The EDELWEISS experiment is one of the dark matter direct detection experiments. It aims to detect WIMP interactions using an array of cryogenic germanium detectors. In the previous EDELWEISS-II phase, the cables and connectors used have been identified as a major source of neutron background in the experiment, which means that further effort aimed at better WIMP-nucleon interaction detection sensitivity requires a new, different cold cabling solution connecting the detectors to the front-end electronics. Motivated by this, a new two-section cold cabling system based on semi-flexible laminated copper and stainless steel cables has been developed for the EDELWEISS- III phase at Oxford. Batches of prototypes have been tested first in a cryostat at Oxford as part of a phonon-scintillation detector module, and then at the LSM underground laboratory in several EDELWEISS-III commissioning runs. Following that, a final set of cabling has been produced and installed in the EDELWEISS-III setup, which is currently conducting a science run aiming to improve its sensitivity reach compared to the previous results. This new cold cabling system has shown similar electrical performance as the previous coaxial cabling when comparing different cold cabling configurations in a commissioning run at LSM. Also, its background contribution is within the EDELWEISS-III requirements, according to radioactivity level tests and Monte Carlo simulations. In addition, the assembled connectors have allowed hundreds of signal tracks to be installed within a few days and the low material and space budget has made the cables compatible with the compact cryostat design. Besides reading out detectors for dark matter detection, prototypes of this cabling solution for a wider application range have also been produced at Oxford. The next generation dark matter direct detection experiments aim to achieve detection sensitivity better by a few orders of magnitude. This requires a target mass at tonne-scale, which converts to thousands of cryogenic detectors. Cryogenic phonon-scintillation detectors used in current dark matter searches can provide excellent performance but they usually require individual tuning and attention, making operation in large-scale experiments difficult. It is also technically challenging to stably produce such detectors in large quantity. Therefore, a scalable, robust novel detector concept for cryogenic phonon- scintillation detectors to be used in future rare event search experiments has been developed in this work. This detector module consists of a phonon detector based on a CaMoO4 scintillating crystal as the target with an attached NTD-Ge sensor as the thermometer, and a light detector based on a low-temperature PMT. To provide the high voltage necessary for PMT operation while ensuring the detector module can be cooled down and that the performance of the phonon detector is unaffected, a high voltage supply system based on a Cockcroft-Walton generator (CWG), a transformer and a small AC input has been designed and tested in the cryostat. The laminated cabling system is chosen for reading out the phonon channel and connecting the CWG and the PMT. A test run has demonstrated that, the high voltage can be provided to the PMT without causing a problem to the detector operation, and it is feasible to operate the low-temperature PMT at a temperature as low as 17 mK. Testing with a cobalt-57 gamma source, the phonon detector and the light detector have achieved resolutions of 1.07 keV and 34.2 keV for the 122.06 keV peak respectively. This is close to the performance of detectors used in the current dark matter direct searches, proving this detector concept can be applied to future large-scale dark matter direct detection experiments and other rare event searches. Using the light channel in this detector setup, the scintillation properties of CaMoO4 has been studied. In this work, the experimental data of its scintillation decay time constant has been extended from the previous 7 K to milli-Kelvin temperatures. The data are interpreted using a three-level model, confirming the existence of a metastable emission level in CaMoO4, and giving various parameters of its emission centre. This suggests that the work related to producing a high voltage supply and demonstrating the excellent performance of a low-temperature PMT could also be attractive to scintillator studies at cryogenic temperatures.

