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

## TL;DR

This paper explores how parity doubling and chiral symmetry restoration in baryons influence the structure of neutron stars, proposing a hybrid model where chiral symmetry is restored before quark deconfinement, affecting star properties.

## Contribution

It introduces a hybrid quark-meson-nucleon model showing chiral symmetry restoration via parity doubling occurs in the hadronic phase before deconfinement in neutron stars.

## Key findings

- Chiral symmetry restoration occurs as a first-order phase transition deep in the hadronic phase.
- High-mass neutron stars can exist without quark deconfinement.
- Parity doubling impacts the mass-radius relation of neutron stars.

## Abstract

Recent lattice QCD studies at vanishing density exhibit the parity-doubling structure for the low-lying baryons around the chiral crossover temperature. This finding is likely an imprint of the chiral symmetry restoration in the baryonic sector of QCD, and is expected to occur also in cold dense matter, which makes it of major relevance for compact stars. By contrast, typical effective models for compact star matter embody chiral physics solely in the deconfined sector, with quarks as degrees of freedom. In this contribution, we present a description of QCD matter based on the effective hybrid quark-meson-nucleon model. Its characteristic feature is that, under neutron-star conditions, the chiral symmetry is restored in a first-order phase transition deep in the hadronic phase, before the deconfinement of quarks takes place. We discuss the implications of the parity doubling of baryons on the mass-radius relation for compact stars obtained in accordance with the modern constraints on the mass from PSR J0348+0432, the compactness from GW170817, as well as the direct URCA process threshold. We show that the existence of high-mass stars might not necessarily signal the deconfinement of quarks.