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

摘要

Data from ({ m 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 (alpha_s(M_{mathrm Z}) = 0.1170 pm 0.0001 ({ m stat}) pm 0.0013 ({ m 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 (alpha_s(M_{ m Z}) = 0.119 pm 0.006 ({ m stat}) pm 0.026 ({ m sys}))(C_{ m A} = 2.93 pm 0.14 ({ m stat}) pm 0.58 ({ m sys}))(C_{ m F} = 1.35 pm 0.07 ({ m stat}) pm 0.26 ({ m sys})) in good agreement with the expectation from QCD.