Measurement of the strong coupling constant α s from global event-shape variables of hadronic Z decays

Abstract

An analysis of global event-shape variables has been carried out for the reaction e +e −→Z 0→hadrons to measure the strong coupling constant α s. This study is based on 52 720 hadronic events obtained in 1989/90 with the ALEPH detector at the LEP collider at energies near the peak of the Z-resonance. In order to determine α s, second order QCD predictions modified by effects of perturbative higher orders and hadronization were fitted to the experimental distributions of event-shape variables. From a detailed analysis of the theoretical uncertainties we find that this approach is best justified for the differential two-jet rate, from which we obtain α s( M Z 2) = 0.121 ± 0.002(stat.)±0.003(sys.)±0.007(theor.) using a renormalization scale ω = 1 2 M Z . The dependence of α s( M Z 2) on ω is parameterized. For scales m b<ω< M Z the result varies by −0.012 +0.007.