Building relativistic mean field models for finite nuclei and neutron stars

TL;DR

This paper develops a relativistic energy density functional model for finite nuclei and neutron stars, using statistical methods to optimize parameters and analyze uncertainties, successfully reproducing key nuclear and astrophysical observations.

Contribution

It introduces the FSUGold2 model, a new relativistic EDF that accurately describes finite nuclei and neutron star properties, with a flexible calibration scheme and covariance analysis.

Findings

The model reproduces finite nuclei ground-state properties.

It accounts for the maximum observed neutron star mass.

Predicts a large neutron-skin thickness in Pb208.

Abstract

Background: Theoretical approaches based on density functional theory provide the only tractable method to incorporate the wide range of densities and isospin asymmetries required to describe finite nuclei, infinite nuclear matter, and neutron stars. Purpose: A relativistic energy density functional (EDF) is developed to address the complexity of such diverse nuclear systems. Moreover, a statistical perspective is adopted to describe the information content of various physical observables. Methods: We implement the model optimization by minimizing a suitably constructed chi-square objective function using various properties of finite nuclei and neutron stars. The minimization is then supplemented by a covariance analysis that includes both uncertainty estimates and correlation coefficients. Results: A new model, FSUGold2, is created that can well reproduce the ground-state properties of finite nuclei, their monopole response, and that accounts for the maximum neutron star mass observed up to date. In particular, the model predicts both a stiff symmetry energy and a soft equation of state for symmetric nuclear matter--suggesting a fairly large neutron-skin thickness in Pb208 and a moderate value of the nuclear incompressibility. Conclusions: We conclude that without any meaningful constraint on the isovector sector, relativistic EDFs will continue to predict significantly large neutron skins. However, the calibration scheme adopted here is flexible enough to create models with different assumptions on various observables. Such a scheme--properly supplemented by a covariance analysis--provides a powerful tool to identify the critical measurements required to place meaningful constraints on theoretical models.