A precise determination of the strong-coupling constant from the recoil of $Z$ bosons with the ATLAS experiment at $\sqrt{s} = 8$ TeV

The coupling constant of the strong force is determined from the transverse-momentum distribution of $Z$ bosons produced in 8 TeV proton-proton collisions at the LHC and recorded by the ATLAS experiment. The $Z$-boson cross sections are measured in the full phase space of the decay leptons using 15.3 million electron and muon pairs, in a dataset collected in 2012 and corresponding to an integrated luminosity of 20.2 fb$^{-1}$. The analysis is based on predictions evaluated at third order in perturbative QCD, supplemented by the resummation of logarithmically enhanced contributions in the low transverse-momentum region of the lepton pairs. The determined value of the strong coupling at the reference scale corresponding to the $Z$-boson mass is $\alpha_\text{s}(m_Z) = 0.1183 \pm 0.0009$. This is the most precise experimental determination of $\alpha_\text{s}(m_Z)$ achieved so far.