, Volume 64, Issue 4, pp 689-703,
Open Access This content is freely available online to anyone, anywhere at any time.
Date: 23 Oct 2009

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 \(\alpha_{\mathrm{s}}(M_{\mathrm{Z}^{0}})\) 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 \(\mathcal{O}(\alpha_{\mathrm{s}}^{4})\) ; from hadronic event shapes and jet production in e+e annihilation, based on \(\mathcal{O}(\alpha_{\mathrm{s}}^{3})\) QCD; from jet production in deep inelastic scattering and from ϒ decays, based on \(\mathcal{O}(\alpha_{\mathrm{s}}^{2})\) 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 $$\alpha_\mathrm{s}(M_{\mathrm{Z}^0})=0.1184\pm 0.0007.$$ The measured values of α s(Q 2), covering energy scales from QM τ =1.78 GeV to 209 GeV, exactly follow the energy dependence predicted by QCD and therefore significantly test the concept of Asymptotic Freedom.