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 boththe 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 TeVusing data recorded by the ATLAS detector at the Large Hadron Collider in 2010. Theseratios are compared to predictions from next-to-leading-order QCD calculations correctedfor non-perturbative eects. From the ratio measured as a function of the transverse momentumof each jet in an event, the value of the strong coupling constant can be extractedin the jet transverse momentum range between 210 GeV and 800 GeV. The value of thestrong coupling constant evolved to the Z boson mass is determined to be s(MZ) = 0:111 0:006(exp:) +0:0160:003(theory). The measurement of the strong coupling constant as a functionof energy scale is consistent with the QCD predictions of its running up to an energy scaleof 800 GeV