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

Abstract

We extend the recently developed hybrid quark-meson-nucleon model by augmenting a six-point scalar interaction and investigate the consequences for neutron-star sequences in the mass-radius diagram. The model has the characteristic feature that, at increasing baryon density, the chiral symmetry is restored within the hadronic phase by lifting the mass splitting between chiral partner states (parity doubling), before quark deconfinement takes place. At low temperature and finite baryon density, the model predicts a first-, second-order chiral phase transition, or a crossover, depending on the expectation value of the scalar field, and a first-order deconfinement phase transition. We discuss two sets of free parameters, which result in compact-star mass-radius relations that are at tension with the combined constraints for maximum-mass (2 M ) and the compactness (GW170817). We find that the most preferable mass-radius relations result in isospin-symmetric phase diagram with rather low temperature for the critical point of the chiral phase transition.

Speed of sound and quark confinement inside neutron stars

“…The model embeds the concept of statistical confinement through the modified Fermi-Dirac distribution functions, where b is the expectation value of an auxiliary scalar field b and α is a dimensionless model parameter. As demonstrated in references [25][26][27][28][29], the parameter α plays also a crucial role in tuning the order of the chiral phase transition. From the definition of n ± and n q , it is evident that, in order to mimic the statistical confinement, the b field should have a nontrivial vacuum expectation value, to suppress quark degrees of freedom at low densities in the confined and to allow for their population at high densities in deconfined phase.…”

“…In this section, we briefly introduce the hybrid QMN model for the chiral symmetry restoration and deconfinement phase transitions [25][26][27][28][29]. The hybrid QMN model is composed of the baryonic parity doublet [30][31][32] and mesons as in the Walecka model [33], as well as quark degrees of freedom as in the standard linear sigma model [34].…”

“…When the chiral symmetry is restored, the masses become degenerate with a common finite mass m ± (σ = 0) = m 0 , which reflects the parity doubling structure of the low-lying baryons. Following the previous studies of the parity-doublet-based models [25][26][27][28][29][37][38][39][40][41][42][43][44][45][46][47], as well as recent lattice QCD results [48,49], we choose a rather large value, m 0 = 700 MeV. The couplings g 1 and g 2 in equation 8can be determined by fixing the fermion masses in the vacuum.…”

“…This is particularly useful for determining a class of effective models in which the low-density and high-density regimes are not treated independently, but rather combined in a consistent unified framework. To this end, we employ the hybrid quark-meson-nucleon (QMN) model [25][26][27][28][29] to quantify the EoS of cold and dense matter under NS conditions. The model has the characteristic feature that, at increasing baryon density, the chiral symmetry is restored within the hadronic phase by lifting the mass splitting between chiral partner states, before the quark deconfinement takes place.…”

The special point on the hybrid star mass–radius diagram and its multi–messenger implications

“…Such models are, however, blind to the quark substructure of strongly interacting matter, which in itself is a subject of many studies (cf. [5][6][7][8] and references therein).…”