Novel sets of coupling expansion parameters for low-energy pQCD

In quantum theory, physical amplitudes are usually presented in the form of a Feynman perturbation series in powers of coupling constant α. However, it is known that these amplitudes are not regular functions at α = 0. For QCD, we propose new sets of expansion parameters w _k ( α _s ) that reflect singularity at α _s = 0 and should be used instead of powers α {_/s ^k }. Their explicit form is motivated by the so-called Analytic Perturbation Theory. These parameters reveal saturation in a strong coupling case at the level α {_/s ^eff} ( α _s [Figure not available: see fulltext.] 1) = w ₁( α _s [Figure not available: see fulltext.] 1) ∼ 0.5. They can be used for the quantitative analysis of divers low-energy amplitudes. We argue that this new picture with non-power sets of perturbation expansion parameters, as well as the saturation feature, is of a rather general nature.