Multiparton Interaction Models and Parton Shower Studies

University dissertation from Department of Astronomy and Theoretical Physics, Lund University

Abstract: This thesis involves phenomenological models for describing high energy particle collisions. One class of these models, multiparton interactions (MPI), deals with the fact that when colliding two hadrons, it is possible for more than one pair of their constituents to interact. The other deals with parton showers, which are used to fill enhanced regions of phase space and give fully exclusive final states. These two components, when part of an event generation framework, help in the simulation of complete high energy collision events. Paper I presents a study of an extension to the MPI model, enhanced screening, where the amount of colour screening in an incoming hadron is increased in those events with a large amount of activity. Paper II presents another extension to the MPI model, rescattering. Commonly, in MPI models, interactions are between pairs of partons which originate from the incoming hadrons. With rescattering, an MPI can instead involve partons which come from previous interactions or shower branchings. Paper III studies the first emission of the parton shower. First, the interface to a next-to-leading generator, POWHEG-hvq, is examined. Second, a dampening of the first shower emission, based on general matrix element arguments is studied. Paper IV again deals with parton showers. Minor changes to the framework are outlined, before a kinematic comparison of the shower is made against 2 to 3 QCD real emission matrix elements. Finally, tunes of the generator are made to both Tevatron and LHC data. Paper V examines an extension to the impact parameter formalism of the MPI model, where the size of an incoming proton is varied depending on the x value of a parton being taken from it.

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