Measurement of multi-jet cross-section ratios and determination of the strong coupling constant in proton-proton collisions at sqrt(s)=7 TeV with the ATLAS detector.

The inclusive three-jet to two-jet cross-section ratio is measured as a function of both the leading jet transverse momentum and the transverse momentum of each jet in an event. These ratios are measured in proton-proton collisions at a centre-of-mass energy of 7 TeV using data recorded by the ATLAS detector at the Large Hadron Collider in 2010. These ratios are compared to predictions from next-to-leading-order QCD calculations corrected for non-perturbative effects. From the ratio measured as a function of the transverse momentum of each jet in an event, the value of the strong coupling constant can be extracted in the jet transverse momentum range between 210~GeV and 800~GeV. The value of the strong coupling constant evolved to the $Z$ boson mass is determined to be $\alpha_{s} (M_Z)$ = $0.111 \pm 0.006({\rm exp}.)~^{\rm + 0.016}_{\rm -0.003}({\rm theory})$. The measurement of the strong coupling constant as a function of energy scale is consistent with the QCD predictions of its running up to an energy scale of 800 GeV.