Determination of the QCD Coupling ${\alpha }_s$

Abstract

Theoretical basics and experimental determinations of the coupling parameter of the Strong Interaction, ${\alpha }_s$, are reviewed. The world average value of ${\alpha }_s$, expressed at the energy scale of the rest mass of the $Z^0$ boson, is determined from analyses which are based on complete NNLO perturbative QCD. The result is ${\alpha }_s(M_Z)=0.1184\pm 0.0031$. No significant deviations or systematic biases of subsamples of experimental results are found. From the observed energy dependence of ${\alpha }_s$, which is in excellent agreement with the expectations of QCD, the number of colour degrees of freedom can be constrained to $N_c=3.03\pm 0.12$.

Comments: 49 pages, 12 figures, to be published in Journal of Physics G