W. H. Dickhoff

Single-particle properties and short-range correlations in nuclear matter

Abstract Ladder correlations are studied with inclusion of hole-hole propagation to all orders. The resulting effective interaction is separated into a forward-going and a backward-going contribution with the numerical use of dispersion relations. This procedure allows the correct calculation of the corresponding self-energy terms. Self-consistency between the ladders and the self-energy is established for the quasi-particle energy. Calculations are performed with semi-realistic interactions derived from Reids soft-core interaction to avoid the appearance of pairing instabilities. A careful study of the complete momentum and energy dependence of the resulting self-energy is made for various densities. Effective mass contributions are studied coming from both the k - and ω-dependence of the self-energy. Accurate calculation of all relevant spectral functions is performed with proper fulfillment of the sum rule and quasi-particle properties are discussed. An important fraction of the single-particle strength is found at very high energy due to the realistic short-range repulsion in the interaction. From the hole spectral function the momentum distribution is calculated at various densities. The depletion due to the influence of short-range correlations around normal nuclear matter density amounts to about 13%.

The quasiparticle interaction, a shield of nuclear matter against pion condensation

Abstract A microscopic calculation is presented for the quasiparticle interaction in nuclear matter. As a starting point a Brueckener G-matrix is used, which is derived from realistic potentials (Reid soft core, HM2Δ). Keeping the full complexity of this interaction, the excitation modes for the different spin-isospin channels are evaluated. The effects of isobar excitation [Δ(3, 3)] are also taken into account. The exchange of the corresponding phonons is added to the bare G-matrix and the influence of this so-called crossed channel renormalization on the interaction of quasiparticles at the Fermi surface is discussed. This renormalization of the particle-hole interaction increases drastically the critical density for pion condensation in nuclear matter.

Nucleon properties in the nuclear medium

Recent developments in the many-body theory of interacting fermions are discussed employing a self-consistent Green function approach. This scheme is outlined and its application to nuclear systems is presented. Special attention is paid to the consistent inclusion of short-range and long-range correlations induced by realistic nucleon-nucleon interactions. Such correlations lead to occupation probabilities which deviate from the simple mean-field or shell-model description. The scheme is extended to incorporate relativistic effects. Also applications of field theoretical models for hadrons in a nuclear medium and their relation to QCD are discussed.

The screening of the particle-hole interaction to all orders☆

Abstract Starting from Brueckners G -matrix the effects of the induced interactions on the particle-hole interaction in nuclear matter are investigated to all orders. Although an iterative solution of the non-linear integral equations, which have to be solved, leads to divergent contributions, it can be shown that the renormalization effects summed to all orders are finite. The complete momentum dependence of the particle-hole interaction is treated which in the Landau limit results in less attractive F 0 and more repulsive G ′ 0 parameters. For finite ph momenta these features are maintained implying that the collectivity in the pion spin-isospin channel is screened also when the renormalization is considered to all orders.

The exchange of effective mesons between nucleons in nuclei

Abstract A study is made of the nuclear matter G -matrix. A method is developed which allows the determination of a local G -operator. This method is applied to analyse the correlations of the G -matrix, i.e. G without the bare interaction V , in terms of meson-exchange-type interactions. For a strong potential (REID) these effective mesons have masses which exceed 800 MeV implying very short-ranged correlations. For a weaker potential these correlations are somewhat longer ranged. The dominant feature of these G -operator correlations is found to be a strong scalar isoscalar attractive interaction which provides the binding of the nuclear system. By comparing with exact results for Landau parameters and binding energies it is shown that the approximation made with this local operator is excellent. A weak density dependence of the correlations is found and the structure of the Landau parameter ǵ is discussed. Finally it is proposed that this G -operator is a suitable object to study finite nuclei.

Microscopic description of heavy-ion scattering in the nuclear matter picture☆

Abstract A microscopic parameter-free calculation of the heavy-ion optical potential is presented. Assuming the dominance of nucleon-nucleon collisions in the reaction dynamics, a description of these processes is translated into a two-Fermi-sphere nuclear matter picture, using a generalized local-density approximation. The calculation of reaction cross sections for the systems 40 Ar + 60 Ni, 120 Sn, 208 Pb which were recently measured at E lab = 1760 MeV , confirms the dominance of NN collisions. As previously shown for light systems, some importance must be attributed to the excitation of collective states which are calculated in coupled channels in describing the reactions at lower energies, although volume effects dominate. For the calculation of the effective NN interaction in the medium ( G -matrix) a two-step procedure is introduced which allows one to calculate the imaginary part of the reaction matrix reliably in spite of its sensitivity to the Pauli operator.

Faddeev treatment of long-range correlations and the one-hole spectral function of 16 O

The Faddeev technique is employed to study the influence of both particle-particle and particle-hole phonons on the one-hole spectral function of

Evidence for short-range correlations in O-16

{}^{16}\mathrm{O}.

Selectivity of the {16}O(e,e'pp) reaction to discrete final states.

Collective excitations are accounted for at a random phase approximation level and subsequently summed to all orders by the Faddeev equations to obtain the nucleon self-energy. An iterative procedure is applied to investigate the effects of the self-consistent inclusion of the fragmentation in the determination of the phonons and the corresponding self-energy. The present results indicate that the characteristics of hole fragmentation are related to the low-lying states of

Momentum and energy distributions of nucleons in finite nuclei due to short-range correlations

{}^{16}\mathrm{O}.