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. (C) 1997 Published by Elsevier Science B.V.