Improved Strong Force Coupling Constant Measurements

The four-loop determination of the strong coupling from fully inclusive observables is reviewed. Special attention is given to the low-energy measurement extracted from the hadronic τ decay width. A recent exhaustive analysis of the ALEPH data, exploring several complementary methodologies with very different sensitivities to inverse power corrections and duality violations, confirms the strong suppression of non-perturbative contributions to Rτ. It gives the value αs(m^2τ)=0.328±0.013, which implies αs(M^2Z)=0.1197±0.0015. The excellent agreement with the direct measurement at the Z peak, αs(M^2Z)=0.1196±0.0030, provides a beautiful test of asymptotic freedom. Together with the most recent lattice average from FLAG and the NNLO determinations from e+e−, PDFs and collider data quoted by the PDG, these two inclusive determinations imply a world average value αs(M^2Z)=0.1180±0.0010.

Antionia Rich, "Precision physics with QCD" (Pre-Print December 15, 2016).

The current global average value for the strong force coupling constant before this new report was 0.1182±0.0012. So, the mean value has dropped by 0.0002 and the precision has increased by 16%.