The pygmy dipole strength, the neutron radius of ${}^{208}$Pb and the symmetry energy

TL;DR

This paper investigates the connection between pygmy dipole strength, neutron skin thickness in ${}^{208}$Pb, and the symmetry energy's density dependence, using theoretical models and experimental implications.

Contribution

It provides a detailed analysis of Skyrme HF+RPA predictions for dipole responses and explores their relation to neutron skin thickness and symmetry energy.

Findings

Correlation between low-energy dipole peak and neutron skin thickness analyzed

Model predictions support or challenge the connection with neutron skin

Implications for parity-violating electron scattering experiments discussed

Abstract

The accurate characterization of the nuclear symmetry energy and its density dependence is one of the outstanding open problems in nuclear physics. A promising nuclear observable in order to constrain the density dependence of the symmetry energy at saturation is the neutron skin thickness of medium and heavy nuclei. Recently, a low-energy peak in the isovector dipole response of neutron-rich nuclei has been discovered that may be correlated with the neutron skin thickness. The existence of this correlation is currently under debate due to our limited experimental knowledge on the microscopic structure of such a peak. We present a detailed analysis of Skyrme Hartree-Fock (HF) plus random phase approximation (RPA) predictions for the dipole response in several neutron-rich nuclei and try to elucidate whether models of common use in nuclear physics confirm or dismiss its possible connection with the neutron skin thickness. Finally, we briefly present theoretical results for parity violating electron scattering on ${}^{208}$Pb at the conditions of the PREx experiment and discuss the implications for the neutron skin thickness of ${}^{208}$Pb and the slope of the symmetry energy.