School of Physics - Theses
Permanent URI for this collection
Search Results
1 - 2 of 2
-
ItemPhenomenology of particle dark matter( 2017)The fundamental nature of dark matter (DM) remains unknown. In this thesis, we explore new ways to probe properties of particle DM across different phenomenological settings. In the first part of this thesis, we overview evidence, candidates and searches for DM. In the second part of this thesis, we focus on model building and signals for DM searches at the Large Hadron Collider (LHC). Specifically, in Chapter 2, the use of effective field theories (EFTs) for DM at the LHC is explored. We show that many widely used EFTs are not gauge invariant, and how, in the context of the mono-W signal, their use can lead to unphysical signals at the LHC. To avoid such issues, the next iteration of a minimal DM framework, called simplified models, are considered. We discuss use of such models at the LHC in Chapter 3, and show that in the context of a renormalizable gauge-invariant theory, any isospin violating effects in mono-W signals cannot be large. In Chapter 4, we discuss an alternative search strategy to mono-X searches at the LHC — in the case that DM does not couple directly to hadrons, the mono-X signature does not exist, and instead a leptophilic DM signature can be probed. We focus on the prospects for leptophilic DM with a spin-1 mediator at the LHC, and discuss constraints from other experiments. In the third part of this thesis, we turn to astrophysical signals of DM. In Chapter 5, we show that a consequence of enforcing gauge invariance in simplified DM models provides a new dominant s-wave DM annihilation process for indirect detection searches, and set limits on the annihilation cross section from Pass 8 observations of the Fermi Gamma-ray Space Telescope. In Chapter 6, we demonstrate the impact of mass generation for simplified models, finding that the relic density and indirect detection constraints, along with the DM interaction types, are strongly dictated by the mass generation mechanism chosen. In Chapter 7, we show that the multi-mediator approach advocated in the previous two chapters can also lead to a new dominant signal, in the form of dark initial state radiation. Finally in Chapter 8, we look to the Sun to find that if DM annihilates to long-lived mediators, the gamma rays and neutrinos produced can be strongly probed by gamma-ray telescopes and observatories Fermi-LAT, HAWC, and LHAASO, as well as neutrino telescopes IceCube and KM3Net. Interestingly, these telescopes can provide the strongest probe of the DM spin dependent scattering cross section, outperforming standard high-energy solar neutrino searches and direct detection experiments by several orders of magnitude.
-
ItemInteracting dark matter: decay and bremsstrahlung processes( 2013)Though there is substantial indirect astrophysical evidence for the existence of dark matter (DM), it has yet to be directly detected. Consequently, little is known about its internal structure. It is possible that there is a small but finite non-gravitational interaction between dark matter and the Standard Model (SM) which may have observable consequences. The purpose of this thesis is the exploration of some of these interactions and consequences. In particular we consider the possibility that dark matter is unstable on long timescales, as motivated by discrepancies between simulation and observation of structure on sub-galactic scales. We also consider the consequences of electroweak radiative corrections to annihilation processes involving dark matter, as such corrections are necessarily present in many well motivated models. We consider this possibility in the contexts of dark matter annihilation in galactic halos, and production in colliders. Chapter 1 provides an introduction to dark matter, including some of its astrophysical and particle aspects. As a motivation for the following sections, we begin by briefly outlining some of the observational evidence for dark matter. We go on to discuss structure formation, and the cold dark matter distribution on galactic scales. Next we discuss the possibility of non-gravitational interactions involving dark matter, including decay, annihilation, scattering off nuclei, and production. Finally we discuss the determination of the relic abundance in the early Universe, including a discussion of models involving coannihilation. Late decaying dark matter has been proposed as a solution to the small scale structure problems inherent to cold dark matter cosmology. In these models the parent dark matter particle is unstable, and decays into a daughter with near degenerate mass, plus a relativistic final state. In Chapter 2 we review the observational constraints on decaying dark matter, and construct explicit particle physics models to realize this scenario. To achieve this, we introduce a pair of fermionic dark matter candidates and a new scalar field, which obey either a Z4, or a U(1) symmetry. Through the spontaneous breaking of these symmetries, and coupling of the new fields to standard model particles, we demonstrate that the desired decay process may be obtained. We also discuss the dark matter production processes in these models. In Chapter 3 we investigate electroweak radiative corrections to dark matter annihilation into leptons, in which a W or Z boson is also radiated. In many dark matter models the annihilation rate into fermions is helicity suppressed. We demonstrate that bremsstrahlung processes can remove this helicity suppression, causing the branching ratios Br($\ell \nu W $), Br($\ell^+\ell^-Z$), and Br($\bar\nu \nu Z$) to dominate over Br($\ell^+\ell^-$) and Br($\bar\nu \nu$). We find this effect to be most significant in the limit where the dark matter mass is nearly degenerate with the mass of the boson which mediates the annihilation process. Finally, in Chapter 4, we investigate a mono-Z process as a potential dark matter search strategy at the Large Hadron Collider (LHC). In this channel a single Z boson recoils against missing transverse momentum attributed to dark matter particles, $\chi$, which escape the detector. For illustrative purposes we consider the process $q\bar{q} -> \chi\chi Z$ in a toy dark matter model, where the Z boson is emitted from either the initial state quarks, or from the internal propagator. We look for muonic decays of the Z, showing the Standard Model backgrounds to this process to be easily removable with modest selection cuts. We compare signal with Standard Model backgrounds and demonstrate that there exist regions of parameter space where the signal may be clearly visible above background in future LHC data.