Chiral symmetry restoration by parity doubling and the structure of neutron stars

TL;DR

This paper models the equation of state for neutron stars using a hybrid quark-meson-nucleon approach, showing how chiral symmetry restoration affects star properties and aligns with recent astrophysical observations.

Contribution

It introduces a novel hybrid model capturing chiral symmetry restoration via parity doubling, predicting neutron star characteristics consistent with current observational constraints.

Findings

Chiral symmetry is restored in a first order transition within the hadronic phase.

Predicted mass-radius relations match observational data from PSR J0437-4715 and GW170817.

The model suggests the direct URCA process occurs above 1.4 solar masses.

Abstract

We investigate the equation of state for a recently developed hybrid quark-meson-nucleon model under neutron star conditions of $\beta-$equilibrium and charge neutrality. The model has the characteristic feature that at increasing baryon density chiral symmetry is restored in a first order transition within the hadronic phase by lifting the mass splitting between chiral partner states, before quark deconfinement takes place. Most important for this study are the nucleon (neutron, proton) and $N(1535)$ states. We present three sets for the two free parameters which result in compact star mass-radius relations in accordance with modern constraints on the mass from PSR~J0437-4715 and on the compactness from GW170817. We also consider the threshold for the direct URCA process for which a new relationship is given and suggest as an additional constraint on the parameter choice of the model that this process shall become operative at best for stars with masses above the range for binary radio pulsars, $M>1.4~M_\odot$.