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Cold dark matter in the form of weakly interacting massive particles (WIMPs) is a favoured explanation to the galactic dark matter puzzle and could account for a large proportion of the missing mass of the Universe. There are currently numerous detectors around the world attempting to observe a WIMP signal. The ZEPLIN-III detector is one such device. Utilising liquid xenon as a target medium, identification is based on extraction of scintillation and electroluminescence signals from the two-phase xenon target caused when WIMPs scatter and has recently completed its first science run (FSR). With no WIMP signal observed, ZEPLIN-III has excluded a WIMP-nucleon spin-independent cross section above 8.1 × 10−8 pb (90% confidence limit) for a WIMP mass of 60 GeV/c2 and also set a 90% confidence upper limit of a pure WIMP-neutron spin-dependent cross section of 1.9 × 10−2 pb for a 55 GeV/c2 WIMP mass. However, the focus of this thesis is the future of the ZEPLIN-III detector with regards to the second science run (SSR). As with all dark matter detectors, background reduction from neutrons and gamma-rays plays a significant part in obtaining competitive WIMP detection sensitivities. The author has contributed significantly to the design, development and testing of a low radioactivity veto for the ZEPLIN-III detector, to be retrofitted in time for the SSR. It will detect neutrons and gamma-rays in coincidence with the ZEPLIN-III target allowing these events to be removed as candidate WIMP events. This thesis describes the author’s contribution to the design, construction, testing and evaluation of the veto. Also discussed is the development of a comprehensive Monte Carlo simulation, utilised to aid in the design process, to determine the background rates emanating from the veto components (and therefore possible impact on the low sensitivity running of ZEPLIN-III), and to provide an accurate estimation of the overall veto efficiency to reject coincident neutrons and gamma-rays. The veto will have a neutron rejection factor of 67%, reducing the expected neutron background in ZEPLIN-III from 0.4 neutrons/year to 0.14 neutrons/year, a significant factor in the event of a possible WIMP observation. In addition to the work performed on the ZEPLIN-III veto, the author has also contributed to the first science run analysis program by profiling the historical evolution of the electron lifetime throughout the FSR, and implementing consideration of this to improve the data quality.

I review theoretical expectations for the probable appearance of galaxies during their formation phase, placing particular emphasis on the uncertainties in these ideas. Recent models suggest that formation may occur relatively recently, but that young galaxies are less spectacular than previously supposed. They may be analogous to recently discovered high red - shift radio galaxies, and indeed they may have been observed directly in faint galaxy counts. I summarise several other lines of evidence which suggest that galaxy formation may have been a recent process. Finally I give preliminary results from a detailed analytic study of the observable properties of young galaxies in a Cold Dark Matter universe. Predictions are given for faint galaxy counts and redshift distributions, and for the galaxy luminosity function.

Cryogenic detectors are an advanced technology for both dark matter and neutrinoless double beta decay searches, having the key advantage of a range of possible absorber materials that can be used for the detectors. Neutron transmutation doped germanium sensors are highly sensitive thermometers ideal for use at milli kelvin temperatures, with a simple repeatable resistance temperature relation. To discriminate between candidate events and background events simultaneous measurements can be made of the energy deposited in the detector as phonons and the energy emitted by the absorber crystal as scintillation light. Phonon detectors with a calcium tungstate or calcium molybdate crystal as the target and an NTD sensor as a thermometer were made in Oxford, along with a light detector with a light-absorbing silicon layer on a sapphire crystal, also with an NTD thermometer. A system of electronics was designed and tested in Oxford to bias and readout the NTD thermometers, while the setup inside the cryostat was developed to provide a thermally and mechanically stable shielded environment for the detectors. As part of this, prototype semi-rigid kapton cabling for use in the EDELWEISS experiment was installed and tested in the cryostat. Three different NTD germanium sensor types were characterized and calibrated in the cryostat and two of these selected for use on the phonon and light detectors. The detectors were operated at temperatures as low as 9 mK and tested with radioactive sources to produce energy spectra. Baseline resolutions of 1.7 keV and 2.5 keV, respectively, were achieved for the calcium molybdate and calcium tungstate phonon detectors. A working scintillation light detector was demonstrated as part of a phonon-scintillation detector module with a suggested application in double-beta decay searches.

