The 2009 World Average of ${\alpha }_s$

Abstract

Measurements of ${\alpha }_s$, the coupling strength of the Strong Interaction between quarks and gluons, are summarised and an updated value of the world average of ${\alpha }_s(M_Z)$ is derived. Building up on previous reviews, special emphasis is laid on the most recent determinations of ${\alpha }_s$. These are obtained from $\tau$-decays, from global fits of electroweak precision data and from measurements of the proton structure function $\F_2$, which are based on perturbative QCD calculations up to $O({\alpha }_s^4)$; from hadronic event shapes and jet production in $\epem$ annihilation, based on $O({\alpha }_s^3)$ QCD; from jet production in deep inelastic scattering and from ${\rm Y}$ decays, based on $O({\alpha }_s^2)$ QCD; and from heavy quarkonia based on unquenched QCD lattice calculations. Applying pragmatic methods to deal with possibly underestimated errors and/or unknown correlations, the world average value of ${\alpha }_s(M_Z)$ results in ${\alpha }_s(M_Z)=0.1184\pm 0.0007$. The measured values of ${\alpha }_s(Q)$, covering energy scales from $Q \equiv \mtau = 1.78$ GeV to 209 GeV, exactly follow the energy dependence predicted by QCD and therefore significantly test the concept af Asymptotic Freedom.

14 pages, 7 figures

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