Diffraction in high-energy collisions

Abstract: This thesis concerns itself with low-momentum-transfer processes in high-energy particle collisions. These processes cannot be described in the framework of the fundamental theory of quantum chromodynamics (QCD), but rely on a pre-QCD approach, the Regge theory. Several models containing various degrees of complexity have been proposed within this framework, some of which have been developed and studied in this thesis. All models have been implemented in a numerical simulation (a Monte Carlo event generator), which allows forprecise predictions of several types of particle collisions at various energies.Paper I: A model for hard diffractive events in pp collisions is proposed. It is based on evaluating a probability for diffraction, but employs an additional dynamical choice of survival of the proposed diffractive events. This dynamical description is the first of its kind and offers an explanation of earlier discrepancies between data and theory.Paper II: Existing models are reviewed and new ones developed for all components of the pp cross section; the total, elastic and diffractive cross sections. The models are implemented in the event generator Pythia 8, thus offering improved predictions for these collisions at LHC energies.Paper III: In this work the model for hard diffraction is extended to photoproduction. The dynamical survival effect is by construction only present in a subset of these collisions and thus puts forward an explanation of the discrepancies between data and theory of hard diffractive dijet events in the photoproduction regime.Paper IV: Here a new model for intial state evolution is implemented and studied in Pythia 8. This model opens up for asymmetrical matter distributions within the colliding particles, and consequences because of these asymmetries are studied within ep, pp, pA and AA collisions. Predictions for the color fluctuations in the crosssections in eA collisions are presented as a first step towards a complete description of electron-ion collisions.