Implications of PREX-2 on the electric dipole polarizability of neutron-rich nuclei

Background: The recent announcement by the PREX collaboration of an unanticipated thick neutron skin in Pb-208 (R-skin(208)) has challenged the understanding of neutron-rich matter in the vicinity of nuclear saturation density. Whereas earlier constraints indicate that the symmetry energy is relatively soft, the PREX-2 result seems to suggest the opposite. Purpose: To confront constraints on the symmetry energy obtained from measurements of the electric dipole polarizability against those informed by the PREX-2 measurement of R-skin(208) and by the correlations that it entails. Methods: Covariant energy density functionals informed by the properties of finite nuclei are used to compute the electric dipole response of Ca-48, Ni-68, Sn-132, and Pb-208. The set of functionals used in this work are consistent with experimental data, yet are flexible enough in that they span a wide range of values of R-skin(208). Results: It is found that theoretical predictions of the electric dipole polarizability that are consistent with the PREX-2 measurement systematically overestimate the corresponding values extracted from the direct measurements of the distribution of electric dipole strength. Conclusions: The neutron skin thickness of Pb-208 extracted from parity violating electron scattering and the electric dipole polarizability measured in photoabsorption experiments are two of the cleanest experimental tools used to constrain the symmetry energy around nuclear saturation density. However, the recent value of R-skin(208) that suggests a fairly stiff symmetry energy stands in stark contrast to the conclusions derived from the electric dipole polarizability. At present, I offer no solution to this dilemma.

Automated summary

The study focuses on the constraints of symmetry energy through the neutron skin thickness of Pb-208 and electric dipole polarizability, finding significant discrepancies between theoretical predictions and experimental values. These experimental tools offer clues to constrain the symmetry energy around nuclear saturation density, but there remains a contradiction in the hardness of the symmetry energy.