Under the umbrella of Theoretical Physics, progress in ‘Beyond the Standard Model’ (BSM) physics proceeds broadly along two main avenues of investigation. The first is concerned with constructing theories that attempt to explain observations, or address theoretical problems, which cannot be explained within the tremendously successful Standard Model (SM) of particle physics. The second involves looking for new ways to observe or test BSM physics, and such tests are usually developed with current experimental hints, or attractive theoretical models, in mind. This thesis contains material which falls under both approaches. Part I is concerned with Supersymmetry (SUSY). We review the basics of SUSY, and the current state of this field, and then present a novel model for SUSY at the TeV scale. This model has a Higgs sector similar to the SM and possesses a continuous U(1)_R symmetry, dramatically suppressing contributions to flavour-changing neutral currents, which can be problematic in SUSY models. After this we demonstrate that if more than one SUSY-breaking sector is present then this could lead to a rich spectrum of states with mass roughly twice the gravitino mass. In particular, if SUSY-breaking in a hidden sector arises dynamically then multiple ‘Goldstini’ and ‘Modulini’ states can arise, which couple to visible sector fields via the ‘Goldstino Portal’. We also demonstrate a new phenomenon which can occur in the context of multiple hidden sectors. If one sector breaks SUSY then this can ‘stimulate’ other sectors into also breaking SUSY, even if they are incapable of doing so on their own. Part II focusses on the matter in our Universe. We review our current understand- ing of how the visible matter in our Universe came into existence, and our current understanding of the nature of dark matter (DM). Following this we describe how DM could potentially be indirectly observed through its effects on cold white dwarf stars. Alternatively, if DM were detected by independent means, then observed cold white dwarfs could be used to place limits on the DM density in globular clusters, giving clues as to how these clusters of stars formed. We then present a new model for the co-generation of both the visible and dark matter in our Universe. This proceeds by generating a particle anti-particle asymmetry in the dark sector, which is then shared with the visible sector. This model predicts the existence of a light, m ≲ 5 eV, scalar particle which derivatively couples to DM, and provides a final state for the symmetric DM component to annihilate away into. Work completed during the period of this D.Phil is contained in [1–8], however only material in [3–6, 8] is presented in this thesis.

The dark matter search CRESST-II completed its most recent run, Run 32, in 2011. Compared to previous runs, the quantity of data taken in this run increased , significantly. In this work, Oxrop, analysis software in use within the CRESST collaboration, is upgraded to analyse this new data. At the same time, Oxrop's internal structure is improved so that it can now handle data from detectors across different experiments consistently. This upgrade was performed with a view to developing Oxrop's candidacy for use with EURECA, a future dark matter experiment. Oxrop is then used to model CRESST-II data .. First, light detector response to scintillation light produced in y interactions in CRESST -Il' s target crystals is examined. A factor influencing detector efficiency is the time constant of scintillation light production, and this light detector examination is performed with a view to extracting the scintillation time constants of the target crystals. A simple model of light detector response of one exponential rise and two exponential decay times is initially considered. It is shown that this simple model does not closely match the light detector response to y interactions in the crystal scintillator. Empirical extensions to this expected model are then made, allowing for additional decay times. These extensions allow the light detector response to crystal scintillator interactions to be well modelled, and allow estimates of the millikelvin y scintillation time of Ca W04 and Zn W04. This model is then also applied to X-ray interactions directly in the light detectors. It is seen that, even with these model extensions, interactions directly in the light detector still show significant tension with the applied model. This implies that direct calibration of light detectors with X-rays is not possible without a further understanding of light detector response, or that future direct calibrations should be done with optical photons. Position dependent effects in Run 32 calibration data are then studied. A phenomenon that has previously been considered as unrelated to position dependence, the anti-correlation effect between phonon and light detector signals, is shown to exhibit a position dependent effect in at least one lightjphonon detector pair under study. Additionally, the collection efficiency of the light detector is shown to be related to the mean interaction position. Collection efficiency is found to reduce when mean interaction position is close to the cylindrical surfaces of CRESST's Ca W04 target crystals. The magnitude of the difference in light collection: efficiency between surface and bulk interactions is also seen to be correlated with high energy light detector resolution. The WIMP-nucleon cross section limits resulting from the CRESST-II commissioning run (2007) are also reanalysed in this work. The original analysis of the commissioning run accounted only for tungsten recoils in the Ca W04 crystals used in CRESST - II. Here, interactions from calcium and oxygen nuclei are also accounted for. The resulting WIMP-nucleon cross section limits were improved at light WIMP masses -0(10 GeV j c2). These limits show a mild tension with a recent dark matter analysis of Run 32, particularly for WIMP masses below 10 GeV j c2. Possible causes of this tension are discussed

