Eur. Phys. J. C (2009) 64: 689-703
https://doi.org/10.1140/epjc/s10052-009-1173-1

Review

The 2009 world average of α _s

MPI für Physik, Föhringer Ring 6, 80805, Munich, Germany

^* e-mail: bethke@mppmu.mpg.de

Received: 10 August 2009
Published online: 23 October 2009

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 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 ; from hadronic event shapes and jet production in e⁺e⁻ annihilation, based on QCD; from jet production in deep inelastic scattering and from ϒ decays, based on 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 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.