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Consistent Dark Matter models for the Large Hadron Collider

Student thesis: Doctoral ThesisDoctor of Philosophy

The unexplained presence of a large component of invisible matter in our universe is a compelling mystery. This Dark Matter (DM) is needed to solve a number of puzzles on astrophysical and cosmological scales, and should be something exotic, something beyond the matter that we currently know about. A compelling answer to the DM puzzle is the idea that DM is a Weakly Interacting Massive Particle (WIMP), which is where the DM is an exotic particle that obtains its currently observed abundance by falling out of thermal equilibrium with the primordial plasma in the early universe. If the strength of the interaction is around the electroweak scale, then just the right abundance of DM is obtained. This gives a good motivation to search for GeV-TeV scale exotic particles that might make up all or part of the DM. 
The Large Hadron Collider (LHC) is a machine purpose built to find new particles in this energy range. Therefore, it is inevitable to want to search for DM at the LHC, and a large number of competing theories are on the market which allow us to do so. In this thesis we will be studying consistent models for DM at the LHC. As we will explain, starting from model-independent Effective Field Theories (EFTs) of DM interactions, a framework of simplified models has been built up. We will explore the constraints LHC data impose on these simplified models, propose more consistent theories, examine the parameter space of these UV-completions, and propose a new LHC search based on an extended version of a simplified model. 
The layout of this thesis is therefore as follows. In the following sections we will summarise the various evidence for DM, before describing competing candidates, including DM EFTs and simplified models. We will then detail the calculation of thermal freeze-out which enumerates how much of the DM budget a particular candidate can account for. Then, we describe searches for DM and exotic particles at the LHC, before including a brief mention of direct and indirect detection experiments. 
The subsequent chapters are based on the Refs. [1–4] of which I am an author. In Chapter 2 we will look at a combination of dijet resonance searches to place bounds on spin-one exotic particles, before applying these constraints to a simplified model at the same time as including information coming from the relic density of DM. In Chapter 3 we continue looking at the spin-one simplified models, but from a theoretical perspective. We propose extensions of these models that are free of gauge anomalies, and investigate the conventional constraints that apply to these models in Chapter 4. In contrast, in Chapter 5 we look at a different simplified model, namely one with a pseudoscalar mediator between the SM and the DM. Here we take an already proposed UV completion, which enables the theory to be fully gauge-invariant, and use this theory to propose a new search channel that can be used by the LHC collaborations to discover this kind of portal to DM. Finally, in Chapter 6 we offer some conclusions and comments on possible future directions.
Original languageEnglish
Awarding Institution
Award date1 May 2019


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