Analytic Perturbation Theory for QCD observables

The connection between ghost–free formulations of RG–invariant perturbation theory in the both Euclidean and Minkowskian regions is investigated. Our basic tool is the “double spectral representation”, similar to the definition of Adler function, that stems from first principles of local QFT. It relates real functions defined in the Euclidean and Minkowskian regions. On this base we establish a simple relation between — The trick of resummation of the π2–terms (known from early 80s) for the invariant QCD coupling and observables in the time-like region and — Invariant Analytic Approach (devised a few years ago) with the “analyticized” coupling αan(Q 2) and nonpower perturbative expansion for observables in the space-like domain which are free of unphysical singularities . As a result, we formulate a self–consistent scheme — Analytic Perturbation Theory (APT) — that relates a renorm–invariant, effective coupling functions αan(Q 2) and α̃(s) , as well as non–power perturbation expansions for observables in both space– and time–like domains, that are free of extra singularities and obey better convergence in the infrared region. Then we consider the issue of the heavy quark thresholds and devise a global APT scheme for the data analysis in the whole accessible space-like and time-like domain with various numbers of active quarks. Preliminary estimates indicate that this global scheme produces results a bit different, sometimes even in the five-flavour region, on a few per cent level for ᾱs – from the usual one, thus influencing the total picture of the QCD parameter correlation.