Improved alpha_s from Tau Decays

TL;DR

This paper updates the measurement of the strong coupling constant alpha_s at the tau mass scale using spectral functions, highlighting the advantages of contour-improved perturbation theory over fixed-order methods and achieving the most precise alpha_s(M_Z) value from tau data.

Contribution

It provides an improved extraction of alpha_s(m_tau) from tau decays, demonstrating the effectiveness of contour-improved perturbation theory and estimating systematic effects.

Findings

Contour-improved perturbation theory shows better convergence than fixed-order.

The measured alpha_s(m_tau) is 0.344 with combined errors.

The evolved alpha_s(M_Z) is the most precise from tau data, consistent with Z decay results.

Abstract

We present an update of the measurement of alpha_s(m_tau) from ALEPH tau hadronic spectral functions. We report a study of the perturbative prediction(s) showing that the fixed-order perturbation theory manifests convergence problems not presented in the contour-improved calculation. Potential systematic effects from quark-hadron duality violations are estimated to be within the quoted systematic errors. The fit result is alpha_s(m_tau) = 0.344 +- 0.005 +- 0.007, where the first error is experimental and the second theoretical. After evolution, the alpha_s(M_Z) determined from tau data is the most precise one to date, in agreement with the corresponding NNNLO value derived from Z decays.