The Large Hadron Collider (LHC) experiments are an excellent tool for the improvement of our knowledge of the Standard Model and the examination of Beyond Standard Model theories. Nonetheless, to maximise the learning-potential of the LHC, clear and precise theoretical predictions are needed, for both the Standard Model and its extensions, to allow critical comparison of these models with data. In particular, given the complexity of the collision environment at the LHC, and the expansive nature of many parameter spaces of Beyond Standard Model theories, computational programs to perform theoretical calculations are increasingly required. The work presented in this thesis fits this role, it is focused on two computational programs developed with the aim of producing such theoretical predictions for LHC phenomenology in two key areas. These are the precision Standard Model predictions of transverse momentum spectra for a wide class of processes at the LHC, and Beyond Standard Model predictions for the decay widths of as-yet undiscovered particles in the context of supersymmetry. Chapter 1 presents a brief chronology and review of the Standard Model. Fol- lowing this, the work reported in this thesis is split into two parts, focused on the two main projects undertaken. Chapters 2, 3 and 4 describe the development of the SoftSusy decay calculator program to determine the partial widths and branching ratios of supersymmetric and Higgs particles in the Minimal Supersymmetric Standard Model and the Next-to-Minimal Supersymmetric Standard Model. The theoretical and phenomenological background, methodology, assumptions, and the vast array of decay modes calculated by the program are described. This is followed by details of the extensive validation of the decay calculator program and a selection of results. Chapter 5 begins the second part of the thesis, providing theoretical background for Chapters 6 and 7, which discuss the newly-developed reSolve program, designed to undertake the theoretically-demanding calculations associated with transverse momentum resummation for a wide range of LHC processes. Details of the methods, assumptions, validation and results for channels so far included are all provided, these show excellent agreement with previous theoretical results and experimental data. Both projects are then summarised in Chapter 8. Further information is provided in the appendices; Appendix A presents explicitly all formulae incorporated into the SoftSusy decay calculator pro- gram; whilst Appendix B provides further details on the theoretical underpinning of the transverse momentum resummation calculations performed by the reSolve program.
