String Effect in $e^+ e^- \to 3$ Jets at the Z Peak

The interjet particle densities in 3-jet events are studied and compared to QCD model predictions. By ordering the jets according to their energy (E1 > E2 > E3) the least energetic jet is the gluon jet with a high probability. Then the ratio R of particle production in the angular region between jets 1 and 3 to the one between jets 1 and 2 is considered. Our results show that the concept of a coherent parton shower as implemented in the JETSET and HERWIG models (combined with a phenomenological fragmentation model) successfully describes the data. Models with an incoherent parton shower or independent fragmentation do not reproduce the string effect observed in the data.