Determination of ${\alpha }_s(M_{\tau }^2)$ from Improved Fixed Order Perturbation Theory

Abstract

We revisit the extraction of ${\alpha }_s(M_{\tau }^2)$ from the QCDperturbative corrections to the hadronic $\tau$ branching ratio, using an improved fixed-order perturbation theory based on the explicit summation of all renormalization-group accessible logarithms, proposed some time ago in the literature. In this approach, the powers of the coupling in the expansion of the QCD Adler function are multiplied by a set of functions $D_n$, which depend themselves on the coupling and can be written in a closed form by iteratively solving a sequence of differential equations. We find that the new expansion has an improved behavior in the complex energy plane compared to that of the standard fixed-order perturbation theory (FOPT), and is similar but not identical to the contour-improved perturbation theory (CIPT). With five terms in the perturbative expansion we obtain in the $\overline{\mathrm{MS}}$ scheme ${\alpha }_s(M_{\tau }^2)=0.338\pm 0.010$, using as input a precise value for the perturbative contribution to the hadronic width of the $\tau$ lepton reported recently in the literature.

Comments: 12 pages, 6 figures, uses revtex; replaced with version corresponding to published version in Physical Review