An Extended Brown-Bolsterli Model For Pygmy Dipole Resonance

We present a model which extends the approach first introduced by Brown and Bolsterli for the study of collectivity of the Giant Dipole Resonance in order to explore the properties of the dipolar states which emerge around the neutron separation energy. The model accounts in an effective manner for the non-uniform density distribution inside the nuclei by defining the core at saturation density, an intermediate density region as well as an exterior, skin-like domain of low density, and generalises our previous model which considered the nucleus as composed only by two subsystems, an isospin symmetric core and excess neutrons. As a consequence, a separable dipole-dipole interaction which encodes the density dependence of the symmetry energy can be introduced. We show that the predictions for the E1 response are quite robust when the number of subsystems is varied, Pygmy Dipole Resonance appearing as a collective dipolar mode which exhausts a fraction of Energy Weighted Sum Rule close to the experimentally observed values.