High-density QCD and instantons

Instantons generate strong nonperturbative interactions between quarks. In vacuum, these interactions lead to chiral symmetry breaking and generate co nstituent quark masses on the order of 300 400 MeV. The observation that th e same forces also provide attraction in the scalar diquark channel leads t o the prediction that cold quark matter is a color superconductor, with gap s as large as similar to 100 MeV. We provide a systematic treatment of colo r superconductivity in the instanton model. We show that the structure of t he superconductor depends on the number of flavors. In the case of two flav ors, we verify the standard scenario, and provide an improved calculation o f the mass gap. For three flavors, we show that the ground state is color-f lavor locked and calculate the chiral condensate in the high-density phase. We show that as a function of the strange quark mass, there is a sharp tra nsition between the two phases. Finally, we go beyond the mean-field approx imation and investigate the role of instanton/anti-instanton molecules, whi ch-in addition to superconducting gap formation-provide a competitive mecha nism for chiral restoration at finite density. (C) 2000 Academic Press.