We introduce a new measurable quantity, RΔϕ, for studies of the
rapidity and transverse momentum dependence of dijet azimuthal decorrelations
in hadron-hadron collisions. In pQCD, RΔϕ is computed as a ratio
of three-jet and dijet cross sections in which the parton distribution
functions cancel to a large extent. At the leading order, RΔϕ is
proportional to αs, and the transverse momentum dependence of can
therefore be exploited to determine αs. We compute the NLO pQCD theory
predictions and non-perturbative corrections for RΔϕ at the LHC
and the Tevatron and investigate the corresponding uncertainties. From this, we
estimate the theory uncertainties for αs determinations based on
RΔϕ at both colliders. The potential of RΔϕ
measurements for tuning Monte Carlo event generators is also demonstrated.Comment: 20 pages, 11 figures, 1 table, submitted to JHE