Skyrme second random-phase approximation description of low-lying dipole states in 48Ca isotope

We apply the second random phase approximation (RPA) with Skyrme interaction to the study of low-lying dipole strength distributions of 48Ca, in the energy region of the excitation spectrum between 5 and 10 MeV. RPA models do not usually predict any presence of strength in this energy region, while experimentally a significant amount of strength is found. The inclusion of the 2 particle -2 hole configurations within the second RPA is found to be very important. Several states are obtained and many of them, despite being essentially 2 particle - 2 hole states, show quite large B(E1) values. These results are in reasonable agreement with other beyond mean-field predictions and with the corresponding experimental measurements. A detailed analysis of the properties of the most collective state is done in terms of its 1 particle -1 hole nature and its transition densities.