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We propose a framework to construct the ground-state energy and density matrix of an N-electron system by solving selfconsistently a set of single-particle equations. The method can be viewed as a non-trivial extension of the Kohn-Sham scheme (which is embedded as a special case). It is based on separating the Green's function into a quasi-particle part and a background part, and expressing only the background part as a functional of the density matrix. The calculated single-particle energies and wave functions have a clear physical interpretation as quasiparticle energies and orbitals., Comment: 12 pages, 1 figure, to be published in Phys. Rev. A

The symmetry energy for nuclear matter and its relation to the neutron skin in finite nuclei is discussed. The symmetry energy as a function of density obtained in a self-consistent Green function approach is presented and compared to the results of other recent theoretical approaches. A partial explanation of the linear relation between the symmetry energy and the neutron skin is proposed. The potential of several experimental methods to extract the neutron skin is examined., Comment: to appear in Phys. Rev. C

A fully self-consistent treatment of short-range correlations in nuclear matter is presented. Different implementations of the determination of the nucleon spectral functions for different interactions are shown to be consistent with each other. The resulting saturation densities are closer to the empirical result when compared with (continuous-choice) Brueckner-Hartree-Fock values. Arguments for the dominance of short-range correlations in determining the nuclear-matter saturation density are presented. A further survey of the role of long-range correlations suggests that the inclusion of pionic contributions to ring diagrams in nuclear matter leads to higher saturation densities than empirically observed. A possible resolution of the nuclear-matter saturation problem is suggested., Comment: 5 pages, 1 figure, to be published in Phys.Rev.Lett

The chiral invariant QHD-III model of Serot and Walecka is applied in the calculation of some meson properties. The electromagnetic interaction is included by extending the symmetry of the model to the local U(1) \times SU(2)_{R} \times SU(2)_{L} group. The minimal and nonminimal contributions to the electromagnetic Lagrangian are obtained in a new representation of QHD-III. Strong decays of the axial-vector meson, a_{1} \to \pi \rho, a_{1} \to \pi \sigma, and the electromagnetic decays \rho \to \pi \pi \gamma, a_{1} \to \pi \gamma and \rho \to \pi \gamma are calculated. The low-energy parameters for the \pi-\pi scattering are calculated in the tree-level approximation. The effect of the auxiliary Higgs bosons, introduced in QHD-III in order to generate masses of the vector and axial-vector mesons via the Higgs mechanism, is studied as well. This is done on the tree level for \pi-\pi scattering and on the level of one-loop diagrams for the a_{1} \to \pi \gamma decay. It is demonstrated that the model successfully describes some features of meson phenomenology in the non-strange sector., Comment: 20 pages, 3 eps figures, RevTeX4

Coherent Compton scattering on light nuclei in the delta resonance region is studied in the impulse approximation and is shown to be a sensitive probe of the in-medium properties of the delta resonance. The elementary amplitude on a single nucleon is calculated from the unitary K-matrix approach developed previously. Modifications of the properties of the delta resonance due to the nuclear medium are accounted for through the self-energy operator of the delta, calculated from the one-pion loop. The dominant medium effects such as the Pauli blocking, mean-field modification of the nucleon and delta masses, and particle-hole excitations in the pion propagator are consistently included in nuclear matter., Comment: 30 pages, 11 figures, accepted for publication in Phys. Rev. C

New lower bounds for the binding energy of a quantum-mechanical system of interacting particles are presented. The new bounds are expressed in terms of two-particle quantities and improve the conventional bounds of the Hall-Post type. They are constructed by considering not only the energy in the two-particle system, but also the structure of the pair wave function. We apply the formal results to various numerical examples, and show that in some cases dramatic improvement over the existing bounds is reached., Comment: 29 pages, 5 figures, to be published in Phys. Rev. A

We treat short-range correlations in nuclear matter, induced by the repulsive core of the nucleon-nucleon potential, within the framework of a self-consistent Green's function theory. The effective in-medium interaction sums the ladder diagrams of both the particle-particle and hole-hole type. The demand of self-consistency results in a set of nonlinear equations which must be solved by iteration. We explore the possibility of approximating the single-particle Green's function by a limited number of poles and residues., Comment: 9 pages, 3 eps-figures; added two tables dealing with calculations including larger sets of BAGEL-poles

