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

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-toleading 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) . The ALEPH Collaboration: Measurements of the strong coupling constant and the QCD colour factors 3 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. The ALEPH Collaboration: Measurements of the strong coupling constant and the QCD colour factors 3 1 Also at CERN, 1211 Geneva 23, Switzerland 2 Now at Université de Lausanne, 1015 Lausanne, Switzerland 3 Also at Dipartimento di Fisica di Catania and INFN Sezione di Catania, 95129 Catania, Italy 4 Now at CERN, 1211 Geneva 23, Switzerland 5 Also Istituto di Cosmo-Geofisica del C.N. R. , Torino, Italy 6 Now at Institut für Experimentelle Kernphysik, Universität Karlsruhe, 76128 Karlsruhe, Germany 7 Supported by CICYT, Spain 8 Supported by the National Science Foundation of China 9 Supported by the Danish Natural Science Research Council 10 Supported by the UK Particle Physics and Astronomy Research Council 11 Supported by the US Department of Energy, grant DEFG0295-ER40896 12 Now at Departement de Physique Corpusculaire, Université de Genève, 1211 Genève 4, Switzerland 13 Supported by the US Department of Energy, grant DEFG03-92ER40689 14 Supported by the Leverhulme Trust 15 Permanent address: Universitat de Barcelona, 08208 Barcelona, Spain 16 Supported by the Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie, Germany 17 Supported by the Direction des Sciences de la Matière, C.E. A 18 Supported by the Austrian Ministry for Science and Transport 19 Now at SAP AG, 69185 Walldorf, Germany 20 Also at Department of Physics, Tsinghua University, Beijing, The People’s Republic of China 21 Now at Département de Physique, Faculté des Sciences de Tunis, 1060 Le Belvédère, Tunisia 22 Now at Groupe d’Astroparticules de Montpellier, Université de Montpellier II, 34095, Montpellier, France 23 Also at Dipartimento di Fisica e Tecnologie Relative, Università di Palermo, Palermo, Italy 24 Now at McKinsey and Compagny, Avenue Louis Casal 18, 1203 Geneva, Switzerland 25 Now at SLAC, Stanford, CA 94309, USA 26 Now at INFN Sezione di Roma II, Dipartimento di Fisica, Universitá di Roma Tor Vergata, 00133 Roma, Italy 27 Now at LBNL, Berkeley, CA 94720, USA 28 Now at Skyguide, Swissair Navigation Services, Geneva, Switzerland 29 Now at Honeywell, Phoenix AZ, USA 30 Also at Groupe d’Astroparticules de Montpellier, Université de Montpellier II, 34095, Montpellier, France 31 Now at Fermilab, PO Box 500, MS 352, Batavia, IL 60510, USA 32 Deceased