Improving Parton Showers with Fixed Order Matrix Elements

Sammanfattning: This thesis considers numerical simulations of high energy particle collisions, and improvements in the description of multiple collimated jets of hadrons in collision events in particular. In the event simulation software called event generators, the structure of multi-jet events is determined through an interplay of the input calculation, parton showers and phenomenological hadronisation models. Prescriptions to imbue existing event generator simulations with more complex input calculations are presented, and
public implementations of the proposed methods provided.

Paper I describes the implementation of an earlier algorithm to include regularised tree-level multi-jet configurations into event generators. The implementation is applied to collisions at LEP, Tevatron and LHC.

Paper II proposes a method to amend cross section changes in traditional multi-jet merging schemes. The implementation of the new scheme is tested for W-boson + jets and QCD Dijet production at the LHC.

Paper III presents extensions of the algorithms of Papers I and II to allow the inclusion of virtual orrections to the scattering processes. The resulting NLO merging methods are applied to W-boson + jets and Higgs-boson + jets at the LHC.