Observation of planar three-jet events in e +e - annihilation and evidence for gluon bremsstrahlung

Topological distributions of charged and neutral hadrons from the reaction e⁺e^- → multihadrons are studied at √ s of about 30 GeV. An excess of planar events is observed at a rate which cannot be explained by statistical fluctuations in the standard two-jet process. The planar events, mostly consisting of a slim jet on one side and a broader jet on the other, are shown actually to possess three-jet structure by demonstrating that the broader jet itself consists of two collinear jets in its own rest system. Detailed agreement between data and predictions is obtained if the process e ⁺e ^-→q q¯g is taken into account. This strongly suggests gluon bremsstrahlung as the origin of the planar three-jet events. By comparison of the data with the qq¯g-model we obtain a value for the strong coupling constant of α_S( q² = 0.17 ± 0.04.