High-density QCD and Instantons

Instantons generate strong nonperturbative interactions between quarks. In vacuum, these interactions lead to chiral symmetry breaking and generate constituent quark masses on the order of 300-400MeV. The observation that the same forces also provide attraction in the scalar diquark channel leads to the prediction that cold quark matter is a color superconductor, with gaps as large as ~100MeV. We provide a systematic treatment of color superconductivity in the instanton model. We show that the structure of the superconductor depends on the number of flavors. In the case of two flavors, we verify the standard scenario, and provide an improved calculation of the mass gap. For three flavors, we show that the ground state is color-flavor 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 transition between the two phases. Finally, we go beyond the mean-field approximation and investigate the role of instanton/anti-instanton molecules, which - in addition to superconducting gap formation - provide a competitive mechanism for chiral restoration at finite density.