M. Waroquier

We consider a simple but nontrivial many-body system interacting through short-range forces, and confirm a property of such systems that was recently proposed on general grounds, namely, that single-particle overlap functions, spectroscopic factors, and separation energies of the bound (A-1)-particle eigenstates can be derived from the one-body density matrix of the A-particle system in its ground state. The basis of natural orbitals for the system is constructed and its properties are discussed. We also investigate the high-momentum content of the bound-state overlap functions and momentum distribution. It is found that the mean field provides a good approximation for the bound-state overlap functions even in the region of large momenta, where the total momentum distribution is already enhanced by several orders of magnitude over the mean-field result. © 1996 The American Physical Society.

The influence of short-range correlations (SRC) on the triple-coincidence (e,e′pp) reactions is studied. The non-relativistic model uses a mean-field potential to account for the distortions that the escaping particles undergo. Apart from the SRC, that are implemented through a Jastrow ansatz with a realistic correlation function, we incorporate the contribution from pion exchange and intermediate Δ₃₃ currents. The (e,e′pp) cross sections are predicted to exhibit a sizeable sensitivity to the SRC. The contribution from the two-nucleon breakup channel to the semi-exclusive ¹²C(e,e′p) cross section is calculated in the kinematics of a recent NIKHEF-K experiment. In the semi-exclusive channel, a selective sensitivity in terms of the missing energy and momentum to the SRC is found.

The role of π- and ϱ-meson exchange currents and dynamic Δ-isobar currents with both π- and ϱ-exchange are studied in two-nucleon knockout reactions. The Gottfried approximation is tested for these different absorption mechanisms by making a comparison between a factorized (Gottfried) model and an unfactorized model. For the unfactorized model, analytical expressions are given for the different absorption mechanisms by using harmonic-oscillator wavefunctions. The formalism is applied to the ¹⁶O(γ,pn) reaction in the photon-energy range Eγ = 60–300 MeV. Over the whole energy range, the ϱ-meson degree of freedom is found not to be negligible. It is demonstrated that the unfactorized treatment leads to a sizeable reduction of the cross section and to a change of the shape of the angular cross section.

A model is presented to describe electromagnetically induced two-nucleon emission processes in a shell-model picture. Distortions in the outgoing nucleon waves are accounted for by performing a partial-wave expansion in a real mean-field potential. The antisymmetry condition for the A-body wave functions is shown to be naturally preserved. The model is used to calculate (γ, pn) and (γ, pp) cross sections off the target nuclei ¹⁶O and ¹²C for photon energies ranging from 50 MeV up to the Δ(1232) isobar threshold. Effects due to the pionic currents and intermediate Δ creation are implemented. The impact of the distortions due to the interaction of the outgoing nucleon waves with the A — 2 core is examined. Hadronic form factors are introduced to regularize the πNN vertices and the sensitivity of the cross section to the pion cutoff mass is examined. The relative contribution of the (γ, pp) and the (γ, pn) channel to the total photoabsorption strength is discussed. Further, the photon energy dependence of the (γ, pp)/(γ, pn) ratio is investigated.