Quark-nuclear hybrid equation of state for neutron stars under modern observational constraints

TL;DR

This paper develops a hybrid equation of state for neutron stars combining hadronic and quark matter phases, fitted to lattice QCD results, and constrained by recent astrophysical observations.

Contribution

It introduces a new hybrid EOS model with a Maxwell construction, incorporating nonlocal NJL quark matter and lattice QCD fitting, constrained by recent neutron star data.

Findings

Speed of sound remains approximately constant at 0.4-0.6 in neutron star cores.

A mu-dependent bag pressure is necessary to satisfy observational constraints.

The model predicts a coexistence phase of color superconductivity and chiral symmetry breaking.

Abstract

We study a family of equations of state for hybrid neutron star matter. The hybrid EOS are obtained by a Maxwell construction of the first-order phase transition between a hadronic phase described by the relativistic density-functional EOS of the "DD2" class with excluded volume effects and a deconfined quark matter phase modeled by an instantaneous nonlocal version of the Nambu-Jona-Lasinio model in SU(2)$_f$ with vector interactions and color superconductivity. The form factor in the nonlocal quark matter model is fitted to lattice QCD results in the Coulomb gauge. Owing to strong coupling in the vector meson and diquark channels, a coexistence phase of color superconductivity and chiral symmetry breaking occurs. Our results show an approximately constant behavior for the squared speed of sound with values of 0.4 - 0.6 in the density region relevant for neutron star interiors. To simultaneously fulfill the constraints from the Neutron Star Interior Composition Explorer radius measurement for PSR J0740+6620 and tidal deformability from GW170817 it is necessary to consider a $\mu$-dependent bag pressure that mimics confinement.