Chromomagnetic Instability and Gluonic Phase in Dense Neutral Quark Matter

The phase structure of hot and/or dense QCD is one of the most exciting current topics in the field of strong interactions. In particular, the properties of cold and dense quark matter are of great interest in astrophysics and cosmology; it is now widely accepted that, at moderate densities of relevance for the interior of compact stars, quark matter is a color superconductor and has a rich phase structure with important implications for compact star physics (for a recent review, see Ref. 1)). Bulk matter in the interior of compact stars should be color and electrically neutral and be in β-equilibrium. In the two-flavor case, these conditions separate the Fermi momenta of up and down quarks and, as a consequence, the ordinary BCS state (2SC) is not always energetically favored over other unconventional states. The possibilities include crystalline color superconductivity2), 3) and gapless color superconductivity (g2SC).4) However, the 2SC/g2SC phases suffer from a chromomagnetic instability, indicated by imaginary Meissner masses of some gluons.5) The instability related to gluons of color 4–7 occurs when the ratio of the 2SC gap over the chemical potential mismatch, ∆/δμ, decreases below a value √ 2. Resolving the chromomagnetic instability and clarifying the nature of true ground state of dense quark matter are central issues in the study of color superconductivity.6), 7), 8), 9), 10), 11), 12), 13) We will describe the results of recent studies of a chromomagnetic instability and a gluonic phase6) (gluonic cylindrical phase II) in dense neutral quark matter.