Application of “optimised” perturbation theory to determination of αs(MZ2) from hadronic event shape observables in e+e− annihilation

We have applied so-called “optimised” perturbation theory to resolve the renormalisation-scale (μ) ambiguity of exact O(αs2) QCD calculations of event shape observables in e+e− → hadrons. We fitted the optimised predictions for 15 observables to hadronic Z0 decay data from the SLD experiment to determine αs(MZ2). Comparing with results using the physical scale μ = Mz we found no reduction in the scatter among αs(MZ2) values from the 15 observables, implying that the O(αs2) predictions with optimised scales are numerically no closer to the exact all-orders results than those with the physical scale.