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

Abstract

Recent lattice QCD studies at vanishing density exhibit the parity-doubling structure for the lowlying 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.

Parity Doubling and the Dense-Matter Phase Diagram under Constraints from Multi-Messenger Astronomy

“…In this section, we briefly introduce the hybrid QMN model for the QCD transitions at finite temperature, density, and arbitrary isospin asymmetry for the application to the physics of neutron stars [15][16][17].…”

“…The parameters g 1 , g 2 , g q are Yukawa-coupling constants, m 0 is the chirally invariant mass of the baryons and is treated as an external parameter (for more details see Ref. [15,17]). The values of those couplings can be determined by fixing the fermion masses in the vacuum (see Table 2).…”

“…In the literature, this strong phase transition has been discussed as due to quark deconfinement [12][13][14]. This conclusion may however be premature since strong phase transitions with a large latent heat occur also within hadronic matter, for instance due to chiral symmetry restoration within the hadronic phase [15]. In the present work, we employ the hybrid quark-meson-nucleon (QMN) model [15][16][17] to explore the implications of dynamical sequential phase transitions at high baryon density on the phenomenology of neutron stars.…”

“…This conclusion may however be premature since strong phase transitions with a large latent heat occur also within hadronic matter, for instance due to chiral symmetry restoration within the hadronic phase [15]. In the present work, we employ the hybrid quark-meson-nucleon (QMN) model [15][16][17] to explore the implications of dynamical sequential phase transitions at high baryon density on the phenomenology of neutron stars. In order to improve the description of nuclear matter properties at the saturation density, we extend the previous hybrid QMN model by including a six-point scalar interaction.…”

Impact of the Nuclear Equation of State on the Stability of Hybrid Neutron Stars

“…Currently, there are only a few approaches that attempt to consistently describe hadron matter at the level of quarks and gluons at high density (cf. for example [7][8][9][10][11]), with the majority of studies relying on models constructed using the two-phase approach [12], i.e., with a separate hadron and quark matter models and a transition between them emulating the effect of deconfinement. A general review of recent developments concerning the EoS in astrophysical applications can be found in [13,14].…”