The process p+d \leftrightarrow 3He + \gamma* at intermediate energies is described using a covariant and gauge-invariant model, and a realistic pd3He vertex. Both photodisintegration of 3He and proton-deuteron capture with production of e+e- pairs are studied, and results for cross sections and response functions are presented. The effect of time-like formfactors on the dilepton cross sections is investigated as well., Comment: 10 pages, 4 figures, Revtex, to be published in Physics Letters B

The spectroscopic factors for the low-lying quasi-hole states observed in the 16O(e,e'p)15N reaction are reinvestigated with a variational Monte Carlo calculation for the structure of the initial and final nucleus. A computational error in a previous report is rectified. It is shown that a proper treatment of center-of-mass motion does not lead to a reduction of the spectroscopic factor for $p$-shell quasi-hole states, but rather to a 7% enhancement. This is in agreement with analytical results obtained in the harmonic oscillator model. The center-of-mass effect worsens the discrepancy between present theoretical models and the experimentally observed single-particle strength. We discuss the present status of this problem, including some other mechanisms that may be relevant in this respect., Comment: 14 pages, no figures, uses Revtex, to be published in Phys. Rev. C 58 (1998)

We present a model to calculate cross sections for electroinduced two-nucleon emission from finite nuclei. Short-range correlations in the wave functions and meson-exchange contributions to the photoabsorption process are implemented. Effects of the short-range correlations are studied with the aid of a perturbation expansion method with various choices of the Jastrow correlation function. The model is used to investigate the relative importance of the different reaction mechanisms contributing to the A(e,e$'$pn) and A(e,e$'$pp) process. Representative examples for the target nuclei $^{12}$C and $^{16}$O and for kinematical conditions accessible with contemporary high-duty cycle electron accelerators are presented. A procedure is outlined to calculate the two-nucleon knockout contribution to the semi-exclusive (e,e$'$p) cross section. Using this technique we investigate in how far semi-exclusive (e,e$'$p) reactions can be used to detect high-momentum components in the nuclear spectral function., Comment: 51 pages, Latex, uses epsf.sty and elsart.sty, 17 figures (in eps format)

A technique to deal with Coulomb electron distortions in the analysis of (e,e'p) reactions is presented. Thereby, no approximations are made. The suggested technique relies on a partial-wave expansion of the electron wave functions and a multipole decomposition of the electron and nuclear current in momentum space. In that way, we succeed in keeping the computational times within reasonable limits. This theoretical framework is used to calculate the quasielastic (e,e'p) reduced cross sections for proton knockout from the valence shells in $^{16}$O, $^{40}$Ca, $^{90}$Zr and $^{208}$Pb. The final-state interaction of the ejected proton with the residual nucleus is treated within an optical potential model. The role of electron distortion on the extracted spectroscopic factors is discussed., Comment: 45 pages, 10 encapsulated postscript figures, Revtex, uses epsfig.sty and fancybox.sty, to be published in Physical Review C

Long-range correlations, which are partially responsible for the observed fragmentation and depletion of low-lying single-particle strength, are studied in the Green's function formalism. The self-energy is expanded up to second order in the residual interaction. We compare two methods of implementing self-consistency in the solution of the Dyson equation beyond Hartree-Fock, for the case of the 16O nucleus. It is found that the energy-bin method and the BAGEL method lead to globally equivalent results. In both methods the final single-particle strength functions are characterized by exponential tails at energies far from the Fermi level., Comment: 11 pages, 3 postscript figures, Revtex, uses epsf.sty, to be published in Physics Letters B

The reduced (e,e$'$p) cross section is calculated for kinematics that probe high missing momenta. The final-state interaction is handled within a non-relativistic many-body framework. One- and two-body nuclear currents are included. Electron distortion effects are treated in an exact distorted wave calculation. It is shown that at high missing momenta the calculated (e,e$'$p) cross sections exhibit a pronounced sensitivity to ground-state correlations of the RPA type and two-body currents. The role of these mechanisms is found to be relatively small at low missing momenta., Comment: 15 pages in REVtex with embedded psfigures

