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

In this thesis we investigate selection effects in astrophysical observations. We demonstrate that the determination of the quasar black hole mass function and Eddington ratio distribution via observations are biased. By modelling the quasar selection function and the black hole mass measurement process we show that one is able to infer the true distribution of physical quantities from observations. We present the intrinsic accretion rates of AGN, the intrinsic accretion rates and the black hole mass function for optically selected quasars up to redshift of two. The results show that the Eddington limit continues to be a real physical limit to black hole accretion. We present a new upper limit of black hole masses from the inferred intrinsic black hole mass function and demonstrate the need of a mass dependent accretion rate in accordance with down sizing. Finally we investigate correlations between radio luminosity and observed black hole mass for optically selected quasars. We first show that mixing of fiat and steep spectrum quasars leads to results that are dependent on the observing frequency, and therefore should be treated with caution. We demonstrate that beaming of the radio core together with an orientation dependent optical selection effect can give rise to a bimodal distribution in the radio luminosities of optically selected quasars.

This thesis presents searches for evidence for Weakly Interacting Massive Particles (WIMPs) and Extra Dimensions in proton-proton collisions recorded by the ATLAS experiment at the CERN Large Hadron Collider (LHC). The WIMP is one of the main candidates to constitute the particle content of Dark Matter. Extra Dimensions are introduced in several theories in order to explain the apparent weakness of gravity when compared to the other interactions in Nature. Theories with WIMPs as well as Extra Dimensions can manifest themselves at the LHC, with experimental signatures characterized by an energetic hadronic jet associated with large missing momentum. These signatures are known as monojet signatures, and are investigated in this thesis.  The first analysis is performed using L = 20.3 fb-1 of proton-proton collisions at √s = 8 TeV recorded in the ATLAS Run 1. The second analysis is performed using L = 3.2 fb-1 of proton-proton collisions at √s = 13 TeV recorded in the ATLAS Run 2. No significant excess over the expected background is found in either of the analyses. New exclusion limits are set at 95% confidence level on Dark Matter particle production. New limits are also set on graviton production in the so-called ADD scenario with Extra Dimensions.

Identifier la nature de la matière sombre est l'un des plus grands problèmes de la physique contemporaine. Si la matière sombre est constituée de particules, on peut espérer la détecter, directement ou indirectement, grâce à des expériences terrestres ou spatiales. Prédire les résultats de ces expériences, ou les interpréter en cas de détection, nécessite une compréhension profonde de la structuration de la matière sombre dans notre Galaxie. En partant de considérations issues de la physique des particules et de la cosmologie, je construits un modèle du halo de matière sombre Galactique contraint dynamiquement qui incorpore une description détaillée des ses inhomogénéités. L'impact des ces inhomogénéités sur les recherches utilisant le rayonnement cosmique est ensuite analysé en détails. J'étudie également une méthode permettant de prédire la distribution dans l'espace des phases des particules de matière sombre, et discute sa possible application aux recherches de matière sombre. Cet outil est ensuite appliqué aux recherches utilisant les électrons et positrons cosmiques, et de nouvelles contraintes très fortes sont obtenues sur les modèles microscopiques de matière sombre. / Understanding the nature of dark matter is one of the greatest challenges of modern physics. If dark matter is made of particles, we can hope to detect it, directly or indirectly, using Earth-based or spatial experiments. Make predictions for the outcome of these experiments, or interpret the results in case of a detection, requires a deep understanding of the structuring of dark matter in our Galaxy. Starting from particle physics and cosmological considerations, I built a dynamically constrained model of the Galactic dark halo including a detailed description of its inhomogeneities. The impact of these inhomogeneities on searches with cosmic rays is then analysed in details. I also study a method allowing to predict the phase-space distribution of dark matter particles, and discuss its possible application to dark matter searches. This method is then applied to searches with cosmic-ray electrons and positrons, and new very stringent constraints are obtained on microscopic models of dark matter.

