The Development of Computational Tools for Theoretical Predictions in Particle Physics at the Large Hadron Collider

Abstract

The Large Hadron Collider (LHC) experiments are an excellent tool for the improvement of our knowledge of the Standard Model and the examination of BeyondStandard Model theories. Nonetheless, to maximise the learning-potential of the LHC,clear and precise theoretical predictions are needed, for both the Standard Model andits 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 natureof many parameter spaces of Beyond Standard Model theories, computational programsto perform theoretical calculations are increasingly required.The work presented in this thesis fits this role, it is focused on two computationalprograms developed with the aim of producing such theoretical predictions for LHCphenomenology in two key areas. These are the precision Standard Model predictionsof transverse momentum spectra for a wide class of processes at the LHC, and BeyondStandard Model predictions for the decay widths of as-yet undiscovered particles in thecontext 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 thetwo main projects undertaken. Chapters 2, 3 and 4 describe the development of theSoftSusy decay calculator program to determine the partial widths and branching ratios of supersymmetric and Higgs particles in the Minimal Supersymmetric StandardModel and the Next-to-Minimal Supersymmetric Standard Model. The theoretical andphenomenological background, methodology, assumptions, and the vast array of decaymodes 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 5begins the second part of the thesis, providing theoretical background for Chapters 6and 7, which discuss the newly-developed reSolve program, designed to undertake thetheoretically-demanding calculations associated with transverse momentum resummation for a wide range of LHC processes. Details of the methods, assumptions, validationand results for channels so far included are all provided, these show excellent agreementwith previous theoretical results and experimental data. Both projects are then summarised in Chapter 8. Further information is provided in the appendices; Appendix Apresents explicitly all formulae incorporated into the SoftSusy decay calculator pro-gram; whilst Appendix B provides further details on the theoretical underpinning of thetransverse momentum resummation calculations performed by the reSolve program.