Scheme Variations of the QCD Coupling and Hadronic τ Decays

The quantum chromodynamics (QCD) coupling alpha(s) is not a physical observable of the theory, since it depends on conventions related to the renormalization procedure. We introduce a definition of the QCD coupling, denoted by a (alpha) over cap (S), whose running is explicitly renormalization scheme invariant. The scheme dependence of the new coupling (alpha) over cap (S) is parametrized by a single parameter C, related to transformations of the QCD scale Lambda. It is demonstrated that appropriate choices of C can lead to substantial improvements in the perturbative prediction of physical observables. As phenomenological applications, we study e(+)e(-) scattering and decays of the tau lepton into hadrons, both being governed by the QCD Adler function.