M. Waroquier

Short-range correlations in (e,e′p) and (e,e′pp) reactions on complex nuclei

J. Ryckebusch, M. Vanderhaeghen, K. Heyde, M. Waroquier
Physics Letters B
350 (1), 1-7
1995
A1
Published while none of the authors were employed at the CMM

Abstract 

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.

Two-body currents in inclusive electron scattering

V. Van der Sluys, J. Ryckebusch, M. Waroquier
Physical Review C
51 (5), 2664-2670
1995
A1
Published while none of the authors were employed at the CMM

Abstract 

The longitudinal and transverse structure functions are calculated for inclusive electron scattering from 12C and 40Ca in the quasielastic and dip region. The microscopic model presented here incorporates two-body currents derived from one-pion exchange and intermediate Δ excitation. The reaction mechanism involves both one-nucleon and two-nucleon knockout processes. It is demonstrated that, even for quasielastic kinematics, two-body currents give a substantial contribution to the transverse structure function. Furthermore, the observed excess of transverse strength in the dip region between the quasielastic and delta peak can be partially ascribed to direct two-nucleon knockout following photoabsorption on a two-body current.

On meson-exchange and Δ-isobar currents in the two-nucleon photoabsorption mechanism

M. Vanderhaeghen, L. Machenil, J. Ryckebusch, M. Waroquier
Nuclear Physics A
580 (4), 551-576
1994
A1
Published while none of the authors were employed at the CMM

Abstract 

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.

Effects of the final-state interaction in (γ, pn) and (γ, pp) processes

J. Ryckebusch, M. Vanderhaeghen, L. Machenil, M. Waroquier
Nuclear Physics A
568 (4), 828-854
1994
A1
Published while none of the authors were employed at the CMM

Abstract 

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.

Absorption mechanisms in photon induced two-body knockout

L. Machenil, M. Vanderhaeghen, J. Ryckebusch, M. Waroquier
Physics Letters B
316 (1), 17-22
1993
A1
Published while none of the authors were employed at the CMM

Abstract 

Calculations have been performed for the 16O(γ, pn) and the 16O(γ, pp) reaction in the photon-energy range Eγ =60–300 MeV. Besides the contribution from the more common photoabsorption on the pionic degrees of freedom, we have investigated the influence of heavier meson exchange (ϱ, σ, ω) and intermediate Δ creation with π and ϱ exchange. Whereas the π meson is found to set the main trends, the ϱ meson is found not to be discardable in a theoretical description of the (γ, pn) reaction. The incorporation of an energy dependence and a decay width in the Δ propagator is observed to be essential in order to arrive at a more realistic description of (γ, NN) reactions at higher photon energies.

Proton hole states in 208Pb, studied by means of a self-consistent solution of the second-order Dyson equation for single-particle propagators

D. Van Neck, M. Waroquier, V. Van der Sluys, K. Heyde
Nuclear Physics A
563 (1), 1-20
1993
A1
Published while none of the authors were employed at the CMM

Abstract 

The single-particle self-energy in finite nuclei is constructed in a microscopic way by means of the Green-function formalism. We present calculations for the 208Pb system which are fully self-consistent up to second order in the residual interaction. This leads to an improved description of damping effects in the spectral function for deeply bound hole states. The theoretical results are compared with experimental (e, e'p) results. Global properties of the single-particle strength distributions for deep hole states, such as the centroids and widths, are well reproduced. Finally some comments are made about the meaning of occupation probabilities for shell-model orbits.

On the relationship between single-particle overlap functions, natural orbitals and the one-body density matrix for many-fermion systems

D. Van Neck, M. Waroquier, K. Heyde
Physics Letters B
314 (3-4), 255-259
1993
A1
Published while none of the authors were employed at the CMM

Abstract 

We consider the groundstate of a system of A interacting fermions and construct relationships connecting the asymptotic behaviour of the one-body density matrix with the overlap functions of the eigenstates in the (A−1)-particle system. It is shown that the knowledge of the exact one-body density matrix is sufficient to determine the overlap functions, spectroscopic factors and separation energies of the bound (A−1)-particle eigenstates. We also derive sumrules linking the overlap functions with the natural orbitals of the system.

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