Neste trabalho, estudamos modelos cosmológicos em que a energia escura foi tratada como um campo de matéria que interage com a matéria escura. Três modelos distintos foram considerados. O primeiro trata tanto a matéria escura fria quanto a energia escura como fluidos perfeitos. O termo de interação entre eles é dado por uma expressão com origem fenomenológica que postulamos existir na equação de balanço entre esses dois fluidos. Dadas as equações no universo plano de Friedmann-Robertson-Walker (FRW), pudemos escrever uma versão covariante para as equações de balanço. Com isso, as equações de balanço em um universo de FRW perturbado linearmente foram obtidas. Isso, por sua vez, permitiu que a estabilidade das equações diferenciais obtidas fosse estudada. O segundo modelo tem origem em modelos de f(R). Esses modelos propõem uma generalização da Relatividade Geral ao considerar a ação da gravidade como um funcional do escalar de Ricci, R. Através de uma transformação conforme, foi possível reinterpretar os modelos de f(R) como modelos em que um campo escalar canônico, que representa a energia escura, interage com os campos da matéria. Através do princípio da ação, obtivemos as equações de movimento e o tensor de energia-momento para nosso sistema. Com o campo escalar sendo interpretado como um fluido perfeito, pudemos, por fim, obter equações de balanço entre fluidos perfeitos tanto no nível de fundo quanto no universo perturbado linearmente. O terceiro modelo começa com a lagrangiana, em um espaço-tempo de FRW, de um campo escalar canônico, que representa a energia escura, e um campo fermiônico de spin-1/2, que representa a matéria escura. Um termo de interação de Yukawa entre esses campos foi postulado existir na lagrangiana. Novamente através do princípio da ação, obtivemos as equações de movimento e o tensor de energia-momento para esses campos. Essas equações de movimento puderam, por fim, ser reescritas como equações de balanço entre fluidos perfeitos tanto no nível de fundo quanto no universo perturbado linearmente. / In this work we studied cosmological models in which the dark energy was treated as a field of matter that interacts with dark matter. Three different models were considered. The first one treats both the cold dark matter and the dark energy as perfect fluids. The interaction term between them is given by a expression with phenomenological origin that we postulated to exist in the balance equations between these two fluids. Given the equations in the flat Friedmann-Robertson-Walker (FRW) universe, we wrote a covariant version of the balance equations. Thus, the balance equations in a linearly perturbed FRW universe were obtained. This, in turn, allowed the stability of the obtained differential equations to be studied. The second model comes from f(R) models. These models propose a generalization of General Relativity by considering the action for gravity as a functional of the Ricci scalar, R. Through a conformal transformation, it was possible to reinterpret the f(R) models as models in which a canonical scalar field, which represents the dark energy, interacts with matter fields. Through the principle of least action, we obtained the equations of motion and the energy-momentum tensor for our system. With the scalar field being interpreted as a perfect fluid, we obtained equations of balance for perfect fluids at both the background level and in the linearly perturbed universe. The third model starts with the Lagrangian, in a FRW space-time, of a canonical scalar field, which represents the dark energy, and of a fermionic field of spin-1/2, which represents the dark matter. A Yukawa interaction term between these fields was postulated to exist in the Lagrangian. Again, through the principle of least action, we obtained the equations of motion and the energy-momentum tensor for these fields. These equations of motion could then be rewritten as balance equations for perfect fluids at both the background level and in the linearly perturbed universe.

In this thesis we go beyond the standard cosmological LCDM model and study the effect of an interaction between dark matter and dark energy. Although the LCDM model provides good agreement with observations, it faces severe challenges from a theoretical point of view. In order to solve such problems, we first consider an alternative model where both dark matter and dark energy are described by fluids with a phenomenological interaction given by a combination of their energy densities. In addition to this model, we propose a more realistic one based on a Lagrangian density with a Yukawa-type interaction. To constrain the cosmological parameters we use recent cosmological data, the CMB measurements made by the Planck satellite, as well as BAO, SNIa, H0 and Lookback time measurements. / Nesta tese vamos além do modelo cosmológico padrão, o LCDM, e estudamos o efeito de uma interação entre a matéria e a energia escuras. Embora o modelo LCDM esteja de acordo com as observações, ele sofre sérios problemas teóricos. Com o objetivo de resolver tais problemas, nós primeiro consideramos um modelo alternativo, onde ambas, a matéria e a energia escuras, são descritas por fluidos com uma interação fenomenológica dada como uma combinação das densidades de energia. Além desse modelo, propomos um modelo mais realista baseado em uma densidade Lagrangiana com uma interação tipo Yukawa. Para vincular os parâmetros cosmológicos usamos dados cosmológicos recentes como as medidas da CMB feitas pelo satélite Planck, bem como medidas de BAO, SNIa, H0 e Lookback time.

Neste trabalho discutimos um modelo baseado em teoria de campos para descrever a energia escura, no qual ela é representada por uma partícula ultra-leve situada em um mínimo metaestável de um potencial. Mostramos que a energia escura neste modelo decai em matéria escura durante o tempo de vida do universo, amenizando o problema da coincidência. / In the present work we discuss a field theory model in which dark energy is described by ultra-light particle situated at a metastable minimum of a potential. We show that dark energy in this model decays into dark matter during a time scale corresponding to the age of the universe, alleviating the coincidence problem.

