Nonlocal QCD vacuum in the instanton model

A new gauge invariant approach to analyze nonlocal properties of quarks and gluons in the physical QCD vacuum is suggested. It is based on the instanton liquid model. The topologically nontrivial solution to the classical Yang-Mills equations in the physical QCD vacuum is found. This solution, called the constraint instanton, decays exponentially at long distances. It is stable only if the interaction of the instanton with the background field is weak and additional constraints are introduced. The estimates of the averaged virtualities of the quarks and gluons in the QCD vacuum are given. The instanton model is shown to satisfactorily describe the nonlocal structure of the QCD vacuum. The two-point vacuum correlator of the gluon field strengths is calculated in the framework of the effective instanton vacuum model.