Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal.

One of the pressing concerns in Dark Matter detection experiments is ensuring that the potential signal from exceedingly rare Dark Matter interactions is not obscured by background from interactions with more common particles. This work focuses on the ways in which alpha particles from primordial isotopes in the DEAP detector components can cause background events in the region of interest for Dark Matter search, based on both Monte Carlo simulations and data from the DEAP-1 prototype detector. The DEAP experiment uses liquid argon as a target for Dark Matter interactions and relies on the organic electroluminescent dye tetraphenyl butadiene (TPB) to shift the UV argon scintillation light to the visible range. The light yield and pulse shape of alpha particle induced scintillation of TPB, which is an essential input parameter for the simulations, was experimentally determined. An initial mismatch between simulated and measured background spectra could be explained by a model of geometric background events, which was experimentally confirmed and informed the design of certain parts of the DEAP-3600 detector that is under construction at the moment. Modification of the DEAP-1 detector geometry based on this model led to improved background rates. The remaining background was well described by the simulated spectra, and competitive limits on the contamination of acrylic with primordial isotopes were obtained. Purity requirements for the DEAP-3600 detector components were based on this work. The design and testing of a novel large area TPB deposition source, which will be used to make TPB coatings for the DEAP-3600 detector, is described. / Thesis (Ph.D, Physics, Engineering Physics and Astronomy) -- Queen's University, 2012-08-09 13:12:52.26

Galaxies and clusters of galaxies are believed to be dominated by non-luminous non-baryonic dark matter. A favored candidate is a new type of Weakly Interacting Massive Particle (WIMP) with a mass of order 100 GeV/c^2. The ZEPLIN II experiment is a WIMP search experiment that attempts to directly detect WIMP interactions using the two-phase xenon approach. The detector measures both scintillation and ionization generated by interactions in a 31 kg liquid xenon target. This approach provides a powerful discrimination between nuclear recoils, as expected from WIMPs, and background electron recoils. In this work, we develop a new X^2 approach to determine the three dimensional event positions in an attempt to improve the background rejection. The optical properties of the PTFE reflectors and the grids of the detector were determined using the Geant4 simulation, and event positions were obtained by finding the best match to the amount of light in each photomultiplier. This was found to greatly improve the position resolution. The approach was then applied to the WIMP search data. It was found that one of the dominating background sources was events from the gas above the anode grid and not from the PTFE walls caused by the small signals as previously thought. WIMP search results were then obtained from the first 31 days of stable ZEPLIN II data using two methods. Although the X^2 method greatly improved position resolution, the number of background events was not significantly altered and the new limit agreed well with the limit published by the collaboration.

The Standard Model of particle physics can account for neither the dark matter dominating the universe's matter density, nor the baryon asymmetry that leads to the visible matter density. This dissertation explores models of new physics that connect dark matter to baryogenesis and can naturally account for the observed quantities of both types of matter. Special emphasis is given to models incorporating new weak-scale physics, as such models often predict signatures at present and upcoming experiments and can potentially be connected to solutions of the hierarchy problem. In one class of models we study, the dark matter abundance is determined by a dark matter asymmetry connected to the baryon asymmetry. In such models, the separate dark matter, baryon, and lepton number global symmetries observed today are individually broken at or above the weak scale and lead to mixing of dark matter and Standard Model fields in the early universe. This can happen generically, with dark matter-visible matter mass mixing induced by large background energies or moduli in the early universe, and can also arise at the electroweak phase transition. Mass mixing models of asymmetric dark matter can readily accommodate dark matter masses ranging from 1 GeV to 100 TeV and expand the scope of possible relationships between the dark and visible sectors. We also consider models of symmetric dark matter in which the annihilation of dark matter particles in the early universe generates the observed baryon asymmetry. This process, called “WIMPy baryogenesis”, naturally accommodates weak-scale dark matter and explains the observed dark matter density with only order-one couplings. WIMPy baryogenesis is a new model of baryogenesis at the weak scale, avoiding problems with high reheat temperatures in supersymmetric theories, and yielding observable consequences in ongoing and future experiments for some models / Physics

