Color superconductivity and dense quark matter

TL;DR

This paper reviews the concept of color superconductivity in dense quark matter, its phases, and relevance for compact stars, highlighting theoretical challenges and the need for effective models at lower densities.

Contribution

It provides an accessible overview of color superconductivity phases, their theoretical basis, and discusses open problems in understanding dense quark matter in astrophysical contexts.

Findings

QCD predicts the color-flavor locked phase at high densities

Homogeneous phases become unstable at lower densities due to flavor asymmetry

Effective models are needed to study matter in compact stars

Abstract

The properties of cold and dense quark matter have been the subject of extensive investigation, especially in the last decade. Unfortunately, we still lack of a complete understanding of the properties of matter in these conditions. One possibility is that quark matter is in a color superconducting phase which is characterized by the formation of a diquark condensate. We review some of the basic concepts of color superconductivity and some of the aspects of this phase of matter which are relevant for compact stars. Since quarks have color, flavor as well as spin degrees of freedom many different color superconducting phases can be realized. At asymptotic densities QCD predicts that the color flavor locked phase is favored. At lower densities where the QCD coupling constant is large, perturbative methods cannot be applied and one has to rely on some effective model, eventually trying to constrain such a model with experimental observations. The picture is complicated by the requirement that matter in the interior of compact stars is in weak equilibrium and neutral. These conditions and the (possible) large value of the strange quark mass conspire to separate the Fermi momenta of quarks with different flavors, rendering homogenous superconducting phases unstable. One of the aims of this presentation is to introduce non-experts in the field to some of the basic ideas of color superconductivity and to some of its open problems.