Measurements of the strong coupling constant and the QCD colour factors using four-jet observables from hadronic Z decays

Abstract

Data from e+e− annihilation into hadrons, taken with the ALEPH detector at the Z resonance, are analyzed. The four-jet rate is studied as a function of the resolution parameter and compared to next-to-leading order calculations combined with resummation of large logarithms. Angular correlations in four-jet events are measured and compared to next-to-leading order QCD predictions. With these observables two different measurements are performed. In a first analysis the strong coupling constant is measured from the four-jet rate yielding αs(MZ)=0.1170±0.0001(stat)±0.0013(sys) . In a second measurement the strong coupling constant and the QCD colour factors are determined simultaneously from a fit to the four-jet rate and the four-jet angular correlations, giving αs(MZ)=0.119±0.006(stat)±0.026(sys) CA=2.93±0.14(stat)±0.58(sys) CF=1.35±0.07(stat)±0.26(sys) in good agreement with the expectation from QCD.