Symmetry energy in dilute matter and the neutron skin

TL;DR

This paper investigates how the neutron skin thickness in nuclei can be explained by revisiting surface tension and diffuseness effects within an extended energy density functional framework, linking nuclear structure and the equation of state in dilute matter.

Contribution

It demonstrates that the neutron skin discrepancy between 48Ca and 208Pb can be resolved by considering surface and structure effects driven by the EoS in dilute matter within a non-relativistic EDF model.

Findings

Revisiting surface tension and diffuseness explains neutron skin differences.

Surface and structure effects are influenced by the EoS in dilute matter.

The approach aligns finite nuclei properties with the nuclear matter equation of state.

Abstract

Energy density functional (EDF) theory provides a unified framework for the description of nuclei and of infinite nuclear matter. In principle, it facilitates direct connections between nuclear data and the nuclear equation of state (EoS). Although in practice traditional nuclear EDF theory has strained to describe finite nuclei and infinite systems at the same time, recently developed extended EDF models overcome many of the limitations of traditional models in that respect. A recent challenge to EDF and EoS studies has come entirely from within nuclear structure, namely how to account both for the relatively thin neutron skin in 48Ca as extracted by the CREX experiment and the relatively thick neutron skin of 208Pb exctracted by the PREX-II experiment. The discrepancy suggests a surface and structure effect. The present study shows that the puzzle can be resolved in a non-relativistic framework by revisiting the nuclear surface tension and diffuseness, as driven in part by the EoS in dilute matter well below the saturation point and in part by the isovector gradient terms and spin-orbit potential. Such effects have no bearing on the EoS near and above saturation.