School of Physics - Theses
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ItemPUMA and MAJICK: cross-matching and imaging techniques for a detection of the epoch of reionisation( 2017)The epoch of reionisation (EoR) is one of the last unknowns in observational cosmology. After recombination, when the Universe cooled sufficiently and collapsed into neutral hydrogen (HI), the Universe was devoid of any light sources. During this cosmic ‘dark age’, the Universe was completely opaque to ultra-violet radiation, due to the abundance of HI. After some time, the very first luminous sources formed via gravitational collapse, and began to ionise the surrounding HI. Eventually the Universe transitioned from opaque to transparent, allowing us once again to peer into the cosmic depths. The exact timing and manner of the EoR has never been observed, and is paramount to confirming our the wealth of theoretical understanding. A new generation of low radio-frequency interferometers have opened a window to explore the EoR, by tracing the evolution of 21 cm radiation from HI. The experiment hinges upon our ability to remove astrophysical foregrounds; extragalactic radio-loud galaxies and galactic diffuse synchrotron emission all conspire to drown out the EoR signal. In the first part of this thesis, we develop a new cross-matching method in order to create the most accurate radio source foreground model possible. We go on to apply this technique to catalogue creation and verification, and investigate the effects of accurate source positions in foreground removal. We comment on how this technique can inform next generation instrument design, such as the upcoming instrument SKA_LOW. In the second part, we investigate averaging interferometric data as a potential method to reduce both the enormous data loads that interferometers produce, and the contamination caused by far-field sources. Averaging inherently causes signal loss, with the amplitude of the loss dependent on the scale and layout of the telescope, and so its impact on a potential EoR measurement must be well understood. We develop simulation and imaging software with new functionality to achieve this.
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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.