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 proton-proton collision 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 million electron and muon pairs, in dataset 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 small 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 $\alpha_s(m_Z) = 0.11828^{+0.00084}_{-0.00088}$. This is the most precise experimental determination of $\alpha_s(m_Z)$ achieved so far.