Variational approach to QCD and its applications

A nonperturbative expansion technique in quantum field theory, variational perturbation theory, and some of its applications are reviewed. Within this method, a quantity under consideration is represented by a so-called varia tional or a "floating" series, whose convergence properties can be controll ed by special parameters. It is shown that, in the case of QCD, there appea rs a new small expansion parameter that obeys an equation whose solutions a re always smaller than unity for any value of the initial coupling constant . The method is applied to the inclusive semileptonic decay of tau leptons.