Test of color reconnection models in Z $\to$ 3 jets

Hadronic Z decays into three jets are used to test QCD models of color reconnection (CR). The sensitive quantity is the rate of gluon jets with a gap in rapidity and zero jet charge. Gluon jets are identified by either energy-ordering or anti-b tagging. The two string-based CR models, one in JETSET(+GAL), the other in ARIADNE, give too high rates and are thus disfavoured by the data. On the other hand, there generators without CR give too low rates. The data can be described with a small value in the range 0.01-0.02 for the R0 parameter of the GAL model. No decision is possible on the cluster-based HERWIG CR model.