Treatment of QCD coupling in Evolution Equations

by Boris Ermolaev (IFMO Inst. of Fine Mechanics and Optics)

Thursday 10 Dec 2009, 11:00 → 12:00 Europe/Zurich

1/1-025 (CERN)

The parametrization $\alpha_s = \alpha_s (Q^2)$ in the DGLAP evolution equations comes from the parametrization $\alpha_s = \alpha_s(k_T^2)$ in every rung of the ladder graphs. Such a parametrization comes from analysis of QCD reactions in the Hard kinematics. I address it as Standard Parametrization. However, our analysis demonstrates that actually this expression should be replaced by an effective coupling $\alpha_s^{eff}$ given by a much more involved expression. Nevertheless, when the starting point $\mu^2$ of the $Q^2$ -evolution is chosen high enough, Standard Parametrization can be used. The criterion is $\mu^2 \gg \Lambda^2_{QCD} \exp(\pi)$