Physics of QCD Instantons

These lectures cover applications of the semiclassical theory to QCD. We start with tunneling through the topological barrier described by the instantons, and review their multiple applications in QCD. The interacting instanton liquid model describes the ensemble of instantons, in its multiple phases. The QCD vacuum happens to be a disordered "liquid", which breaks spontaneously chiral symmetry. The high-T phase is kind of a "molecular gas" which does not break any symmetries. High density phase is a color superconductor: here instantons form chains or diquarks. As recent applications we briefly discuss correlation functions and pion and nucleon form-factors. We discuss in some detail also the fate of instantons at large number of colors N_c. The second set of topics are applications of the semi-classical theory of high energy collisions: instead of tunneling through the barrier one has in this case excitation from under the barrier to it. These methods were recently applied to the old problem of soft Pomeron, and especially for explanations of puzzling behaviors of heavy ion collisions observed recently at Relativistic Heavy Ion Collider in Brookhaven.