Constraints on the symmetry energy and neutron skins from experiments and theory

TLDR

The symmetry energy term in the nuclear equation of state influences phenomena in nuclear astrophysics, structure, and reactions. The study seeks to determine this symmetry energy to better understand dense matter in neutron stars and to examine the most widely used theoretical models that predict its value and slope. The authors analyze laboratory experiments that constrain the symmetry energy and its density dependence near normal nuclear matter density, and evaluate theoretical models predicting its slope parameter. The constraints from nuclear properties and electroweak/hadronic probes are experimentally consistent, and comparison with theory demonstrates that three‑body forces can be quantified.

Abstract

The symmetry energy contribution to the nuclear equation of state impacts various phenomena in nuclear astrophysics, nuclear structure, and nuclear reactions. Its determination is a key objective of contemporary nuclear physics, with consequences for the understanding of dense matter within neutron stars. We examine the results of laboratory experiments that have provided initial constraints on the nuclear symmetry energy and on its density dependence at and somewhat below normal nuclear matter density. Even though some of these constraints have been derived from properties of nuclei while others have been derived from the nuclear response to electroweak and hadronic probes, within experimental uncertainties-they are consistent with each other. We also examine the most frequently used theoretical models that predict the symmetry energy and its slope parameter. By comparing existing constraints on the symmetry pressure to theories, we demonstrate how contributions of three-body forces, which are essential ingredients in neutron matter models, can be determined.