Microscopic description of the GDR and PDR modes: An application to the ⁴⁸Ca case

The study of the dipole response in atomic nuclei is among the main ELI-NP goals. One of the advantages of ELI-NP is the possibility to use polarized gamma rays, which allow to separate unambiguously the electric and magnetic strength. The ELI-NP facility will provide detailed strength distributions (fine structure), gamma-decay branches, spin and parity of the nuclear dipole response with an important impact both on nuclear structure and astrophysical studies. The theoretical description of the nuclear response requires advanced many body techniques able to go beyond the so called mean-field approximation. In this paper, we will describe a widely employed approach for the microscopic description of the nuclear response, namely the Random Phase Approximation. After that, the need to go beyond this approach will be discussed and an extension of this method will be introduced. An application of these methods will be presented to analyze the dipole strength in ⁴⁸Ca. Results are illustrated with two Skyrme parametrizations, SGII and SLy4 and compared with the experimental data recently measured. The results obtained with the SGII interaction are particularly satisfactory. In this case, the low–lying strength below the neutron threshold is extremely well reproduced and the giant dipole resonance is described in a very satisfactory way especially in its spreading and fragmentation. Spreading and fragmentation are produced in a natural way within such a theoretical model by the coupling of one particle–one hole and two particle–two hole configurations.