Color superconductivity in dense quark matter

TL;DR

This paper reviews the theoretical understanding of color superconductivity in dense quark matter, focusing on the CFL phase, its properties, and implications for neutron star observations.

Contribution

It provides a comprehensive review of the calculations and physical properties of various color-superconducting phases, especially the CFL phase, in dense quark matter.

Findings

CFL phase is a superfluid and electromagnetic insulator.

Effective theories describe low-energy excitations in the CFL phase.

Transport properties influence neutron star signatures.

Abstract

Matter at high density and low temperature is expected to be a color superconductor, which is a degenerate Fermi gas of quarks with a condensate of Cooper pairs near the Fermi surface that induces color Meissner effects. At the highest densities, where the QCD coupling is weak, rigorous calculations are possible, and the ground state is a particularly symmetric state, the color-flavor locked (CFL) phase. The CFL phase is a superfluid, an electromagnetic insulator, and breaks chiral symmetry. The effective theory of the low-energy excitations in the CFL phase is known and can be used, even at more moderate densities, to describe its physical properties. At lower densities the CFL phase may be disfavored by stresses that seek to separate the Fermi surfaces of the different flavors, and comparison with the competing alternative phases, which may break translation and/or rotation invariance, is done using phenomenological models. We review the calculations that underlie these results, and then discuss transport properties of several color-superconducting phases and their consequences for signatures of color superconductivity in neutron stars.