Multiparticle Production in Strong Interactions

University dissertation from Department of Theoretical Physics, Lund University

Abstract: An alternative formulation of the Lund fragmentation model for string states without gluons is presented. It is shown that the fragmentation process can be factorised into a product of transition operators with well-defined eigenfunctions and corresponding eigenvalues. The physical properties of these functions are studied and possible applications are suggested. A fragmentation model for multigluon strings is defined and investigated in detail. The model fulfils the Lund area law on an event to event basis and it is formulated in a way which makes it suitable for a Monte Carlo implementation. A Monte Carlo fragmentation program, called ALFS, based upon this exact implementation of the Lund area law is analysed and compared with the PYTHIA event generator. We find that inclusive distributions are the same but that there are differences on an event to event basis. We demonstrate that the hadrons produced in this model can be grouped into (almost) planar regions in momentum space called Coherence Chains. A qualitative study of the properties of the Coherence Chains is performed using the above mentioned ALFS Monte Carlo fragmentation program. We find that the distribution in the number of hadrons in a Coherence Chain has stability properties which allows us to base a model for the Bose-Einstein effect upon the concept of Coherence Chains. The two particle correlation function for 2, 3 and 4 jet events is studied. The thesis also contains a phenomenological investigation of a set of distributions obtained from the perturbative QCD parton cascades as they are implemented in the Monte Carlo event generators ARIADNE and PYTHIA. In an analysis built upon a generalised rapidity range, we observe a set of independent entities, that we call Generalised Dipoles. In the Lund model, the Generalised Dipoles correspond to connected plaquettes along a surface characteristic to the string state. We demonstrate that there is a close relationship between the Generalised Dipoles and the above mentioned Coherence Chains.

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