The contributions from $^{12}$C$(e,e'pn)$ and $^{12}$C$(e,e'pp)$ to the semi-exclusive $^{12}$C$(e,e'p)$ cross section have been calculated in an unfactorized model for two-nucleon emission. We assume direct two-nucleon knockout after virtual photon coupling with the two-body pion-exchange currents in the target nucleus. Results are presented at several kinematical conditions in the dip and $\Delta$(1232) regions. The calculated two-nucleon knockout strength is observed to account for a large fraction of the measured $(e,e'p)$ strength above the two-nucleon emission threshold., Comment: 12 Revtex pages, 4 postscript figures (available upon request), University of Gent preprint SSF94-02-01

The similarities in the experimental indications for multinucleon mechanisms in $(\gamma,p)$ and $(e,e'p)$ processes are pointed out. For both types of reactions, the substantial role of two-nucleon emission processes for transitions to high excitation energies in the residual nucleus is stressed. A microscopic model for the calculation of the two-body knockout contributions to the inclusive $(\gamma,N)$ reaction is presented. It is based on an unfactorized formalism for the calculation of electromagnetically induced two-nucleon emission cross sections. The model is shown to yield a reasonable description of the overall behaviour of the $^{12}$C$(\gamma,p)$ and $^{12}$C$(\gamma,n)$ data at high excitation energies in the residual nucleus. In the calculations effects from non-resonant and resonant pion exchange currents are included. Photoabsorption on these currents are predicted to produce the major contributions to the exclusive $^{16}$O$(\gamma,n_0)^{15}$O process at photonenergies above the pion threshold. Double differential cross sections for photon induced $pp$ and $pn$ emission from $^{16}$O are calculated and compared with the data., Comment: 18 pages, Revtex 3.0, 13 figures (available upon request), SSF93-10-01

The response functions for the unpolarized $(e,e'p)$ and polarized $(\vec{e},e'p)$ reaction are calculated for medium-heavy nuclei under quasifree conditions. The formalism presented here incorporates two-body currents related to meson-exchange and the $\Delta (1232)$ excitation. The final-state interaction of the outgoing nucleon with the residual nucleus is handled in a HF-RPA formalism. The sensitivity of the results to the two-body currents is discussed for the five structure functions in quasielastic $(\vec{e},e'p)$ scattering off the target nuclei $^{16}O$ and $^{40}Ca$. A selective sensitivity to the two-body currents is obtained in the longitudinal-transverse interference term $W_{LT}$ where two-body currents can explain part of the discrepancy between the impulse-approximation calculations and the data., Comment: 12 pages in LaTex with 5 figures (available on request), INW0703

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 wavefunctions is shown to be naturally preserved. The model is used to calculate ($\gamma$,pn) and ($\gamma$,pp) cross sections off the target nuclei $^{16}$O and $^{12}$C for photon energies ranging from 50 MeV up to the $\bigtriangleup$(1232) isobar threshold. Effects due to the pionic currents and intermediate $\bigtriangleup$ 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 $\pi$NN vertices and the sensitivity of the cross section to the pion cut-off mass is examined. The relative contribution of the ($\gamma$,pp) and the ($\gamma$,pn) channel to the total photoabsorption strength is discussed. Further, the photon energy dependence of the ($\gamma$,pp)/($\gamma$,pn) ratio is investigated., Comment: 30 pages, 11 figures (available from the authors upon request), INW0701

Calculations have been performed for the $^{16}$O($\gamma$,pn) and the $^{16}$O($\gamma$,pp) reaction in the photon-energy range $E_{\gamma}$ = 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 ($\rho, \sigma, \omega$) and intermediate $\Delta$ creation with $\pi$ and $\rho$ exchange. Whereas the $\pi$ meson is found to set the main trends, the $\rho$ meson is found not to be discardable in a theoretical description of the ($\gamma$,pn) reaction. The incorporation of an energy dependence and a decay width in the $\Delta$ propagator is observed to be essential in order to arrive at a more realistic description of ($\gamma$,NN) reactions at higher photon energies., Comment: 10 pages, 5 figures in seperate postscript file, Submitted to Phys. Lett. B. - INW9306II