Pygmy resonance and torus mode within Vlasov dynamics

TL;DR

This paper investigates the pygmy dipole resonance and torus mode in neutron-rich nuclei using Vlasov dynamics, showing the resonance's generic nature and its relation to toroidal modes, with results aligning with experimental data.

Contribution

It demonstrates that the pygmy resonance is a universal phenomenon in neutron-rich nuclei, independent of shell effects, using a Vlasov equation approach with a realistic energy functional.

Findings

Pygmy resonance appears in all neutron-rich nuclei studied.

Resonance energies and transition probabilities agree with experiments.

Pygmy resonance relates to low-lying isoscalar toroidal modes.

Abstract

The pygmy dipole resonance in neutron-rich nuclei is studied within the framework of the Vlasov equation which is solved numerically. The interaction used in the Thomas-Fermi ground state and in the Vlasov equation is derived from an energy functional which correctly describes the equation of state of nuclear matter and neutron matter. It is found that the pygmy resonance appears in the electric dipole response of all nuclei with strong neutron excess, the energies and transition probabilities being in reasonable agreement with experimental results. Since the Vlasov equation does not account for any shell effects, this indicates that the existence of the pygmy resonance is a generic phenomenon and does not rely on the specific shell structure. Besides the electric dipole response, the isoscalar toroidal response is calculated. The transition densities and velocity fields are discussed. A comparison of the peak positions and velocity fields suggests that the pygmy resonance can be identified with one of the low-lying modes excited by the isoscalar toroidal operator.