Abstract
A self-consistent calculation is performed within an equation of motion method to study the electric dipole response in the neutron rich Sn-132 with special attention at the low-energy excitations associated with the pygmy dipole resonance. A basis of one- plus two-phonon states is constructed by solving iteratively a set of equations of motion and adopted to bring the nuclear Hamiltonian to diagonal form.
The phonons are generated in a Tamm-Dancoff approximation using a Hartree-Fock basis. The Hamiltonian is composed of an intrinsic kinetic term and an optimized two-body chiral potential.
If complemented with a weak phenomenological density dependent term, such a potential improves substantially the description of the dipole response as compared to other realistic interactions. The two-phonon states play an important role in determining the fine structure of the giant dipole resonance and in enhancing the fragmentation of the strength at low energy, in fair agreement with the available data.
Insights into the nature of the excitations building up the pygmy resonance are gained from the analysis of the phonon composition and shell structure of the states.