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The coupled Dyson-Schwinger equations for the gluon and ghost propagators areinvestigated in the Landau gauge using a two-loop improved truncation thatpreserves the multiplicative renormalizability of the propagators. In thistruncation all diagrams contribute to the leading order infrared analysis. Theinfrared contributions of the nonperturbative two-loop diagrams to the gluonvacuum polarization are computed analytically, and this reveals that infraredpower behaved propagator solutions only exist when the squint diagramcontribution is taken into account. For small momenta the gluon and ghostdressing functions behave respectively like (p^2)^{2\kappa} and(p^2)^{-\kappa}, and the running coupling exhibits a fixed point. The values ofthe infrared exponent and fixed point depend on the precise details of thetruncation. The coupled ghost-gluon system is solved numerically for allmomenta, and the solutions have infrared behaviors consistent with thepredictions of the infrared analysis. For truncation parameters chosen suchthat \kappa=0.5, the two-loop improved truncation is able to produce solutionsfor the propagators and running coupling which are in very good agreement withrecent lattice simulations.Comment: 41 pages, LateX; minor corrections; accepted for publication in  Few-Body System

few-body systems

Two-Loop Improved Truncation of the Ghost-Gluon Dyson-Schwinger Equations: Multiplicatively Renormalizable Propagators and Nonperturbative Running Coupling