Evidence for the triple- gluon vertex from measurements of the QCD colour factors in Z decay into four jets

A sample of 4148 four-jet events observed in the ALEPH-detector at LEP in 1989 and 1990 is used to test the underlying gauge group of strong interactions. A fit to the ratios of the 'colour factors' CF, NC and TF, which determine the differential cross sections, yields NC/CF=2.24±0.32stat±0.24syst and TF/CF=0.58±0.17stat±0.23syst. This is in agreement with the values expected from QCD: NC/CF=2.25 and TF/CF=0.375. The non-zero value of NC/CF constitutes direct evidence for the existence of the triple-gluon coupling and excludes any abelian gauge theory by more than five standard deviations. (orig.)