In this Thesis, I explore the formation and evolution of stellar spiral structures embedded in realistic dark matter haloes with very high resolution simulations. I first study the impact of the shape of the dark matter haloes. I find that non-adiabatic changes to the dark matter halo shape, commonly found in cosmological simulations due to the assembly history of haloes, can trigger strong two-armed grand-design spiral structures extending from the inner disc to the outer region. The nature of the spiral structures is found to be consistent with kinematic density waves based on the study of their power spectra. Such grand-design spiral structures may help the formation of transient multi-armed spiral structures if the self-gravity in disc is strong enough. Evolution of spiral structures is similar when the disc and the halo are misaligned, although warps develop additionally. I further find a strong correlation between the torque strength from the halo and the strength of the corresponding spiral structures. In the second part of my Thesis I then study the influence of subhaloes by including them from realistic cosmological simulations. I identify five different massive subhaloes that hit the central region of the disc, two out of which hit the disc twice. Aside from disc heating, three distinct generations of spiral structures are found in the stellar disc, which can be related to different subhaloes. For each generation, counter-rotating single-armed spiral structures develop first. They wind up very quickly before two-armed spiral structures become prominent. These spiral structures are again identified as kinematic density waves. We find that rather than interacting with the disc through resonances, subhaloes preferentially trigger spiral structures impulsively, due to their relatively short impact time with the disc. The strength of spiral structures can be related to the integrated strength of the torque generated by subhaloes. The correlation between the torque strength exerted by a triaxial dark matter halo and by subhaloes and the spiral strength may provide constraints on the distribution of dark matter.

In this thesis, I develop and combine strong lensing and dynamical probes of the mass of early-type galaxies (ETGs) in order to improve our understanding of their dark and luminous mass structure and evolution. Firstly, I demonstrate that the dark matter halo of our nearest brightest cluster galaxy (BCG), M87, is centrally cored relative to the predictions of dark-matter-only models, and suggest an interpretation of this result in terms of dynamical heating due to the infall of satellite galaxies. Conversely, I find that the haloes of a sample of 12 field ETGs are strongly cusped, consistent with adiabatic contraction models due to the initial infall of gas. I suggest an explanation for these differences in which the increased rate of merging and accretion experienced by ETGs in dense environments leads to increased amounts of halo heating and expansion, such that the signature of the halo's initial contraction is erased in BCGs but retained in more isolated systems. Secondly, I find evidence that the stellar-mass-to-light ratio declines with increasing radius in both field and cluster ETGs. With M87, I show that the strength of this gradient cannot be explained by trends in stellar metallicity or age if the stellar initial mass function (IMF) is spatially uniform, but that an IMF which becomes increasing bottom-heavy towards the galaxy centre can fully reproduce the inference on the stellar mass. Finally, I use the sizes, stellar masses and luminous structures of two samples of massive ETGs at redshift z ~ 0.6 to set constraints on the mechanisms of ETG growth. I find that ETGs in dense cluster environments already lie on the local size-mass relation at this redshift, contrary to their isolated counterparts, and suggest that this may be evidence for their accelerated growth at early times due to the higher incidence of merger events in clusters. I also show that massive compact ETGs at this redshift are composed of a compact, red, spheroidal core surrounded by a more extended, diffuse, bluer envelope, which may be a structural imprint of their ongoing inside-out growth. Overall, the studies presented in this thesis suggest a coherent scenario for ETG evolution which is dominated by hierarchical processes.

Over much of the last century evidence has been building for a new component of our universe that interacts primarily through gravitation. Known as cold dark matter, this non-luminous source is predicted to constitute 83% of matter and 26% of mass-energy in the universe. Experiments are currently searching for dark matter via its possible creation in particle colliders, annihilation in high-density regions of the universe, and interactions with Standard Model particles. So far dark matter has eluded detection so its composition and properties remain a mystery. Weakly interacting massive particles (WIMPs) are hypothetical elementary particles that interact on the scale of the weak nuclear force. They naturally satisfy predictions from extensions of the Standard Model, and are one of the most favored dark matter candidates. A number of direct detection experiments dedicated to measuring their predicted interactions with atomic nuclei have been constructed over the last 25 years. Liquid xenon dual phase time projection chambers (TPCs) have led the field for spin-independent WIMP searches at WIMP masses of >10 GeV/c^2 for most of the last decade. XENON1T is the first tonne-scale TPC, and with 278.8 days of dark matter data has set the strictest limits on WIMP-nucleon interaction cross sections above WIMP masses of 6 GeV/c^2, with a minimum of 4.1 x10^{-47} cm^2 at 30 GeV/c^2. XENON1T and the analysis that led to this result are discussed, with an emphasis on electronic and nuclear recoil calibration fits, which help discriminate between background and WIMP-like events. Interactions in liquid xenon produce light and charge that are measured in TPCs. These signals are attenuated by electronegative impurities including O_2 and H_2O, which are homogeneously distributed throughout the liquid xenon. The decrease in observables enlarges the uncertainty in our analysis, and can decrease our sensitivity. Methods on measuring the charge loss are presented, and a physics model that describes the behavior of the electronegative impurity concentration over the lifetime of XENON1T is derived. The model is shown to successfully explain the more than two years of data.