$\alpha_s$ status and perspectives (2018)

Abstract

The latest experimental and theoretical developments in the high-precision determination of the strong coupling $\alpha_s$ are briefly reviewed. Six groups of observables: (i) lattice QCD data, (ii) hadronic $\tau$ decays, (iii) deep-inelastic $e^\pm p$ data and parton distribution functions (PDF) fits, (iv) event shapes and jet rates in $e^+e^-$ collisions, (v) Z boson hadronic decays, and (vi) top-quark cross sections in pp collisions, are used to extract the current world-average at the Z pole mass, $\alpha_s(m_Z^2) = 0.1181 \pm 0.0011$ at next-to-next-to-leading-order (NNLO), or beyond, accuracy. Additional NNLO extractions have recently appeared based on new lattice studies, the $R(s)$ ratio in $e^+e^-\to \mbox{hadrons}$, updated PDF fits, energy-energy correlations in $e^+e^-$ collisions, jet cross sections in $e^\pm p$ collisions, and the full set of $pp\to t\bar{t}$ cross sections at the LHC. Inclusion of these new data into the world-average would slightly increase its value and reduce its uncertainty to $\alpha_s(m_Z^2) = 0.1183 \pm 0.0008$. Future $\alpha_s$ extraction perspectives with permille uncertainties at future high-luminosity $e^+e^-$ machines -- via W and Z hadronic decays, parton fragmentation functions, and photon $F_2(x,Q^2)$ structure function in $\gamma\gamma$ collisions -- are also discussed.