Measurement of the triple gluon vertex from double quark tagged 4-jet events

The 4-jet events collected by the DELPHI experiment at LEP1 in 1992, 1993 and 1994 are analysed to determine the contribution of the triple-gluon vertex. Two of the four jets are tagged as jets from b- or c-quarks using lifetime and lepton transverse momentum information. The 4-jet contributions from double-gluon bremsstrahlung, the triple-gluon vertex, and secondary quark-antiquark pair production then yield significantly different two-dimensional distributions in the generalized Nachtmann Reiter angle versus the opening angle of the two secondary jets. These distributions are used to fit CACF, the ratio of the coupling strength of the triple-gluon vertex to that of gluon bremsstrahlung, and NCNA, the ratio of the number of quark colours to the number of gluons, giving CACF = 2.51 ± 0.28 and NCNA = 0.38 ± 0.10 in agreement with the values expected from QCD: CACF = 94 = 2.25 and NCNA = 38 = 0.375.