Coherent Soft Particle Production in Z Decays into Three Jets

Low-energy particle production perpendicular to the event plane in three-jet events produced in Z decays in e+e- annihilation is measured and compared to that perpendicular to the event axis in two-jet events. The topology dependence of the hadron production ratio is found to agree with a leading-order QCD prediction. This agreement and especially the need for the presence of a destructive interference term gives evidence for the coherent nature of gluon radiation. Hadron production in three-jet events is found to be directly proportional to a single topological scale function of the inter-jet angles. The slope of the dependence of the multiplicity with respect to the topological scale was measured to be: 2.211 +/- 0.014 (stat.) +/- 0.053 (syst.) in good agreement with the expectation given by the colour-factor ratio C_A / C_F = 9/4. This result strongly supports the assumption of local parton-hadron duality, LPHD, at low hadron momentum.