First Determination of the Strong Coupling Constant from the ttbar Cross Section

Approximate next-to-next-to-leading-order QCD predictions for the cross section of top-quark pair production are used together with different parton distribution functions to extract the strong coupling constant, $\alpha_S$, from the cross section measured by the CMS experiment in proton-proton collisions at a center-of-mass energy of 7 TeV. This is the first determination of $\alpha_S$ from top-quark production. It probes the strong coupling with remarkable precision and in an energy regime that has been accessible to only a small number of $\alpha_S$ measurements so far. The extracted value of $\alpha_S (m_Z)$ is $0.1178^{+0.0046}_{-0.0040}$, in good agreement with the current world average.