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

摘要

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 α_s(M_z) = 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 α_s(M_Z) = 0.119 ± 0.006(stat) ± 0.026(sys) C_A = 2.93 ± 0.14(stat) ± 0.58(sys) C_F = 1.35 ± 0.07(stat) ± 0.26(sys) in good agreement with the expectation from QCD.