The 2009 World Average of $α_s$

Siegfried Bethke

Paper

The 2009 World Average of $α_s$

Abstract

Measurements of

, the coupling strength of the Strong Interaction between quarks and gluons, are summarised and an updated value of the world average of

is derived. Building up on previous reviews, special emphasis is laid on the most recent determinations of

. These are obtained from

-decays, from global fits of electroweak precision data and from measurements of the proton structure function

, which are based on perturbative QCD calculations up to

; from hadronic event shapes and jet production in

annihilation, based on

QCD; from jet production in deep inelastic scattering and from

decays, based on

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

results in

. The measured values of

, covering energy scales from

GeV to 209 GeV, exactly follow the energy dependence predicted by QCD and therefore significantly test the concept af Asymptotic Freedom.