Cosmological constraints from galaxy surveys are as accurate as our understanding of the relative distributions of dark matter and galaxies, known as galaxy bias. Weak gravitational lensing is a powerful probe of galaxy bias, since the distortion in the shapes of distant galaxies, called shear, is directly related to the dark matter distribution, which can be compared to the galaxy field. I look at the galaxy clustering amplitude relative to the dark matter field, quantified by the galaxy bias b, as well as the cross-correlation coefficient r, which tells us how correlated the positions of galaxies are with the dark matter. In this thesis I present several techniques to constrain galaxy bias through weak lensing, using both numerical simulations and observational data. The most commonly used method, using aperture statistics, is shown to be subject to serious systematics in the presence of noisy data and scale- and time dependence in the galaxy bias. A local comparison technique is introduced, where the foreground distribution is used to predict the shear in the background, to which it is compared. The technique is tested with simulations, concluding that it requires high quality data. A model fitting approach is proposed, based on the McDonald (2006) galaxy bias model. The two parameters of this model, a large scale bias, b1, and a parameter, b2, that quantifies the scale dependence of the bias, are insufficient in the presence of stochasticity. Therefore, R is introduced as an additional parameter to take this into account. I present galaxy bias constraints for two spectroscopic galaxy samples: the Baryon Oscillations Spectroscopic Survey (BOSS) and the WiggleZ Dark Energy Survey (WiggleZ), applying the traditional aperture method and the model fitting approach to the Red Sequence Cluster Lensing Survey (RCSLenS). Both techniques strongly suggest that galaxies trace mass, but in a complicated way, with differences in scale- and time dependence between the samples considered. The WiggleZ galaxy bias is found to be around b ~ 1:2, depending on redshift and scale, and has a low cross-correlation coefficient of r ~ 0:5 at small scales. The BOSS samples have higher bias with scale dependence around b ~ 2:0 and show no sign of stochasticity, finding r to be close enough to unity to be explained within a deterministic scenario. The observations are in line with previous galaxy bias measurements from lensing data. The thesis incorporates work on the X-ray Luminosity Function (XLF) of galaxy clusters, measured from the Wide Angle ROSAT Pointed Survey (WARPS). Evolution is quantified with a likelihood analysis and I conclude that it is driven by a decreasing number density of high luminosity clusters with redshift, while the bulk of the cluster population remains nearly unchanged out to redshift z ~ 1:1, as expected in a low density Universe. I conclude by investigating the impact of my galaxy bias measurements from BOSS and WiggleZ on the growth rate of structure, as extracted from Redshift Space Distortions (RSD). The imperfect correlation between the galaxy and matter field, as quantified by R and b2, leads to an underestimation of the true growth rate under the assumption of a linear bias. Therefore, in order to constrain galaxy bias and gravity simultaneously, future cosmological redshift surveys require high quality lensing data.

An ever-growing body of evidence suggests that dark matter exists and is abundant in our universe. Although the direct detection of dark matter has yet to be realized, the intensity of the experimental and theoretical search continues to amplify. The question is no longer whether dark matter exists, but rather what is its fundamental nature and how can it be known. Many large-scale, international experiments are actively searching for one class of dark matter candidates, weakly interacting massive particles (WIMPs). While indirect searches, such as those looking for the creation of dark matter in particle accelerators or for the Standard Model byproducts of dark matter annihilation, are contributing significantly to our understanding of the properties WIMPs may have, direct searches, such as those using cryogenic liquids and solids to look for scattering, have produced the most stringent limits on the properties of WIMPs. Liquid xenon (LXe) detectors continue to lead the field in the search for the direct detection of WIMPs. The success of experiments using LXe relies upon decades of measurements of the fundamental properties of LXe itself, as well as thorough characterization of the detectors that utilize this amazing element. One frontier of LXe detectors is at low energies. Next-generation LXe detectors, such as XENON1T, require a better understanding of the response of LXe to particle interactions as a function of electric field, as well as more precise measurements of the radioactive backgrounds that contribute to low-energy electronic recoil interactions. In this thesis, I describe details of efforts to characterize the stability of the XENON100 detector during its primary dark matter search periods in 2010-2012. I examine the electronic recoil data for any indications of periodic behavior, and compare the XENON100 result with a dark matter annual modulation claim by DAMA/LIBRA. I also describe the design, construction, and performance of a dedicated experiment to measure the low-energy properties of LXe, in particular the energy and electric field dependence of the response of LXe to electronic recoils. Finally, I describe the design and performance of an atom trap trace analysis device for assaying the levels of radioactive krypton in LXe dark matter detectors.