Einstein's theory of general relativity has been the accepted theory of gravity for nearly a century but how well have we really tested it? The laws of gravity have been probed in our solar system to extremely high precision using several different tests and general relativity has passed each one with flying colours. Despite this, there are still some mysteries it cannot account for, one of which being the recently discovered acceleration of the universe and this has prompted a theoretical study of modified theories of gravity that can self-accelerate on large scales. Indeed, the next decade will be an exciting era where several satellites will probe the structure of gravity on cosmological scales and put these theoretical predictions to the test. Despite this, one must still worry about the behaviour of gravity on smaller scales and the vast majority of these theories are rendered cosmologically uninteresting when confronted with solar system tests of gravity. This has motivated the study of theories that differ from general relativity on large scales but include screening mechanisms which act to hide any modifications in our own solar system. This then presents the problem of being able to distinguish these theories from general relativity. In the last few years, astrophysical scales have emerged as a new and novel way of probing these theories. These scales encompass the mildly non-linear regime between galactic and cosmological scales where the astrophysical objects have not yet joined the Hubble flow. For this reason, the screening mechanism is active but not overly efficient and novel effects may be present. Furthermore, these tests do not require a large sample of galaxies and hence do not require dedicated surveys; instead they can piggyback on other experiments. This thesis explores a class of theories of screened modified gravity which are scalar-tensor theories where the field is conformally coupled to matter via the metric and includes chameleon and symmetron models as well as those that screen using the environment-dependent Damour-Polyakov effect. The thesis is split into two parts. The first is aimed at searching for new and novel astrophysical probes and using them to place new constraints on the model parameters. In particular, we derive the equations governing hydrodynamics in the presence of an external gravitational field that includes the modifications of general relativity. Using this, we derive the equations governing the equilibrium structure of stars and show that unscreened stars are brighter and hotter than their screened counterparts owing to the larger nuclear burning rate in the core needed to combat the additional inward force. These theories have the property that the laws of gravity are different in unscreened galaxies from our own. This means that the inferred distance to an unscreened galaxy using a stellar effect that depends on the law gravity will not agree with a measurement using a different method that is insensitive gravitational physics. We exploit this property by comparing the distances inferred using pulsating Cepheid variable stars, tip of the red giant branch stars and water masers to place new constraints on the model parameters that are three orders of magnitude stronger than those previously reported. Finally, we perturb the equations of modified gravity hydrodynamics to first order and derive the equations governing the oscillations of stars about their equilibrium structure. By solving these equations we show that unscreened stars are more stable to small perturbations than screened stars. Furthermore, we find that the oscillation period is far shorter than was previously estimated and this means that the current constraints can potentially be improved using previous data-sets. We discuss these new results in light of current and future astrophysical tests of modified gravity. The final part of this thesis is dedicated to the search for supersymmetric completions of modified theories of gravity. There have been recent investigations into the quantum stability of these models and there is evidence that they may suffer from quantum instabilities. Supersymmetric theories enjoy powerful non-renormalisation theories that may help to avoid these issues. For this reason, we construct a framework for embedding these models into global supersymmetry and investigate the new features this introduces. We show how supersymmetry is broken at a scale set by the ambient density and that, with the exception of no-scale models, supergravity corrections already constrain the model parameters to levels where it is not possible to probe the theories with astrophysics or laboratory experiments. Next, we construct a class of supersymmetric chameleon models and investigate their cosmology. In particular, we find that they are indistinguishable from the LCDM model at the background level but that they may show deviations in the cold dark matter power spectrum that can be probed using upcoming experiments. Finally, we introduce a novel mechanism where a cosmological constant in the form of a Fayet-Illiopoulos term can appear at late times and investigate the constraints this imposes on the model parameter space.

We present a detailed strong lensing (SL) mass reconstruction of the core of the galaxy cluster MACS J2129.4-0741 (z(cl) = 0.589) obtained by combining high-resolution Hubble Space Telescope photometry from the CLASH (Cluster Lensing And Supernovae survey with Hubble) survey with new spectroscopic observations from the CLASH-VLT (Very Large Telescope) survey. A background bright red passive galaxy at z(sp) = 1.36, sextuply lensed in the cluster core, has four radial lensed images located over the three central cluster members. Further 19 background lensed galaxies are spectroscopically confirmed by our VLT survey, including 3 additional multiple systems. A total of 31 multiple images are used in the lensing analysis. This allows us to trace with high precision the total mass profile of the cluster in its very inner region (R < 100 kpc). Our final lensing mass model reproduces the multiple images systems identified in the cluster core with high accuracy of 0.4 arcsec. This translates to a high-precision mass reconstruction of MACS 2129, which is constrained at a level of 2 per cent. The cluster has Einstein parameter Theta(E) = (29 +/- 4) arcsec and a projected total mass of M-tot (< Theta(E)) = (1.35 +/- 0.03) x 10(14) M-circle dot within such radius. Together with the cluster mass profile, we provide here also the complete spectroscopic data set for the cluster members and lensed images measured with VLT/Visible Multi-Object Spectrograph within the CLASH-VLT survey.

