Techniques of evaluation of QCD low-energy physical quantities with running coupling with infrared fixed point

Abstract: Perturbative QCD (pQCD) running coupling a(Q^2) (=alpha_s(Q^2)/pi) is expected to get modified at low spacelike momenta 0 < Q^2 < 1 GeV^2 so that, instead of having unphysical (Landau) singularities it remains smooth and finite there, due to infared (IR) fixed point. This behavior is suggested by: Gribov-Zwanziger approach, Dyson-Schwinger equations (DSE) and other functional methods, lattice calculations, light-front holographic mapping AdS/CFT modified by a dilaton background, and by most of the analytic (holomorphic) QCD models. All such couplings, A(Q^2), differ from the pQCD couplings a(Q^2) at |Q| > 1 GeV by nonperturbative (NP) terms, typically by some power-suppressed terms ~1/(Q^2)^N. Evaluations of low-energy physical QCD quantities in terms of such A(Q^2) couplings (with IR fixed point) at a level beyond one-loop are usually performed with (truncated) power series in A(Q^2). We argue that such an evaluation is not correct, because the NP terms in general get out of control as the number of terms in the power series increases. The series consequently become increasingly unstable under the variation of the renormalization scale, and have a fast asymptotic divergent behavior compounded by the renormalon problem. We argue that an alternative series in terms of logarithmic derivatives of A(Q^2) should be used. Further, a Pad\'e-related resummation based on this series gives results which are renormalization scale independent and show very good convergence. Timelike low-energy observables can be evaluated analogously, using the integral transformation which relates the timelike observable with the corresponding spacelike observable.