The PICASSO project is a direct dark matter search experiment located 2070 metres underground in SNOLAB. Superheated droplets of Freon (C4F10) are used as the active mass, providing a target for the incoming neutralinos. Recoiling nuclei deposit energy in the superheated Freon droplets, triggering a phase transition, the pressure waves of which can be detected using piezo-electric sensors. Previously published limits using an exposure of 1.98 +/- 0.19 kg day obtained a peak spin-dependent cross section exclusion limit for neutralino-proton interactions of 1.31 pb at a neutralino mass of 29 GeV/c^2 at a 90% confidence level. Improvements in the detectors installed in the underground experiment have provided 20.99 +/- 0.25 kg day for analysis and improvements in the analysis method have produced an exclusion limit of 2.9 X 10^(-2) pb at a neutralino mass of 16.7 GeV/c^2. In addition, a thorough study of the backgrounds, corrections and systematic uncertainties has been included, indicating that this limit does not exceed 3.5 X 10^(-2) pb when considering the one sigma error on the uncertainty band. / Thesis (Ph.D, Physics, Engineering Physics and Astronomy) -- Queen's University, 2008-08-29 11:42:31.428

SuperCDMS is a leading direct dark matter search experiment which uses solid state detectors (Ge crystals) at milliKelvin temperatures to look for nuclear recoils caused by dark matter interactions in the detector. ‘Weakly Interacting Massive Particles’ (WIMPs) are the most favoured dark matter candidate particles. SuperCDMS, like many other direct dark matter search experiments, primarily looks for WIMPs. The measurement of both the ionization and the lattice vibration (phonon) signals from an interaction in the detector allow it to discriminate against electron recoils which are the main source of background for WIMP detection. SuperCDMS currently operates about 9 kgs worth of germanium detectors at the Soudan underground lab in northern Minnesota. In its next phase, SuperCDMS SNOLAB, it plans to use 100-200 kg of target mass (Ge) which would allow it to probe more of the interesting and unexplored parameter space for WIMPs predicted by theoretical models. The SuperCDMS Queen’s Test Facility is a detector testing facility which is intended to serve detector testing and detector research and development purposes for the SuperCDMS experiment. A modified detector called the ‘HiZIP’ (Half-iZIP), which is reduced in complexity in comparison to the currently used iZIP (interleaved Z-sensitive Ionization and Phonon mediated) detectors, is studied in this thesis. The HiZIP detector design also serves to discriminate against background from multiple scatter events occurring close to the surfaces in a single detector. Studies carried out to compare the surface event leakage in the HiZIP detector using limited information from iZIP data taken at SuperCDMS test facility at UC Berkley produce a highly conservative upper limit of 5 out of 10,000 events at 90% confidence level. This upper limit is the best among many different HiZIP configurations that were investigated and is comparable to the upper limit calculated for an iZIP detector in the same way using the same data. A real HiZIP device operated at Queen’s Test Facility produced an exposure limited 90% upper limit of about 1 in 100 events for surface event leakage. The data used in these studies contain true nuclear recoil events from cosmogenic and ambient neutrons. This background was not subtracted in the calculation of the upper limits stated above and hence they are highly conservative. A surface event source was produced by depositing lead-210 from radon exposure onto a copper plate. This source was then used to take data for a surface event discrimination study of the HiZIP detector operated at Queen’s Test Facility. A study of the contribution of the noise from capacitive crosstalk between charge sensors in a HiZIP detector configuration was investigated, confirming the expectation that no significant drop in performance is to be expected due to this effect. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2013-09-30 23:48:49.375

