Extraction of characteristic constants in QCD with perturbative and nonperturbative methods

Abstract

Part I is devoted to the extraction of the QCD coupling from a bound state approach at low energy scales, where unphysical singularities make the RG-improved pQCD useless. Theoretical results on the meson spectrum based on a Bethe-Salpeter formalism adjusted for QCD have been exploited for the determination of experimental values of α_s below 1GeV by comparison with the data. Results quantitatively confirm the recently developed Analytic Perturbation Theory approach to IR phenomena down to 200 MeV. Below this scale the extracted values give a hint about the vanishing of the QCD coupling as Q^2 goes to zero. In part II the possibility of inferring the spin-dependent gluon distribution from scaling violations is addressed by performing a NLO analysis of inclusive DIS data in pQCD. In addition, this work presents a phenomenological study of open-charm photoproduction in the polarized case, which is tailored to the COMPASS experiment in view of upcoming data. It is shown that current inclusive data allow a reliable determination of the gluon first moment, whereas the x-shape is significantly affected by the assumed functional form. On the other hand, studies of charm pseudo-data reveal that the open-charm approach should be capable to reduce the bias induced by the choice of input densities, and to pin down the x-shape of the gluon polarization.