$α_s$ from the updated ALEPH data for hadronic $τ$ decays

Abstract

We extract the strong coupling $α_s(m_τ^2)$ from the recently updated ALEPH non-strange spectral functions obtained from hadronic $τ$ decays. We apply a self-consistent analysis method, first tested in the analysis of OPAL data, to extract $α_s(m_τ^2)$ and non-perturbative contributions. The analysis yields $α_s^{\rm FO}(m_τ^2)=0.296\pm0.010 $, using Fixed Order Perturbation Theory (FOPT), and $α^{\rm CI}_s(m_τ^2)= 0.310\pm0.014$, using Contour Improved Perturbation Theory (CIPT). The weighted average of these results with those previously obtained from OPAL data give $α_s^{\rm FO}(m_τ^2)=0.303\pm 0.009$ and $α_s^{\rm CI}(m_τ^2)=0.319\pm 0.012$, which gives, after evolution to the $Z$ boson mass scale, $α^{\rm FO}_s(m_Z^2)=0.1165\pm0.0012 $ and $α_s^{\rm CI}(m_Z^2)=0.1185\pm0.0015 $, respectively. We observe that non-perturbative effects limit the accuracy with which $α_s$ can be extracted from $τ$ decay data.

5 pages, Talk given at the 18th International Conference in Quantum Chromodynamics (QCD 15, 30th anniversary), 29 june - 3 july 2015, Montpellier, France