Dynamical Higgs mechanism without elementary scalars: a lesson from instantons

The generation of gauge-dependent fermion vacuum condensates in Yang-Mills theory would dynamically break the gauge symmetry and thus provide an alternative to the Higgs mechanism in unified theories. We explore a simple example: instanton-induced quark pair (diquark) condensation in QCD with 2 flavors and Nc colors. We do find diquark condensates in the vacuum, but only for Nc = 2, where they are equivalent to the standard quark-antiquark condensate. At Nc ≥ 3 diquark condensates exist as meta-stable saddle points of the instanton-induced effective QCD action and may strongly affect the properties of matter under extreme conditions. The scalar diquark excitation, which is a Goldstone boson at Nc = 2, becomes unbound for Nc ≥ 3.