Speed of sound in dense strong-interaction matter

TL;DR

This paper investigates how the speed of sound varies in dense strong-interaction matter, especially considering the effects of a color-superconducting gap, revealing a maximum at certain densities and linking gap properties to observable features.

Contribution

It provides a detailed analysis of the speed of sound in dense matter accounting for color-superconducting gaps, highlighting the impact of gap derivatives on the density dependence.

Findings

Speed of sound approaches asymptotic value at high densities.

Presence of a gap increases the speed of sound below asymptotic density.

A maximum in the speed of sound is suggested at intermediate densities.

Abstract

We study the speed of sound in strong-interaction matter at zero temperature and in density regimes which are expected to be governed by the presence of a color-superconducting gap. At (very) high densities, our analysis indicates that the speed of sound approaches its asymptotic value associated with the non-interacting quark gas from below, in agreement with first-principles studies which do not take the presence of a color-superconducting gap into account. Towards lower densities, however, the presence of a gap induces an increase of the speed of sound above its asymptotic value. Importantly, even if gap-induced corrections to the pressure may appear small, we find that derivatives of the gap with respect to the chemical potential can still be sizeable and lead to a qualitative change of the density dependence of the speed of sound. Taking into account constraints on the density dependence of the speed of sound at low densities, our general considerations suggest the existence of a maximum in the speed of sound. Interestingly, we also observe that specific properties of the gap can be related to characteristic properties of the speed of sound which are indirectly constrained by observations.