Microscopic calculations of the enhancement factor in the electric dipole sum rule

Correlated basis function perturbation theory with state-dependent correlations is used to calculate the nuclear photoabsorp- tion enhancement factor K in the electric dipole sum rule for some realistic models of nuclear matter. The contribution due to 2p-2h admixtures in the ground state wave function turns out to be only a few percent of the unperturbed value. The values obtained for K are about 1.8 at experimental equilibrium density and increase almost linearly with density. We also give estimates of K for finite nuclei, obtained within the local density approximation framework. The surface effects give a contribution which is - 20% of the volume term. state of the non-relativistic hamiltonian having V as nuclear potential and D z = ~Ei= 1 ,A rizZi is the z component of the electric dipole operator, with riz being the third component of the isospin opertor for the ith nucleon. The theoretical estimates (2) of K do not depend very much on the realistic interaction adopted, and are more than a factor of two larger than the experimental value (3), Kex p = 0.76 + 0.10, obtained from the integrated photo- nuclear cross sections up to the rr-meson production threshold. It is important to know how much of this discre- pancy is due to effects not explicitly taken into account in the Bethe-Levinger sum rule, like tail corrections of the integrated cross section, higher multipoles and dipole retardation effects, and how much is due to the in- adequacy of the variational wave function used in the calculation. In this letter we present the results obtained for K when the variational ground state is corrected by adding 2p2h correlated basis functions (CBF) components to it. The 2p2h admixtures are calculated by using second order CBF perturbation theory (4--6). The CBF states are normalized but not orthogonal, and are given by (koi) = F( (bi)/(cb i (F+Fltbi )1/2, (2) where I(I)i) are Fermi gas states and F = S H

S-pairing in neutron matter. I. Correlated Basis Function Theory

S-wave pairing in neutron matter is studied within an extension of correlated basis function (CBF) theory to include the strong, short range spatial correlations due to realistic nuclear forces and the pairing correlations of the Bardeen, Cooper and Schrieffer (BCS) approach. The correlation operator contains central as well as tensor components. The correlated BCS scheme of Ref. [Nucl. Phys. A363 (1981) 383], developed for simple scalar correlations, is generalized to this more realistic case. The energy of the correlated pair condensed phase of neutron matter is evaluated at the two--body order of the cluster expansion, but considering the one--body density and the corresponding energy vertex corrections at the first order of the Power Series expansion. Based on these approximations, we have derived a system of Euler equations for the correlation factors and for the BCS amplitudes, resulting in correlated non linear gap equations, formally close to the standard BCS ones. These equations have been solved for the momentum independent part of several realistic potentials (Reid, Argonne v_{14} and Argonne v_{8'}) to stress the role of the tensor correlations and of the many--body effects. Simple Jastrow correlations and/or the lack of the density corrections enhance the gap with respect to uncorrelated BCS, whereas it is reduced according to the strength of the tensor interaction and following the inclusion of many--body contributions.Comment: 20 pages, 8 figures, 1 tabl

Introduction to modern methods of quantum many-body theory and their applications

This invaluable book contains pedagogical articles on the dominant nonstochastic methods of microscopic many-body theories - the methods of density functional theory, coupled cluster theory, and correlated basis functions - in their widest sense. Other articles introduce students to applications of these methods in front-line research, such as Bose-Einstein condensates, the nuclear many-body problem, and the dynamics of quantum liquids. These keynote articles are supplemented by experimental reviews on intimately connected topics that are of current relevance. The book addresses the striking

Theoretical Nuclear Physics in Italy

Proceedings of the 8th Conference on Problems in Theoretical Nuclear Physics, Cortona, 18-20 October 2000, 408 pagg

Electromagnetic response in nuclear matter and complex nuclei

SIGLEITItal