Pygmy Resonances and Neutron Skins

TL;DR

This paper investigates the electric dipole strength distribution in neutron-rich 68Ni, highlighting the emergence of Pygmy resonances and their correlation with neutron-skin thickness, suggesting dipole polarizability as a neutron skin proxy.

Contribution

It provides a detailed analysis of Pygmy resonances in 68Ni and their relation to neutron-skin thickness, using relativistic RPA with various effective interactions.

Findings

Pygmy strength exhausts 5-8% of EWSR in 68Ni.

Strong correlation between dipole polarizability and neutron-skin thickness.

Dipole polarizability can serve as a proxy for neutron skin.

Abstract

Motivated by a recent experiment, the distribution of electric dipole strength in the neutron-rich 68Ni isotope was computed using a relativistic random phase approximation with a set of effective interactions that - although well calibrated - predict significantly different values for the neutron-skin thickness in 208Pb. The emergence of low-energy "Pygmy" strength that exhausts about 5-8% of the energy weighted sum rule (EWSR) is clearly identified. In addition to the EWSR, special emphasis is placed on the dipole polarizability. In particular, our results suggest a strong correlation between the dipole polarizability of 68Ni and the neutron-skin thickness of 208Pb. Yet we find a correlation just as strong and an even larger sensitivity between the neutron-skin thickness of 208Pb and the fraction of the dipole polarizability exhausted by the Pygmy resonance. These findings suggest that the dipole polarizability may be used as a proxy for the neutron skin.