There is compelling observational evidence for the existence of dark matter in the Universe, including our own Galaxy, which could possibly consist of weakly interacting massive particles (WIMPs) not contained in the standard model (SM) of particle physics. WIMPs may get gravitationally trapped inside heavy celestial bodies of ordinary matter. The Sun is a nearby candidate for such a capture process which is driven by the scattering of WIMPs on its nuclei. Forming an over-density at the Sun's core the WIMPs would self-annihilate yielding energetic neutrinos, which leave the Sun and can be detected in experiments on Earth. The cubic-kilometer sized IceCube neutrino observatory, constructed in the clear glacial ice at the Amundsen-Scott South Pole Station in Antarctica offers an excellent opportunity to search for this striking signal. This thesis is dedicated to the search for these solar dark matter signatures in muon neutrinos from the direction of the Sun. Newly developed techniques based on hit clustering and hit-based vetos allow more accurate reconstruction and identification of events in the detector and thereby a stronger rejection of background. These techniques are also applicable to other IceCube analyses and event filters. In addition, new approaches to the analysis without seasonal cuts lead to improvements in sensitivity especially in the low-energy regime (<=100 GeV), the target of the more densely instrumented DeepCore sub-array. This first analysis of 369 days of data recorded with the completed detector array of 86 strings revealed no significant excess above the expected background of atmospheric neutrinos. This allows us to set strong limits on the annihilation rate of WIMPs in the Sun for the models probed in this analysis. The IceCube limits for the spin-independent WIMP-proton scattering cross-section are the most stringent ones for WIMP masses above 100 GeV. / IceCube

<p>Recent measurements show that dark matter makes up at least one fifth of the total energy density of the Universe. The nature of the dark matter is one of the biggest mysteries in current particle physics and cosmology.</p><p>Big Bang nucleosynthesis limits the amount of baryonic matter that can exist, and shows that the dark matter has to be non-baryonic. Particle physics provides some candidates for non-baryonic matter that could solve the dark-matter problem, weakly interacting massive</p><p>particles (WIMPs) being the most popular. If these particles were created in the early Universe a substatial relic abundance would exist today. WIMPs in our galactic halo could be gravitationally bound in the Solar System and accumulate inside heavy bodies like the Earth. Supersymmetric extensions to the Standard Model give a viable WIMP dark matter candidate in the form of the lightest neutralino. This thesis describes an indirect search for WIMPs by the neutrino signature from neutralino annihilation at the core of the Earth using the AMANDA detector. As opposed to previous dark matter searches with AMANDA, this work focuses on the hypothesis of a relatively light WIMP particle with mass of 50-250GeV/c²</p><p>The AMANDA neutrino telescope is an array of photomultiplier tubes installed in the clear glacier ice at the South Pole which is used as Cherenkov medium. Data taken with AMANDA during the period 2001-2003 is analyzed. The energy threshold of the detector is lowered by the use of a local correlation trigger, and the analysis is taylored to select vertically upgoing low energy events. No excess above the expected atmospheric neutrino background is found. New limits on the flux of muons from WIMP annihilations in the center of the Earth are calculated.</p>

Dark Matter (DM) is believed to consist ofWeakly Interactive Massive Particles (WIMPs) which interact only through gravity and the weak nuclear force. These particles can become trapped in gravitational wells such as the Sun and a theoretical value of the capture rate can be calculated. At high particle density the WIMPs annihilating spontaneously into Standard Model (SM) particles. Due to particle equilibrium the total annihilation rate can be related to the capture rate by a simple expression. This report will focus on calculating the capture rate and the related annihilation rate as well as calculating the neutrino 　ux of the Sun. At rst we will give a brief introduction to cosmology and a theoretical argument for the WIMPs as the prime DM candidate. Then we will look at the theoretical background and the mechanism through which WIMPs become trapped and evaporate or annihilate. Finally we will perform a numerical analysis of the WIMP cycle within the Sun and calculate the capture rate for a variety of theoretical WIMP masses. We will look at the capture rate due to scattering both by hydrogen nucleii and by more massive elements. The Scattering by hydrogen will be the prime contributor to the total capture rate and is the only spin dependent contribution.

Despite making up over 80% of the matter in the universe, very little is known about dark matter. Its only well-established property is that it interacts gravitationally, but does not interact with ordinary matter through any of the other known forces. Specific details such as the number of dark matter particles, their quantum properties, and their interactions remain elusive and are only loosely constrained by experiments. In this dissertation I describe a novel search for a particular type of dark matter that couples preferentially to heavy quarks, using LHC proton-proton collisions at ATLAS. With a model-independent framework, comparisons are made to results obtained from other dark matter searches, and new limits are set on various interaction strengths.