Measurement of the strong coupling constant αs 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-→Z0→hadrons to measure the strong coupling constant αs. 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 αs, 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 αs(MZ 2) = 0.121 ± 0.002(stat.)±0.003(sys.)±0.00..