Scale Setting Using the Extended Renormalization Group and the Principle of Maximum Conformality: the QCD Coupling Constant at Four Loops

TL;DR

The paper tackles the renormalization-scale and -scheme ambiguity in perturbative QCD by developing extended renormalization group equations that introduce a universal coupling to capture scale- and scheme-dependence. It then establishes a scheme-independent, order-by-order PMC/BLM scale-setting framework up to NNLO, enabling the absorption of nonconformal βi terms into the running coupling and yielding scheme-independent predictions. A four-loop application to R_{e^+e^-}(Q) demonstrates improved perturbative convergence and provides explicit extractions of asymptotic scales Λ and α_s(M_Z), illustrating the practical impact of the method. Overall, the work offers a robust, scheme-independent approach to scale setting and commensurate scale relations that can enhance precision in QCD phenomenology and collider analyses.

Abstract

A key problem in making precise perturbative QCD predictions is to set the proper renormalization scale of the running coupling. The extended renormalization group equations, which express the invariance of physical observables under both the renormalization scale- and scheme-parameter transformations, provide a convenient way for estimating the scale- and scheme-dependence of the physical process. In this paper, we present a solution for the scale-equation of the extended renormalization group equations at the four-loop level. Using the principle of maximum conformality (PMC) / Brodsky-Lepage-Mackenzie (BLM) scale-setting method, all non-conformal $\{β_i\}$ terms in the perturbative expansion series can be summed into the running coupling, and the resulting scale-fixed predictions are independent of the renormalization scheme. Different schemes lead to different effective PMC/BLM scales, but the final results are scheme independent. Conversely, from the requirement of scheme independence, one not only can obtain scheme-independent commensurate scale relations among different observables, but also determine the scale displacements among the PMC/BLM scales which are derived under different schemes. In principle, the PMC/BLM scales can be fixed order-by-order, and as a useful reference, we present a systematic and scheme-independent procedure for setting PMC/BLM scales up to NNLO. An explicit application for determining the scale setting of $R_{e^{+}e^-}(Q)$ up to four loops is presented. By using the world average $α^{\bar{MS}}_s(M_Z) =0.1184 \pm 0.0007$, we obtain the asymptotic scale for the 't Hooft associated with the $\bar{MS}$ scheme, $Λ^{'tH}_{\bar{MS}}= 245^{+9}_{-10}$ MeV, and the asymptotic scale for the conventional $\bar{MS}$ scheme, $Λ_{\bar{MS}}= 213^{+19}_{-8}$ MeV.