Variational approach to QCD and its applications

A nonperturbative expansion technique in quantum field theory< variational perturbation theory, and some its applications are reviewed. Within this method a quantity under consideration is represented by the so-called variational or floating series, the convergence properties of which can be controlled by special parameters. It is shown that in the case of QCD a new small expansion parameter is appeared and obeyed an equation whose solutions are always smaller than unity for any value of the initial coupling constant. The method is applied to the inclusive semileptonic decay of the τ-lepton