Instantons in the nonperturbative QCD vacuum

The effect of nonperturbative fields on instantons in QCD is investigated. The nonperturbative vaccum is described in terms of nonlocal gauge-invariant vaccum expectation values of gluon fields. An effective action for instantons is obtained in the bilocal approximation, and it is shown that a stochastic background gluon field leads to an infrared stabilization of instantons. The dependence of a characteristic instanton size on the magnitude of the gluon condensate and on the correlation length in the nonperturbative vaccum is found. The size distribution of instantons that is obtained here is compared with the results of lattice calculations. (C) 2004 MAIK "Nauka/Interperiodica".