed particles are obtained. It is shown that while in many cases these results imply a weakening of the effective interaction, this is not the case for 1 S0 interactions deep in the Fermi sea. [S0556-2813~99!06612-1