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

TL;DR

This paper introduces an improved fixed-order perturbation theory for extracting alpha_s from tau decay data, which sums renormalization-group logarithms for better convergence and compares favorably with existing methods.

Contribution

The authors develop a novel expansion method for the Adler function that improves convergence and behavior in the complex plane, offering a refined approach to determine alpha_s from tau decays.

Findings

Improved behavior of the new expansion in the complex energy plane.

Alpha_s(M_tau^2) determined as 0.338 ± 0.010 using the new method.

Comparable but distinct results from standard FOPT and CIPT approaches.

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 ${\bar{\rm MS}}$ 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.