Determination of the strong coupling constant ${\alpha }_s(m_Z)$ from measurements of the total cross section for top-antitop quark production

We present a determination of the strong coupling constant ${\alpha }_s(m_Z)$ using inclusive top-quark pair production cross section measurements performed at the LHC and at the Tevatron. Following a procedure first applied by the CMS collaboration, we extract individual values of ${\alpha }_s(m_Z)$ from measurements by different experiments at several centre-of-mass energies, using QCD predictions complete in NNLO perturbation theory, supplemented with NNLL approximations to all orders, and suitable sets of parton distribution functions. The determinations are then combined using a likelihood-based approach, where special emphasis is put on a consistent treatment of theoretical uncertainties and of correlations between various sources of systematic uncertainties. Our final combined result is $\alpha_s(m_Z) = 0.1177^{+0.0034}_{-0.0036}$.