Merging Parton Showers and Matrix Elements

Abstract: This thesis considers models for describing high energy particle collisions. Existing phenomonological models are modified and new implementations made in order to achive a better description of states with several hard jets. To predict final state hadrons, parton showers are used together with pheonomenological hadronization models. These models describe a wide range of observables, but have the shortcomming that they do not describe states with several jets well. Jets are better described using matrix elements, which then needs to be merged with the parton shower description. Paper I extends one of the earlier merging algorithms to hadron collisions. The resulting algorithm is applied to W plus jets production at the Tevatron. Paper II is a comparison of several different algorithms from merging parton shower and matrix elements. The algorithms are compared for a range of observables for W plus jets production at the Tevatron and the LHC. The systematics of each algorithms is also studied. Paper III deals with the more simple case of e+e- to jets, where the theory is better understood. Here the most simple matrix element is used for the merging and results are studied for four different algorithms. Three of the algorithms are shown to have problems not previously discussed in the literature. Paper IV presents an extension to the merging algorithms, that allows for including matrix elements with loops. The algorithm is applied to e+e- with matrix element contributions of up to order alphas^2.