Renormalization-group improvement of effective actions beyond summation of leading logarithms

Invariance of the effective action under changes of the renormalization scale mu leads to relations between those (presumably calculated) terms independent of mu at a given order of perturbation theory and those higher-order terms dependent on logarithms of mu. This relationship leads to differential equations for a sequence of functions, the solutions of which give closed form expressions for the sum of all leading logs, next to leading logs, and subsequent subleading logarithmic contributions to the effective action. The renormalization group is thus shown to provide information about a model beyond the scale dependence of the model's couplings and masses. This procedure is illustrated using the phi(6)(3) model and Yang-Mills theory. In the latter instance, it is also shown by using a modified summation procedure that the mu dependence of the effective action resides solely in a multiplicative factor of g(2)(mu) (the running coupling). This approach is also shown to lead to a novel expansion for the running coupling in terms of the one-loop coupling that does not require an order-by-order redefinition of the scale factor Lambda(QCD). Finally, logarithmic contributions of the instanton size to the effective action of an SU(2) gauge theory are summed, allowing a determination of the asymptotic dependence on the instanton size rho as chi goes to infinity to all orders in the SU(2) coupling constant. (C) 2003 Elsevier Science B.V. All rights reserved.