Extraction of α_s from deep inelastic scattering at large x

We present an analysis of the role of the running coupling constant at the intersection of perturbative and nonperturbative QCD. Although the approaches that have been considered so far in these two regimes appear to be complementary to each other, a unified description might derive through the definition of the effective coupling, as they both provide ways of analyzing its freezing at low values of the scale. We extract the effective coupling from all available experimental data on the unpolarized structure function of the proton, $F_2^p$, at large values of Bjorken $x$, including the resonance region. We suggest that parton-hadron duality observed in this region can be explained if nonperturbative effects are included in the coupling constant. The outcome of our analysis is a smooth transition from perturbative to nonperturbative QCD physics, embodied in the running of the coupling constant at intermediate scales.