Determination of the QCD Coupling from ALEPH $τ$ Decay Data

We present a comprehensive study of the determination of the strong coupling from $τ$ decay, using the most recent release of the experimental ALEPH data. We critically review all theoretical strategies used in previous works and put forward various novel approaches which allow to study complementary aspects of the problem. We investigate the advantages and disadvantages of the different methods, trying to uncover their potential hidden weaknesses and test the stability of the obtained results under slight variations of the assumed inputs. We perform several determinations, using different methodologies, and find a very consistent set of results. All determinations are in excellent agreement, and allow us to extract a very reliable value for $α_s(m_τ^2)$. The main uncertainty originates in the pure perturbative error from unknown higher orders. Taking into account the systematic differences between the results obtained with the CIPT and FOPT prescriptions, we find $α_{s}^{(n_f=3)}(m_τ^2) = 0.328 \pm 0.013$ which implies $α_{s}^{(n_f=5)}(M_Z^{2}) = 0.1197\pm 0.0015$.

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