Developing the Angantyr model and extending colour reconnection for heavy-ion collisions in Pythia

Sammanfattning: This thesis is composed of four papers, the first two show the attempts made to extend the Pythia event generator to simulate heavy-ion collision events. The other two papers show methods to improve the hadronization mechanism in the Pythia model for a bettter agreement with the experimental data of pp and heavy-ion collisions.Paper I introduces a new model named Angantyr for heavy-ion collision events simulation using the Pythia machinery. We developed a technique for sophisticated stacking of the pp-like sub-collisions, so that the Pythia model can simulate pA and AA type heavy-ion collision events. We show that the Angantyr model can reproduce the general distributions of the produced particles in pp and heavy-ion collision events.Paper II extends the QCD inspired colour reconnection mechanism in Pythia by introducing spatial constrains and extends the hadronization mechanism involving junction systems. The extended QCD colour reconnection model is used to improve the integration of the stacked pp-like sub-collisions as a single heavy-ion event in the Angantyr model by introducing a global colour reconnection. We show that the upgraded model can reporduce the general distribution of the produced particles in pp collision events and qualitatively in heavy-ion collision events. We also show that the model can reproduce the strangeness enhancement like feature in the baryon sector even though the model has no special treatment for the strangeness enhancement.Paper III investigates the baryon angular correlations and the hadronization mechanism using Pythia. It high-lights the need for improvement in the string fragmentation mechanism to reproduce the baryon angular correlations. We propose a hypothesis for the jets sub-structure studies as an outcome of our investigation.Paper IV improves the formation of junction like setups in the QCD inspired colour reconnection and the fragmentation of the junction configurations in the string fragmentation. We show that the changes made in this paper improves the charm baryons production rate in pp and heavy-ion collision events.