Analysis of Meson Exchange and Isobar Currents in (e,e'p) Reactions from O-16

An analysis of the effects of meson exchange and isobar currents in exclusive (e,e'p) processes from O-16 under quasi-free kinematics is presented. A model that has probed its feasibility for inclusive quasi-elastic (e,e') processes is considered. Sensitivity to final state interactions between the outgoing proton and the residual nucleus is discussed by comparing the results obtained with phenomenological optical potentials and a continuum nuclear shell-model calculation. The contribution of the meson-exchange and isobar currents to the response functions is evaluated and compared to previous calculations, which differ notably from our results. These two-body contributions cannot solve the puzzle of the simultaneous description of the different responses experimentally separated. Copyright 1999 by The American Physical SocietyComment: 5 pages, plus 3 PS figures. To be published in Phys. Rev. C Updated figure

Meson Exchange Currents in (e,e'p) recoil polarization observables

A study of the effects of meson-exchange currents and isobar configurations in $A(\vec{e},e'\vec{p})B$ reactions is presented. We use a distorted wave impulse approximation (DWIA) model where final-state interactions are treated through a phenomenological optical potential. The model includes relativistic corrections in the kinematics and in the electromagnetic one- and two-body currents. The full set of polarized response functions is analyzed, as well as the transferred polarization asymmetry. Results are presented for proton knock-out from closed-shell nuclei, for moderate to high momentum transfer.Comment: 44 pages, 18 figures. Added physical arguments explaining the dominance of OB over MEC, and a summary of differences with previous MEC calculations. To be published in PR

Quaiselastic scattering from relativistic bound nucleons: Transverse-Longitudinal response

Predictions for electron induced proton knockout from the $p_{1/2}$ and $p_{3/2}$ shells in $^{16}$O are presented using various approximations for the relativistic nucleonic current. Results for the differential cross section, transverse-longitudinal response ($R_{TL}$) and left-right asymmetry $A_{TL}$ are compared at $|Q^2|=0.8$ (GeV/c)$^2$ corresponding to TJNAF experiment 89-003. We show that there are important dynamical and kinematical relativistic effects which can be tested by experiment.Comment: 10 pages, including 2 figures. Removed preliminary experimental data from the figure

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

Effects of Short-Range Correlations in (e,e'p) reactions and nuclear overlap functions

A study of the effects of short-range correlations over the (e,e'p) reaction for low missing energy in closed shell nuclei is presented. We use correlated, quasi-hole overlap functions extracted from the asymptotic behavior of the one-body density matrix, containing central correlations of Jastrow type, up to first-order in a cluster expansion, and computed in the very high asymptotic region, up to 100 fm. The method to extract the overlap functions is checked in a simple shell model, where the exact results are known. We find that the single-particle wave functions of the valence shells are shifted to the right due to the short-range repulsion by the nuclear core. The corresponding spectroscopic factors are reduced only a few percent with respect to the shell model. However, the (e,e'p) response functions and cross sections are enhanced in the region of the maximum of the missing momentum distribution due to short-range correlations.Comment: 45 pages, 15 figure

On the role of the effective interaction in quasi-elastic electron scattering calculations

The role played by the effective residual interaction in the transverse nuclear response for quasi-free electron scattering is discussed. The analysis is done by comparing different calculations performed in the Random--Phase Approximation and Ring Approximation frameworks. The importance of the exchange terms in this energy region is investigated and the changes on the nuclear responses due to the modification of the interaction are evaluated. The calculated quasi-elastic responses show clear indication of their sensibility to the details of the interaction and this imposes the necessity of a more careful study of the role of the different channels of the interaction in this excitation region.Comment: 16 pages, 4 Postscript figure

Mean-field calculations of quasi-elastic responses in 4He

We present calculations of the quasi-elastic responses functions in 4He based upon a mean-field model used to perform analogous calculations in heavier nuclei. The meson exchange current contribution is small if compared with the results of calculations where short-range correlations are explicitly considered. It is argued that the presence of these correlations in the description of the nuclear wave functions is crucial to make meson exchange current effects appreciable.Comment: uuencoded file containing 7 LaTex peges plus 3 ps figures. To be published in Physical Review