Retinal diseases such as age-related macular degeneration (AMD) are the major cause of blindness in the developed world. Early diagnosis of these diseases is difficult as symptoms appear only at advanced stages. Nevertheless, several studies suggest that impairment of dark adaptation (the ability of the retina to adapt to low lighting) is a cue to AMD. Dark adaptation is the result of the regeneration of light sensitive pigments after having reacted to light (bleaching). This PhD aims at developing a tool for objective measurements of the quantity of photopigment and the kinetics of dark adaptation. This work comprises a thorough review of the absorbing and reflecting properties of the different ocular structures, giving rise to a new model of retinal (or fundus) reflectance. This model provides a detailed description of the different pathways of light through the photoreceptor layer and was able to explain measurements and findings of the literature, in particular the effect of the photoreceptor matrix interstices. An extensive study of the influence of the different model parameters on the total fundus reflectance led to the proposal of a new objective and comparable measure of quantity of photopigment (QoP). This measure is obtained by fitting a constrained version of the new model to a double density difference (DDD) measurement (i.e. the logarithmic difference between reflectances of a retinal area in bleached and dark adapted states). This approach was validated by correctly fitting several DDD measurements from the literature. Future experimental studies are needed to confirm the relevance of the new QoP measure and specify its application in clinical diagnosis. Several fundus reflectometry instruments have been able to measure the DDD in human eyes however because of practical and technological limitations none of these instruments were suitable for clinical use. Here, these limitations are discussed and two new imaging fundus reflectometers are presented. Developed respectively by 4D Optics Ltd. and the Vision Research group at the University of Manchester, these two systems, based on modified fundus cameras, are ongoing development work towards clinically suitable imaging fundus reflectometry. Example data obtained with these two instruments exhibits aberrant points and low signal to noise ratio (SNR). The main issues encountered were camera noise and stability, uneven retinal illumination, and subject’s eye movements and changes of alignment. It is believed that these issues can be overcome with current technologies. One important impediment to the use of the dark adaptation experiment in clinical practice is the time it takes for photopigments to completely regenerate (up to 40 min in normal eyes). A theoretical data analysis strategy using the new model of fundus reflectance and the Marhoo, Lamb and Pugh model of photopigment regeneration kinetics is proposed to rapidly diagnose an abnormal regeneration, hence reducing considerably the duration of the experiment. This idea has not been tested on experimental data but may become relevant once better quality measurements of DDD are obtained.

