The 2009 world average of α s

Measurements of α s, the coupling strength of the Strong Interaction between quarks and gluons, are summarised and an updated value of the world average of αₛ(M_Z⁰) is derived. Special emphasis is laid on the most recent determinations of α s. These are obtained from τ-decays, from global fits of electroweak precision data and from measurements of the proton structure function F2, which are based on perturbative QCD calculations up to O(αₛ⁴) ; from hadronic event shapes and jet production in e+e− annihilation, based on O(αₛ³) QCD; from jet production in deep inelastic scattering and from ϒ decays, based on O(αₛ²) QCD; and from heavy quarkonia based on unquenched QCD lattice calculations. A pragmatic method is chosen to obtain the world average and an estimate of its overall uncertainty, resulting in αₛ(MZ⁰)=0.1184± 0.0007. The measured values of α s(Q 2), covering energy scales from Q≡M τ=1.78 GeV to 209 GeV, exactly follow the energy dependence predicted by QCD and therefore significantly test the concept of Asymptotic Freedom.