On the Behavior of the Effective QCD Coupling alpha_tau(s) at Low Scales
  The hadronic decays of the tau lepton can be used to determine the effective charge alpha_tau(m^2_tau') for a hypothetical tau-lepton with mass in the range 0 < m_tau' < m_tau. This definition provides a fundamental definition of the QCD coupling at low mass scales. We study the behavior of alpha_tau at low mass scales directly from first principles and without any renormalization-scheme dependence by looking at the experimental data from the OPAL Collaboration. The results are consistent with the freezing of the physical coupling at mass scales s = m^2_tau' of order 1 GeV^2 with a magnitude alpha_tau ~ 0.9 +/- 0.1. 
Stanley J. Brodsky (1), Sven Menke (1 and 2), Carlos Merino (1 and 3),   Johan Rathsman (4) ((1) SLAC, Stanford University, ...