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 C A C F , the ratio of the coupling strength of the triple-gluon vertex to that of gluon bremsstrahlung, and N C N A , the ratio of the number of quark colours to the number of gluons, giving C A C F = 2.51 ± 0.28 and N C N A = 0.38 ± 0.10 in agreement with the values expected from QCD: C A C F = 9 4 = 2.25 and N C N A = 3 8 = 0.375 .