Zero-temperature thermodynamics of dense asymmetric strong-interaction matter

TL;DR

This paper investigates the zero-temperature phase structure of dense asymmetric matter with two quark flavors, revealing a first-order phase transition relevant for neutron stars and analyzing the behavior of the speed of sound in such matter.

Contribution

It provides the first estimate of the phase transition and speed of sound in dense asymmetric matter using microscopic strong-interaction constraints, relevant for astrophysics.

Findings

First-order phase transition from color-superconducting to ungapped quark matter at high density.

Speed of sound exceeds the noninteracting quark gas value in neutron-star matter.

Presence of a maximum in the speed of sound at around 10 times nuclear saturation density.

Abstract

Employing constraints derived from the microscopic theory of the strong interaction, we estimate the zero-temperature phase structure of dense isospin-asymmetric matter with two quark flavors. We find indications that strong-interaction matter along trajectories relevant for astrophysical applications undergoes a first-order phase transition from a color-superconducting phase to an ungapped quark-matter phase when the density is increased. Such a phase transition is found to be absent in isospin-symmetric matter. Moreover, by taking into account constraints from $\beta$-equilibrium, charge neutrality, and color neutrality, we provide an estimate for the speed of sound in neutron-star matter. Notably, we observe that the speed of sound in neutron-star matter exceeds the asymptotic value associated with the noninteracting quark gas and even increases towards lower densities across a wide range, in agreement with recent results for isospin-symmetric matter. Considering results from studies based on chiral effective field theory at low densities, our findings suggest the existence of a maximum in the speed of sound for $n/n_0 \lesssim 10$, where $n_0$ is the nuclear saturation density.