The 2009 world average of α s

Abstract

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 αsMZ0 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 γ 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 QΞM τ =1.78 GeV to 209 GeV, exactly follow the energy dependence predicted by QCD and therefore significantly test the concept of Asymptotic Freedom. © 2009 Springer-Verlag/Società Italiana di Fisica.