Determination of the strong coupling from hadronic tau decays using renormalization group summed perturbation theory

We determine the strong coupling constant α_s from the τhadroni width using a renormalization group summed (RGS) expansion of the QCD Adler function. The main theoretical uncertainty in the extraction of α_s is due to the manner in which renormalization group invariance is implemented, and the as yet uncalculated higher order terms in the QCD perturbative series. We show that new expansion exhibits good renormalization group improvement and the behaviour of the series is similar to that of the standard CIPT expansion. The value of the strong coupling in {\bar{\rm MS}} scheme obtained with the RGS expansion is α_s(M_τ^2)= 0.338 \pm 0.010. The convergence properties of the new expansion can be improved by Borel transformation and analytic continuation in the Borel plane. This is discussed elsewhere in these proceedings.

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