Nils Paar

Exotic modes of excitation in atomic nuclei far from stability

We review recent studies of the evolution of collective excitations in atomic nuclei far from the valley of β-stability. Collective degrees of freedom govern essential aspects of nuclear structure, and for several decades the study of collective modes such as rotations and vibrations has played a vital role in our understanding of complex properties of nuclei. The multipole response of unstable nuclei and the possible occurrence of new exotic modes of excitation in weakly bound nuclear systems, present a rapidly growing field of research, but only few experimental studies of these phenomena have been reported so far. Valuable data on the evolution of the low-energy dipole response in unstable neutron-rich nuclei have been gathered in recent experiments, but the available information is not sufficient to determine the nature of observed excitations. Even in stable nuclei various modes of giant collective oscillations had been predicted by theory years before they were observed, and for that reason it is very important to perform detailed theoretical studies of the evolution of collective modes of excitation in nuclei far from stability. We therefore discuss the modern theoretical tools that have been developed in recent years for the description of collective excitations in weakly bound nuclei. The review focuses on the applications of these models to studies of the evolution of low-energy dipole modes from stable nuclei to systems near the particle emission threshold, to analyses of various isoscalar modes, those for which data are already available, as well as those that could be observed in future experiments, to a description of charge-exchange modes and their evolution in neutron-rich nuclei, and to studies of the role of exotic low-energy modes in astrophysical processes.

Quasiparticle random phase approximation based on the relativistic Hartree-Bogoliubov model

The relativistic quasiparticle random phase approximation (RQRPA) is formulated in the canonical single-nucleon basis of the relativistic Hartree-Bogoliubov (RHB) model. For the interaction in the particle-hole channel effective Lagrangians with nonlinear meson self-interactions are used, and pairing correlations are described by the pairing part of the finite-range Gogny interaction. The RQRPA configuration space includes the Dirac sea of negative-energy states. Both in the particle-hole and particle-particle channels, the same interactions are used in the RHB calculation of the ground state and in the matrix equations of the RQRPA. The RHB+RQRPA approach is tested in the example of multipole excitations of neutron-rich oxygen isotopes. The RQRPA is applied in the analysis of the evolution of the low-lying isovector dipole strength in Sn isotopes and

Collectivity of the low-lying dipole strength in relativistic random phase approximation

N=82

Electric dipole polarizability and the neutron skin

isotones.

Quasiparticle random phase approximation based on the relativistic Hartree-Bogoliubov model. II. Nuclear spin and isospin excitations

Abstract The relativistic random phase approximation is applied in the analysis of the evolution of the isovector dipole response in nuclei with a large neutron excess. The self-consistent framework of relativistic mean-field theory, which has been very successfully applied in the description of ground-state properties of nuclei far from the valley of β -stability, is extended to study the possible onset of low-energy collective isovector dipole modes in nuclei with extreme isospin values.

Hartree-Fock and many body perturbation theory with correlated realistic NN interactions

The recent high-resolution measurement of the electric dipole (E1) polarizability {alpha}{sub D} in {sup 208}Pb [A. Tamii et al. Phys. Rev. Lett. 107 062502 (2011)] provides a unique constraint on the neutron-skin thickness of this nucleus. The neutron-skin thickness r{sub skin} of {sup 208}Pb is a quantity of critical importance for our understanding of a variety of nuclear and astrophysical phenomena. To assess the model dependence of the correlation between {alpha}{sub D} and r{sub skin}, we carry out systematic calculations for {sup 208}Pb, {sup 132}Sn, and {sup 48}Ca based on the nuclear density functional theory using both nonrelativistic and relativistic energy density functionals. Our analysis indicates that whereas individual models exhibit a linear dependence between {alpha}{sub D} and r{sub skin}, this correlation is not universal when one combines predictions from a host of different models. By averaging over these model predictions, we provide estimates with associated systematic errors for r{sub skin} and {alpha}{sub D} for the nuclei under consideration. We conclude that precise measurements of r{sub skin} in both {sup 48}Ca and {sup 208}Pb - combined with the recent measurement of {alpha}{sub D} - should significantly constrain the isovector sector of the nuclear energy density functional.

Toroidal dipole resonances in the relativistic random phase approximation

The proton-neutron relativistic quasiparticle random-phase approximation (PN-RQRPA) is formulated in the canonical single-nucleon basis of the relativistic Hartree-Bogoliubov model, for an effective Lagrangian characterized by density-dependent meson-nucleon couplings. The model includes both the T = 1 and T = 0 pairing channels. Pair configurations formed from the fully or partially occupied states of positive energy in the Fermi sea, and the empty negative-energy states from the Dirac sea, are included in PN-RQRPA configuration space. The model is applied to the analysis of charge-exchange modes: isobaric analog resonances and Gamow-Teller resonances.

Pygmy dipole resonances in the relativistic random phase approximation

We employ correlated realistic nucleon-nucleon interactions for the description of nuclear ground states throughout the nuclear chart within the Hartree-Fock approximation. The crucial short-range central and tensor correlations, which are induced by the realistic interaction and cannot be described by the Hartree-Fock many-body state itself, are included explicitly by a state-independent unitary transformation in the framework of the unitary correlation operator method (UCOM). Using the correlated realistic interaction

Giant Quadrupole Resonances in 208Pb, the nuclear symmetry energy and the neutron skin thickness

\VO_{; ; \UCOM}; ;

Electric dipole polarizability in 208Pb: Insights from the droplet model

resulting from the Argonne V18 potential, bound nuclei are obtained already on the Hartree-Fock level. However, the binding energies are smaller than the experimental values because long-range correlations have not been accounted for. Their inclusion by means of many-body perturbation theory leads to a remarkable agreement with experimental binding energies over the whole mass range from \elem{; ; He}; ; {; ; 4}; ; to \elem{; ; Pb}; ; {; ; 208}; ; , even far off the valley of stability. The observed perturbative character of the residual long-range correlations and the apparently small net effect of three-body forces provides promising perspectives for a unified nuclear structure description.