Parton Cascades and Hadronisation in High Energy Processes

Abstract: The Lund string fagmentation model was reformulated as a process along the boundary curve of the string called the directrix, in order to treat multigluon strings according to the Lund model area law. It was found that the hadrons in the final state could be collected into several groups, in such a way that the energy-momenta of the hadrons in a group lie in a plane in the Minkowski space within small transverse fluctuations. The origin of these structures, named Coherence Chains, was traced to properties of gluon radiation in coherent parton cascades. The distribution of the size of gluon-gluon dipoles was found to be stable against changes in the global event properties as well as changes in the resolution scale, when the size was defined relative to the resolution scale. These properties were explained as consequences of a balance between increased probability for gluon emission because of the running coupling constant in QCD, which results in a decrease of the size of individual dipoles, and a compensating increase of the size measure, owing to the smaller resolution scale. These properties of the colour dipoles translated into stability properties of certain more abstract entities which are relevant to the new approach to string fragmentation. These were named Generalised Dipoles and they were identified as the chief contributing factors for similar properties of the coherence chains. The proporties of the coherence chains were used to extend an interference based model for Bose-Einstein correlations into a model applicable to multigluon strings. The extended model was used to investigate the two particle correlation function for identical pions in multijet events. The hadronisation model was further developed to include effects of limitted available phase space for hadron formation in systems with small invariant mass.