Measurement of dijet azimuthal decorrelations in $pp$ collisions at $\sqrt{s}=8$ TeV with the ATLAS detector and determination of the strong coupling

A measurement of the rapidity and transverse momentumdependence of dijet azimuthal decorrelationsis presented, using the quantity RΔϕ. The quantity RΔϕ specifies the fraction of the inclusive dijet events in which the azimuthal opening angle of the two jets with the highest transverse momenta is less than a given value of the parameter Δϕmax. The quantity RΔϕ is measured in proton-proton collisions at s=8  TeV as a function of the dijet rapidity interval, the event total scalar transverse momentum, and Δϕmax. The measurement uses an event sample corresponding to an integrated luminosity of 20.2  fb-1 collected with the ATLAS detector at the CERN Large Hadron Collider. Predictions of a perturbative QCDcalculation at next-to-leading order in the strong coupling with corrections for nonperturbative effects are compared to the data. The theoretical predictions describe the data in the whole kinematic region. The data are used to determine the strong coupling αS and to study its running for momentum transfersfrom 260 GeV to above 1.6 TeV. Analysis that combines data at all momentum transfersresults in αS(mZ)=0.1127-0.0027+0.0063.