Indirect methods in nuclear astrophysics with relativistic radioactive beams

The pygmy dipole resonance (PDR) is studied in various medium-heavy nuclei by using a Gogny interaction in a self-consistent Hartree-Fock plus Random Phase Approximation method. We compare the details of the PDR structure with those of the giant dipole resonance (GDR). In the PDR protons and neutrons vibrate in phase, and the main contributions are given by particle-hole excitations involving the neutrons in excess. On the contrary, in the GDR protons and neutrons vibrate out of phase, and all the nucleons are involved in the excitation. The values of the parameters we used to define the collectivity of an excitation indicate that the PDR is less collective than the GDR. We also investigate the role of the residual interaction in the appearance of the PDR and we find a subtle interplay with the shell structure of the nucleus.

Pygmy and giant electric dipole responses of medium-heavy nuclei in a self-consistent random-phase approximation approach with a finite-range interaction

Reactions with radioactive nuclear beams at relativistic energies have opened new doors to clarify the mechanisms of stellar evolution and cataclysmic events involving stars and during the big bang epoch. Numerous nuclear reactions of astrophysical interest cannot be assessed directly in laboratory experiments. Ironically, some of the information needed to describe such reactions, at extremely low energies (e.g., keVs), can only be studied on Earth by using relativistic collisions between heavy ions at GeV energies. In this contribution, we make a short review of experiments with relativistic radioactive beams and of the theoretical methods needed to understand the physics of stars, adding to the knowledge inferred from astronomical observations. We continue by introducing a more detailed description of how the use of relativistic radioactive beams can help to solve astrophysical puzzles and several successful experimental methods. State-of-the-art theories are discussed at some length with the purpose of helping us understand the experimental results reported. The review is not complete and we have focused most of it to traditional methods aiming at the determination of the equation of state of symmetric and asymmetric nuclear matter and the role of the symmetry energy. Whenever possible, under the limitations of our present understanding of experimental data and theory, we try to pinpoint the information still missing to further understand how stars evolve, explode, and how their internal structure might be.

A self-consistent approach in the problem of taking into account quasiparticle-phonon interaction provides a high predictive power and is free from adjustable parameters (this is of crucial importance for astrophysics). Moreover, it is consistent and makes it possible to take into account new effects. A brief survey of the results obtained within this approach on the basis of Skyrme or Fayans functionals by employing the smallness parameter g2, where g is the phonon-production amplitudes, with allowance for tadpole effects is presented. The contribution of quasiparticle-phonon interaction to ground-state electromagnetic moments of odd nuclei; second-order anharmonic effects in g2, including quadrupole moments of the first 2+ and 3− states and EL transitions between one-phonon states; third-order anharmonic effects; pygmy dipole and giant resonances; and the contribution of quasiparticle-phonon interaction to radiative properties of nuclear reactions are considered. For magic and semimagic nuclei, additional effects and structures arising in nuclear features because of quasiparticle-phonon interaction are discussed along with new—that is, three- and four-quasiparticle ground-state—correlations. Earlier unknown values of the above features, including the features of the pygmy dipole resonance in neutron-rich nickel isotopes, are predicted. It is shown that, in all of the aforementioned problems, the contributions of quasiparticle-phonon interaction is sizable, is of fundamental importance, and is necessary for explaining experimental data.

Results of Microscopic Self-Consistent Theory of Quasiparticle—Phonon Interaction in Nuclei

We study the Coulomb excitation of pygmy dipole resonances (PDR) in heavy ion reactions at 100 MeV/nucleon and above. The reactions Ni 68 + Au 197 and Ni 68 + Pb 208 are taken as practical examples. Our goal is to address the question of the influence of giant resonances on the PDR as the dynamics of the collision evolves. We show that the coupling to the giant resonances affects considerably the excitation probabilities of the PDR, a result that indicates the need of an improved theoretical treatment of the reaction dynamics at these bombarding energies.

/pdf/dynamical-coupling-of-pygmy-and-giant-resonances-in-4l8gn52pks.pdf

Dynamical Coupling of Pygmy and Giant Resonances in Relativistic Coulomb Excitation

Many-body Green functions are a very efficient formulation of the many-body problem. We review the application of this method to nuclear physics problems. The formulas which can be derived are of general applicability, e.g., in self-consistent as well as in nonself-consistent calculations. With the help of the Landau renormalization, one obtains relations without any approximations. This allows to apply conservation laws which lead to important general relations. We investigate the one-body and two-body Green functions as well as the three-body Green function and discuss their connection to nuclear observables. The generalization to systems with pair correlations are also presented. Numerical examples are compared with experimental data.

Many-body Green functions in nuclear physics

Electric dipole strength below the particle emission threshold both in stable nuclei and short-lived isotopes has received increasing interest due to its astrophysical impact. In analogy to the giant dipole resonance, this strength is commonly referred to as pygmy resonance. Coulomb dissociation of neutron-rich unstable isotopes and nuclear resonance fluorescence photon scattering have begun to provide systematic data on electric dipole strength in various isotope chains. We review the present state of the art of theoretical approaches and point out some open problems. We emphasize the necessity of a simultaneous theoretical treatment of the nucleon separation energies and the energetically low-lying dipole strength because the presently available data do not exclude a non-collective nature of the pygmy strength.

Pygmy dipole resonances

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Many-body Green functions in nuclear physics

Pygmy dipole resonances