Orientador: Pedro Cunha de Holanda / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-24T10:31:04Z (GMT). No. of bitstreams: 1 Miguez_BrunoSilvaRodriguez_D.pdf: 3341752 bytes, checksum: 929d02bdf7d42cd3826d597a0b6ceca8 (MD5) Previous issue date: 2014 / Resumo: Uma das grandes fronteiras da física atual é a identificação da Matéria Escura, que seria responsável por cerca de 25% da densidade do universo. Diversos candidatos a Matéria Escura foram propostos, entre eles os WIMPs (Weakly Interacting Massive Particle). A Colaboração Xenon tem como objetivo a detecção direta de matéria escura através de colisões elásticas com núcleos de xenônio, monitorados em uma câmara de projeção temporal. Os primeiros detectores da colaboração Xenon (Xenon10 e Xenon100) obtiveram grande sucesso impondo os limites mais restritivos para seção de choque WIMP-nucleon quando foram publicados seus resultados. Atualmente o terceiro detector da colaboração, o Xenon1T, está em construção e é esperado que ele verifique seções de choque até duas ordens de grandeza abaixo dos limites atuais. O Xenon1T possuirá um sistema de veto ativo, o Water Tank. A câmara de projeção temporal do Xenon1T será localizada dentro de um detector que busca, através da detecção de outros produtos, identificar nêutrons rápidos produzidos através da interação de múons com as rochas ou estrutura ao redor do detector. As paredes internas do Water Tank são cobertas com uma folha refletora DF2000MA para aumentar a captação de luz. Nesta tese foi estudada a taxa de eventos gerada pela resposta da folha DF2000MA à radioatividade do aço que compõe a estrutura do Water Tank e seu impacto no funcionamento do sistema de veto. A taxa destes eventos que gerariam um sinal no Water Tank seria da ordem de 10-4 Hz, muito abaixo da taxa de operação planejada para o Water Tank (? Hz), não sendo portanto um problema. Outro estudo realizado foi sobre o sinal gerado por neutrinos de supernovas através de espalhamento coerente com os núcleos. Supernovas próximas seriam responsável por menos de 10 eventos concentrados em poucos segundos, muito distintos do fundo esperado de recuos nucleares no Xenon1T, da ordem de 0.1 por ano. Sendo o sinal de uma supernova facilmente reconhecido durante a análise / Abstract: Actually an important frontier on physics is the Dark Matter identification. The Dark Matter is responsible for 80% Universe matter density on Universe. Different Dark Matter candidates have been proposed, among them the WIMPs (Weakly Interacting Massive Particle). The Xenon Collaboration have as goal the direct detection of Dark Matter by observation of elastic scattering on xenon nuclei. The first two Xenon phases achieved great sucess with the most constraining limits on WIMP-nuclei cross section at publishing time. Today the third detector (Xenon1T) is under construction and it expect to probe cross section two orders below the actual limits. One big difficult to increase the detector mass is the signal to noise ratio. The Xenon detectors keep record of excitation and ionization energy ratio to discriminate between electronic recoils and nuclear recoils. WIMPs should cause nuclear recoil by coherent elastic scattering on atomic nuclei. The nuclear recoil selection allows a huge increase on sensibility, once the background rate due to nuclear recoil on Xenon1T is five orders lower than the eletronic recoil one. Neutrinos and neutrons can produce nuclear recoils mimicating the WIMP signal. The Xenon1T will have an active veto system, the Water Tank. The Xenon1T time projection chamber will be placed inside a water tank monitored by photomultipliers. It will detect subproducts of muon interactions on rocks around the detector and tag the nuclear recoil due to fast neutrons produced by muons. To increase the light capture in the internal walls of Water Tank will be covered by a reflective foil, the DF2000MA. We studied the event rate due to the DF2000MA response to the alpha radioactivity of structural steel and its impact on veto system. The rate of these events that would generate a signal in veto system obtained by us was around 10-4 Hz, much lower than the operational planned rate (? Hz). Then this response will not be a problem to veto system. Another study was about the supernova neutrinos signal on Xenon1T. Neutrinos can produce nuclear recoils by coherent elastic scattering, mimicating the WIMP signal. We obtained ? 10 events due to a supernova at 8.5 kpc. Furthermore these events would be concentrated in seconds, much different from the Xenon1T nuclear recoil background (? 0.1/year). Then the supernova neutrino signal would be easilly recognized on analysis level / Doutorado / Física / Doutor em Ciências

