Merging NLO QCD corrections with parton shower simulations

Currently Monte Carlo event generators are playing a very important role for the simulation of signal and background processes at the Large Hadron Collider at CERN, in Geneva. For some observables it will be very important to consistently improve the accuracy of the predictions to higher orders in the perturbative expansion. In this direction, several approaches are possible. In the present thesis we consider the standard Herwig++ parton shower as well as a new, coherent dipole-type shower along with an automated way of matching. The work is focussed on the positive weight next-to-leading-order matching formalism (POWHEG), which combines parton shower simulation and next-to-leading-order calculation in a consistent way which only produces positive weight events. The POWHEG scheme is used in both approaches of parton showers algorithm and for three different processes. The POWHEG method is applied to Deep Inelastic Scattering (DIS) and the related Higgs boson production via vector-boson fusion process in the Herwig++ Monte Carlo event generator. The simulation contains a full implementation of the truncated shower required to correctly model soft emissions in an angular-ordered parton shower. Furthermore, we present a method, based on the POWHEG matching scheme, to simulate photon production processes at next-to-leading-order. This technique is applied to the simulation of diphoton production in hadron-hadron collisions. The simulation includes both the photon fragmentation contribution and a full implementation of the truncated shower. The new, coherent dipole-type parton shower is tested in its final-initial and initial-final radiation by simulating radiation within the POWHEG formalism for the DIS process. We describe the NLO phase-space that is used for the calculation of the NLO momenta and phase-space integration of differential cross section. The present work is concluded with a comparison of the simulations of the two showers for the DIS process.