Density Dependence of the Symmetry Energy in the Post PREX-CREX Era

TL;DR

This paper investigates the conflicting experimental results from PREX and CREX regarding the symmetry energy's density dependence, develops new models to reconcile these findings, and explores implications for neutron star properties.

Contribution

The authors calibrate three new covariant energy density functionals that fit nuclear data and reconcile PREX and CREX constraints, providing a unified description of the symmetry energy.

Findings

Models suggest a stiff equation of state at high densities.

Reconciliation of PREX and CREX results within a single theoretical framework.

Predictions for neutron star properties consistent with the new models.

Abstract

The recently published CREX results suggest a rather peculiar picture for the density dependence of the symmetry energy. Whereas PREX favors a large neutron skin thickness in $^{208}$Pb, thereby suggesting a stiff equation of state, CREX suggests instead a much softer equation of state. This discrepancy has caused a large spur in the theoretical community since no model has been able to simultaneously reproduce within $1\sigma$ the PREX and CREX results. Motivated by a novel correlation between a CREX observable and a combination of bulk symmetry energy parameters, we calibrate three new covariant energy density functionals that reproduce binding energies and charge radii of spherical nuclei - and also accommodate the constraints imposed by PREX and CREX. Given that these models suggest a stiff equation of state at high densities, predictions for neutron star properties are also discussed.