Yeung, Shek = 暗能量和暗物質模型課題 / 楊碩. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2009. / Includes bibliographical references (p. 88-91). / Abstracts in English and Chinese. / Yeung, Shek = An neng liang he an wu zhi mo xing ke ti / Yang Shuo. / Chapter 1 --- Overview of Cosmology --- p.1 / Chapter 1.1 --- The Friedmann-Robertson-Walker Metric and the Friedmann Equations --- p.1 / Chapter 1.2 --- The Hubble constant and the Expansion of the Universe --- p.3 / Chapter 1.3 --- The Energy Contents in the Universe --- p.4 / Chapter 1.4 --- Cosmological Observations --- p.8 / Chapter 2 --- Review of CMB Physics --- p.13 / Chapter 2.1 --- The Anisotropy Spectrum --- p.13 / Chapter 2.2 --- The Boltzmann Equations and Einstein Field Equation --- p.15 / Chapter 2.3 --- Initial Conditions --- p.18 / Chapter 2.4 --- Inhomogeneities of Matters --- p.21 / Chapter 2.5 --- Inhomogeneities and Anisotropies of Photons --- p.23 / Chapter 2.5.1 --- Tightly Coupled Limit of the Boltzmann equations --- p.23 / Chapter 2.5.2 --- Diffusion Damping --- p.25 / Chapter 2.5.3 --- Free Streaming --- p.25 / Chapter 2.5.4 --- Cosmological Parameters --- p.26 / Chapter 3 --- Extra Dimension Model with Casimir Effect --- p.28 / Chapter 3.1 --- Introduction --- p.28 / Chapter 3.2 --- Extra Dimension Model --- p.29 / Chapter 3.3 --- The Casimir Effect --- p.34 / Chapter 3.4 --- Results and Conclusion --- p.40 / Chapter 4 --- Decaying Sterile Neutrino Model --- p.44 / Chapter 4.1 --- Introduction --- p.44 / Chapter 4.2 --- The Model --- p.46 / Chapter 4.3 --- Results and Conclusion --- p.61 / Chapter 5 --- Summary of the Thesis --- p.86 / Bibliography --- p.88

Wong, Ka Wing = 含有暗物質核心的白矮星的物理性質 / 黃家榮. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2011. / Includes bibliographical references (leaves 81-86). / Abstracts in English and Chinese. / Wong, Ka Wing = Han you an wu zhi he xin de bai ai xing de wu li xing zhi / Huang Jiarong. / Abstract --- p.ii / Acknowledgement --- p.iv / List of Abbreviations --- p.v / List of Figures --- p.vi / Table of Contents --- p.xi / Chapter 0 --- Introduction --- p.1 / Chapter 1 --- White Dwarfs --- p.5 / Chapter 1.1 --- Introduction --- p.5 / Chapter 1.2 --- Observation --- p.6 / Chapter 1.3 --- Mass-Radius Relationship & Mass Limit --- p.9 / Chapter 1.4 --- Type Ia Supernovae --- p.14 / Chapter 2 --- Dark Matter --- p.16 / Chapter 2.1 --- Introduction --- p.16 / Chapter 2.2 --- Observational Evidences --- p.17 / Chapter 2.3 --- Dark Matter Candidates and their Categorization --- p.21 / Chapter 3 --- Moon-sized White Dwarf with a Dark Matter Core --- p.26 / Chapter 3.1 --- Introduction --- p.26 / Chapter 3.2 --- Model --- p.29 / Chapter 3.3 --- Results --- p.32 / Chapter 3.3.1 --- Typical Density Profile --- p.32 / Chapter 3.3.2 --- M-R Curves --- p.33 / Chapter 3.3.3 --- Scaling in n --- p.35 / Chapter 3.3.4 --- BPS Equation of State --- p.40 / Chapter 3.4 --- Discussion --- p.41 / Chapter 4 --- Newtonian Hydrodynamic Simulation of a Spherically Symmetric Star --- p.44 / Chapter 4.1 --- Introduction --- p.44 / Chapter 4.2 --- WENO Scheme --- p.46 / Chapter 4.3 --- Runge-Kutta Time Discretization --- p.48 / Chapter 4.4 --- Fluid Dynamics without Gravity in One Dimension --- p.49 / Chapter 4.4.1 --- Riemann Problem Tests --- p.50 / Chapter 4.5 --- Spherically Symmetric Fluid Dynamics without Gravity --- p.54 / Chapter 4.5.1 --- Diffusion Problem --- p.55 / Chapter 4.6 --- Spherically Symmetric Star --- p.57 / Chapter 4.6.1 --- Radial Oscillation of a White Dwarf --- p.58 / Chapter 4.6.2 --- Radial Oscillation of a White Dwarf with a Point-Sized Dark Matter Core --- p.64 / Chapter 4.7 --- Discussion --- p.68 / Chapter 5 --- Newtonian Hydrodynamic Simulation of a Spherically Symmetric Two-Fluid Star --- p.69 / Chapter 5.1 --- Introduction --- p.69 / Chapter 5.2 --- Spherically Symmetric Two-Fluid Stars --- p.70 / Chapter 5.2.1 --- A White Dwarf with a Dark Matter Core --- p.71 / Chapter 5.3 --- Discussion --- p.77 / Chapter 6 --- Summary --- p.79 / Bibliography --- p.81