Neutron stars within the SU(2) parity doublet model

TL;DR

This paper models neutron star matter using the SU(2) parity doublet framework, revealing a smooth chiral transition at high densities and constraining neutron star properties, but highlighting limitations in describing vacuum states.

Contribution

It introduces a mean field and relativistic Hartree approximation analysis of neutron stars within the SU(2) parity doublet model, emphasizing chiral symmetry restoration in dense matter.

Findings

Chiral symmetry restoration occurs at about twice nuclear saturation density.

The model predicts a smooth crossover transition to the chiral restored phase.

Constraints on the model are derived from neutron star maximum mass observations.

Abstract

The equation of state of beta-stable and charge neutral nucleonic matter is computed within the SU(2) parity doublet model in mean field and in the relativistic Hartree approximation. The mass of the chiral partner of the nucleon is assumed to be 1200 MeV. The transition to the chiral restored phase turns out to be a smooth crossover in all the cases considered, taking place at a baryon density of just $2\rho_0$. The mass-radius relations of compact stars are calculated to constrain the model parameters from the maximum mass limit of neutron stars. It is demonstrated that chiral symmetry starts to be restored, which in this model implies the appearance of the chiral partners of the nucleons, in the center of neutron stars. However, the analysis of the decay width of the assumed chiral partner of the nucleon poses limits on the validity of the present version of the model to describe vacuum properties.