Precision physics with QCD

Departament de Física Teòrica, IFIC, Universitat de València – CSIC Edifici d’Instituts de Paterna, Apt. Correus 22085, E-46071, València, Spain

^a e-mail: Antonio.Pich@ific.uv.es

Published online: 22 March 2017

Abstract

The four-loop determination of the strong coupling from fully inclusive observables is reviewed. Special attention is given to the low-energy measurement extracted from the hadronic τ decay width. A recent exhaustive analysis of the ALEPH data, exploring several complementary methodologies with very different sensitivities to inverse power corrections and duality violations, confirms the strong suppression of non-perturbative contributions to R_τ. It gives the value α_s(m_τ²) = 0.328, which implies α_s(M_Z²) = 0.1197 ± 0.0015. The excellent agreement with the direct measurement at the Z peak, α_s(M_Z²) = 0.1196 ± 0.0030, provides a beautiful test of asymptotic freedom. Together with the most recent lattice average from FLAG and the NNLO determinations from e⁺e⁻, PDFs and collider data quoted by the PDG, these two inclusive determinations imply a world average value α_s(M_Z²) = 0.1180 ± 0.0010.