Na última década, o extraordinário progresso nas observações astronômicas (distâncias com supernovas (SNe Ia), espectros de potência da matéria e da radiação cósmica de fundo (RCF), determinação do brilho de aglomerados de galáxias, etc.) aliado com importantes desenvolvimentos teóricos, transformaram a Cosmologia numa das fronteiras mais excitantes da ciência contemporânea. Nesta tese, diferentes testes observacionais são utilizados para vincular alguns cenários cosmológicos acelerados (com e sem energia escura), todos eles definidos no contexto teórico da Relatividade Geral. Inicialmente, para uma grande classe de modelos com decaimento do vácuo, investigamos os vínculos provenientes da existência de objetos velhos em altos redshifts. No modelo de Chen e Wu generalizado, encontramos que o limite para o parâmetro livre descrevendo a taxa do decaimento do vácuo é 0,21 < n < 0,81. Este resultado descarta o modelo de Chen e Wu original (n=2) e também o modelo de concordância cósmica, LCDM (n=0). Além disso, quando incluímos o fluido bariônico em nossa análise do modelo de Wang e Meng, obtemos para seu parâmetro livre um limite inferior, epsilon > 0,231, um valor em desacordo com estimativas independentes baseadas em SNe Ia, RCF e o brilho de Raios-X de aglomerados. Propusemos também um teste estatístico com base nas idades estimadas para uma amostra de 13 galáxias velhas em altos redshifts. Através de uma análise conjunta envolvendo as idades das galáxias e as oscilações acústicas dos bárions (BAO), vinculamos o valor da constante de Hubble no contexto do modelo LCDM plano. Considerando um tempo de incubação adotado por diferentes autores, obtemos h=0,71±0,04 (1 sigma), um resultado de acordo com observações independentes baseadas em Cefeidas (obtidas com o Hubble Space Telescope) e outras estimativas mais recentes. Outro resultado interessante foi obtido através de uma análise termodinâmica para uma classe de modelos com interação no setor escuro (matéria escura-energia escura). Contrariamente ao que se pensava até então, encontramos que a termodinâmica permite que a matéria escura decaia em energia escura, contanto que ao menos uma das componentes possua um potencial químico não-nulo. Como complemento, mostramos que, para um termo de interação específico, dados de SNe Ia, BAO e RCF favorecem o decaimento da matéria escura com ~ 93% de confiança estatística. Investigamos também o comportamento do redshift de transição em diferentes cosmologias, com e sem energia escura, e mostramos que essa quantidade pode ter uma variação extrema dependendo do modelo cosmológico subjacente. Finalmente, discutimos também um novo modelo cosmológico cuja aceleração em baixos redshifts é determinada pela criação de partículas da matéria escura fria. O modelo representa uma redução do setor escuro, isto é, não tem energia escura, contém apenas um parâmetro livre e satisfaz os vínculos de Supernovas do tipo Ia tão bem quanto o modelo LCDM padrão. / In the last decade, the extraordinary progress of the astronomical observations (distances with supernovas, matter and cosmic background radiation (CBR) power spectrum, X-ray surface brightness of galaxy clusters, etc) associated with important theoretical developments turned Cosmology one of the most exciting frontiers of contemporary science. In this thesis, different observational tests are used to constrain several cosmological accelerating scenarios (with and without dark energy), all of them defined in the theoretical framework of General Relativity. Initially, for a large class of decaying vacuum models, we investigate the constraints provided by the existence of old high redshift objects. In the model proposed by Chen and Wu, we find that the limit for the free parameter describing the decay rate of the vacuum fluid is 0.21 < n < 0.81. This result ruled out the original Chen and Wu model (n = 2) and also the cosmic concordance model, LCDM (n = 0). Further, when we include the baryonic fluid in our analysis of the Wang and Meng model, we find for its free parameter a lower bound, epsilon > 0.231, a value in disagreement with independent estimates based on SNe Ia, CMB (shift parameter) and the X-ray surface brightness of galaxy clusters. We also propose a new cosmological statistical test based on the estimated ages of 13 old high redshift galaxies. By performing a joint analysis involving the ages of the galaxies and the baryon acoustic oscillations (BAO) probe, we constrain the value of the Hubble parameter in the context of the flat LCDM model. For an incubation time adopted by different authors, we find h = 0.71 ± 0.04 (1 sigma), a result in agreement with independent observations based on Cepheids (obtained with the Hubble Space Telescope) and other recent estimations. Another interesting result has been derived from a thermodynamic analysis for a class of models endowed with interaction in the dark sector (dark matter and dark energy). In contrast with some results appearing in the literature, we show that the decaying of cold dark matter into dark energy is not forbidden by thermodynamics, provided that the chemical potential of one component is different from zero. As a complement, we also show (for a specific term describing the interaction) that this kind of decaying is favored by SNe Ia, BAO and CMB data with ~ 93% of statistical confidence. We also investigate in detail the behavior of the transition redshift for different cosmologies (with and without dark energy). It is found that such a quantity may have an extreme variation that depends on the underlying cosmological model. Finally, we also discuss a new cosmological model whose acceleration at low redshifts is determined by the creation of cold dark matter particles. The model represents a reduction of the dark sector, that is, it has no dark energy, contains only one free parameter and satisfies the Supernovae type Ia constraints with the same precision of the standard LCDM model.

