Novel features of asymmetric nuclear matter from terrestrial experiments and astrophysical observations of neutron stars

TL;DR

This paper develops new nuclear interaction models to reconcile discrepancies between terrestrial nuclear experiments and astrophysical observations of neutron stars, revealing a softening of the nuclear symmetry energy at high densities.

Contribution

It introduces effective interactions with meson mixing and coupling effects to better match experimental and observational data on nuclear matter and neutron stars.

Findings

Nuclear symmetry energy softens at twice the saturation density.

Models reconcile PREX-2 and NICER observations.

Role of meson interactions in dense nuclear matter.

Abstract

The accurate measurement of neutron skin thickness of $^{208}$Pb by the PREX Collaboration suggests a large value of the nuclear symmetry energy slope parameter, $L$, whereas the smaller $L$ is preferred to account for the small neutron-star radii from NICER observations. To resolve this discrepancy between nuclear experiments and astrophysical observations, new effective interactions have been developed using relativistic mean-field models with the isoscalar- and isovector-meson mixing. We investigate the effects of $\delta$-nucleon coupling and $\sigma$--$\delta$ mixing on the ground-state properties of finite nuclei, as well as the characteristics of isospin-asymmetric nuclear matter and neutron stars. Additionally, we explore the role of the quartic $\rho$-meson self-interaction in dense nuclear matter to mitigate the stiff equation of state for neutron stars resulting from the large $\delta$-nucleon coupling. It is found that the nuclear symmetry energy undergoes a sudden softening at approximately twice the saturation density of nuclear matter, taking into account the PREX-2 result, the recent NICER observation of PSR J0437$-$4715, and the binary neutron star merger, GW170817.