Rapidity-Alignment and p_T Compensation of Particle Pairs in Hadronic Z0   Decays
  Observation is made of rapidity-alignment of K+K- and ppbar pairs which results from their asymmetric orientation in rapidity, with respect to the direction from primary quark to antiquark. The K+K- and ppbar data are consistent with predictions from the fragmentation string model. However, the ppbar data strongly disagree with the conventional implementation of the cluster model. The non-perturbative process of `gluon splitting to diquarks' has to be incorporated into the cluster model for it to agree with the data. Local conservation of p_T between particles nearby in rapidity (i.e., p_T compensation) is analysed with respect to the thrust direction for pi+pi-, K+K-, and ppbar pairs. In this case, the string model provides fair agreement with the data. The cluster model is incompatible with the data for all three particle pairs. The model with its central premiss of isotropically-decaying clusters predicts a p_T correlation not seen in the data. 
The DELPHI Collaboration, J. Abdallah, et al