αs and the τ hadronic width

Different choices exist for the renormalisation group resummation in the determination of αs from hadronic τ decays: namely fixed-order (FOPT) and contour-improved perturbation theory (CIPT). The two approaches lead to systematic differences in the resulting αs. On the basis of a model for higher-order terms in the perturbative series, which incorporates well-known structure from renormalons, it is found that while CIPT is unable to account for the fully resummed series, FOPT smoothly approaches the Borel sum. Employing the model to determine αs yields αs(Mτ)=0.316±0.006, which after evolution leads to αs(MZ)=0.1180±0.0008.