In this dissertation, a low energy theory approach is applied to the studies of Dark Matter direct detection experiments and two-dimensional Quantum Chromodynamics (QCD) spectra. We build a general framework of non-relativistic effective field theory of Dark Matter direct detection using non-relativistic operators. Any Dark Matter particle theory can be translated into the coefficients of an effective operator and any effective operator can be related to a most general description of the nuclear response. Response functions are evaluated for common Dark Matter targets. Based on the effective field theory we perform an analysis of the experimental constraints on the full parameter space of elastically scattering Dark Matter. We also formulate an analytic approach to solving two-dimensional gauge theories. We find that in theories with confinement, in a conformal operator basis, the decoupling of high scaling-dimension operators from the low-energy spectrum occurs exponentially fast in their scaling-dimension. Consequently the low-energy spectrum of a strongly coupled system like QCD can be calculated using a truncated conformal basis, to an accuracy parametrized exponentially by the cutoff dimension. We apply the conformal basis approach in two models, a two-dimensional QCD with an adjoint fermion at large N, and a two-dimensional QCD with a fundamental fermion at finite N. It is shown that the low energy spectrum converges efficiently in both cases.

Dans les dernières décennies, un modèlecosmologique, dénommé “modèle ACDM” ,a émergé et plusieurs observations ont montréqu’il est consistant avec les données. Toutefois, ilsexistent des problèmes encore ouverts ; en particuliernous ne connaissons pas la cause de l’accélérationobservée de l’expansion de l’Univers. De nombreuxmodèles alternatifs ont été proposés.Cette thèse traite d’une approche pour contraindreles alternatives au modèle ACDM dénommée “EffectiveTheory of Dark Energy”. Elle s’applique àtoutes les théories où un seul degré de liberté scalaireest ajouté aux deux degrés de liberté de la RelativitéGénérale. Ces théories sont appelées “théoriestenseur-scalaire”.Le chapitre 1 résume la plus générale classe de théoriestenseur-scalaire viables actuellement connue.Le chapitre 2 introduit la théorie efficace de l’énergienoire. L’inclusion dans la théorie efficace descouplages généraux entre le secteur gravitationnelle(métrique et champ scalaire) et la matière est décritdans le chapitre 3. Ceci inclut aussi la possibilitéque différentes espèces interagissent de façondifférente avec la gravité. La possibilité d’analyseren détail la stabilité de la théorie est explicité dansle chapitre 4.Un objectif de cette thèse est aussi l’investigationdes effets observables des déviations du modèleLCDM. Le chapitre 5 considère la possibilité d’uneinteraction entre la matière noire et l’énergie noire.Ce chapitre étudie les effets d’une telle interactionet, plus généralement, analyse la capacité des observationsfutures à imposer des contraintes sur lesparamètres libres de la théorie efficace. Le chapitre 6 se concentre sur les effets observationnels de lathéorie “ beyond-Horndeski” , en démontrant quedans ce théorie il existe un melange cinétique entrela matière et le champ scalaire. Ce melange donnelieu à un effets particuliers: l’affaiblissement de lagravité aux échelles des grands structures. / In the last decades, a cosmological model that fits observations through a vast range of scales emerged. It goes under the name of ACDM. However, there are still challenging questions that remain unanswered by this model, such as what causes the observed accelerated expansion of the universe. Hence, many alternative models have been proposed. This thesis concerns an approach to test such models known as “Effective Theory of Dark Energy” . It applies to all models where general relativity is modified by adding a single scalar degree of freedom, called “ scalar-tensor theories”. In Chapter 1 I introduce the most general classof such theories currently known. In Chapter 2, I resume the Effective theory of dark energy. The inclusion of a general coupling between matter and the gravitational sector is the subject of Chapter 3. Chapter 4 analyses in details the stability of different classes of theories.Another goal of the thesis is to study the observable effects of deviations from ACDM. In Chapter5, I consider the possibility of an interaction between dark matter and dark energy and I analyse the constraining power of future surveys on the free parameters of the theory. Chapter 6 focuses on the observational effects of theories where a kinetic mixing between matter and the scalar field exists.This gives a peculiar effect, namely the weakening of gravity at large scale structure scales.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第22028号 / 理博第4532号 / 新制||理||1651(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 向山 信治, 准教授 Antonio De Felice, 教授 鶴 剛 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

Uncovering the nature of dark matter is one of the most pressing problems in 21st century cosmology. Despite overwhelming evidence that dark matter exists and vigorous experimental efforts to detect it, dark matter has evaded detection and its fundamental nature remains shrouded in mystery. Indirect dark matter detection experiments search for Standard Model byproducts of dark matter annihilation or decay. At low energies, cosmic antideuterons provide an especially clean dark matter signature, since the production of low-energy antideuterons from conventional astrophysical processes is highly suppressed. The General Antiparticle Spectrometer (GAPS) is an Antarctic balloon experiment designed to search for low-energy cosmic antinuclei as signatures of dark matter. GAPS is optimized to detect low-energy antideuterons, as well as to provide unprecedented sensitivity to low-energy antiprotons and antihelium nuclei. GAPS uses a novel approach to detect antinuclei, based on the formation, decay, and annihilation of exotic atoms. At least three GAPS long-duration balloon (LDB) flights are planned, with the first launch date anticipated for December 2022. The core of the GAPS instrument is a particle tracker, comprised of >1000 lithium-drifted silicon (Si(Li)) detectors, that provides particle tracking and X-ray spectroscopy capabilities. In order to preserve the long-term performance of the tracker, the Si(Li) detectors require a surface passivation coating to protect against environmental contamination. In this thesis, I cover four main areas of my research: prototype Si(Li) detector fabrication and performance evaluation; development of a surface passivation technique to ensure the long-term stability of GAPS flight detectors; calculation of the GAPS antihelium sensitivity using particle tracking; and prediction of the antihelium exotic atom X-ray energies and yields for future identification studies. I discuss the prototype fabrication work that was carried out at Columbia, which led to the successful mass-production of large-area Si(Li) detectors for the GAPS LDB flights. I report the research and development of a surface passivation method to protect the GAPS flight detectors from environmental contamination. I then describe the calibration scheme for the GAPS Si(Li) detectors, and a simulation study that I conducted to disentangle the contribution of Compton scattering and intrinsic detector performance on the observed spectra. I then move on to discuss the simulation studies used to determine the performance capabilities of GAPS. I describe the benchmarking of the hadronic annihilation products in antinucleus-nucleus annihilations in Geant4. I review the exotic atom cascade model used to determine the X-rays produced by antiprotonic and antideuteronic exotic atoms, and discuss my work extending this model to describe the de-excitation of antihelium exotic atoms. Finally, I present the first GAPS antihelium nuclei sensitivity study, based on full instrument simulation, event reconstruction, and realistic atmospheric influence simulations.