The universe is filled with a thermal glow called the cosmic microwave background that comes from the hot plasma which filled it early on. Measurements of this background made by the NASA satellite WMAP have determined the age, geometry and composition of the universe with new precision, determining that the universe today is dominated by a dark energy that is causing it to expand ever more rapidly. The mission has also determined that baryonic matter--the atoms and molecules we see around us--only form a few percent of the total energy density of the universe today, and has determined the epoch at which the first stars formed. Recent results give a tantalizing picture of the first very small fraction of a second in the "big bang". Six years after its launch WMAP remains healthy and the data continue to pour in. This talk will explain to a general audience what this experiment tells us about how the universe began and what it is made out of.

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal

Galaxy clusters, the most massive gravitationally bound objects in the universe, provide an important setting for exploring the structure and interactions of matter in the cosmos. When galaxy clusters merge, there is ample opportunity to examine interactions between densely-packed halos of luminous and dark matter; thus, understanding the dynamics of merging clusters provides insight into understanding properties of dark matter. This paper examines the galaxy cluster 1 RXS J0603.3+4214 (“Toothbrush Cluster”), incorporating information about the polarization of its associated radio relics into Monte Carlo simulations to constrain knowledge about its inclination angle, time since collision, and the velocity and separation distance between its subclusters. We find that the collision velocity, time since merger, and 3D separation between subclusters are well-constrained, which allows for more accurate analysis of the history of the merger. This type of constraint could be applied to a variety of merging systems. Additionally, this constraint may allow opportunity for exploring the validity of different models of dark matter.

On the 4th of July 2012, the discovery of a new scalar particle with a mass of order 125 GeV was announced by the ATLAS and CMS collaborations. An important era is now opening: the precise determination of the properties of the produced particle. This is of extreme importance in order to establish that this particle is indeed the relic of the mechanism responsible for the electroweak symmetry breaking and to pin down effects of new physics if additional ingredients beyond those of the Standard Model are involved in the symmetry breaking mechanism. In this thesis we have tried to understand and characterize to which extent this new scalar field is the Standard Model Higgs Boson and set the implications of this discovery in the context of Supersymmetric theories and dark matter models.In a first part devoted to the Standard Model of particle physics, we discuss the Higgs pair production processes at the LHC and the main output of our results is that they al-low for the determination of the trilinear Higgs self-coupling which represents a first important step towards the reconstruction of the Higgs potential and thus the final verifica-tion of the Higgs mechanism as the origin of electroweak symmetry breaking.The second part is about Supersymmetric theories. After a review of the topics one of our result is to set strong restrictions on Supersymmetric models after the Higgs discov-ery. We also introduce a new approach which would allow experimentalists to efficiently look for supersymmetric heavy Higgs bosons at current and next LHC runs.The third part concerns dark matter. We present results which give strong constraints on Higgs-portal models. We finally discuss alternative non-thermal dark matter scenario. Firstly, we demonstrate that there exists a tight link between the reheating temperature and the scheme of the Standard Model gauge group breaking and secondly we study the genesis of dark matter by a Z' portal.

We study the structure and substructure of Lambda-CDM halos using a suite of high-resolution cosmological N-body simulations. Our analysis of the substructure population of dark matter halos focuses on their mass and peak circular velocity functions, as well as their spatial distribution and dynamics. In our analysis, we consider the whole population of subhalos physically associated with the main halo, defined as those that have, at some time, crossed within the virial radius of the main progenitor. We find that this population extends beyond 3 times the virial radius and includes objects on unorthodox orbits, several of which travel at velocities approaching the nominal escape speed from the system. We trace the origin of these unorthodox orbits to the tidal dissociation of bound groups, which results in the ejection of some systems along tidal streams. This process primarily influences low-mass systems leading to clear mass-dependent biases in their spatial distribution and kinematics: the lower the subhalo mass at accretion time the more concentrated and kinematically hotter their descendant population. When quantified in terms of present day subhalo mass these trends disappear, presumably due to the increased effect of dynamical friction and tidal stripping on massive systems. We confirm several of these results using the ultra-high resolution Aquarius simulations, which extend the dynamic range of the subhalo mass function by nearly 3 orders of magnitude. Using these simulations we confirm that the substructure mass function follows a power-law, $dN/dM\propto M^{-1.9}$, and exhibits very little halo-to-halo scatter. This implies that the total mass in substructure within a given halo is bounded to a small fraction of the total halo mass, with the smooth component dominating the halo inner regions. Using the Aquarius simulations we study the structure of galaxy-sized Lambda-CDM halos. We find that the spherically averaged density profiles become increasingly shallow toward the halo center, with no sign of converging to an asymptotic power-law; a radial dependence accurately described by the Einasto profile. In our highest resolution run we resolve scales approaching 100 pc, at which point the maximum asymptotic slope is $\approx -0.89$, confidently ruling out recent claims for cusps as steep as $r^{-1.2}$. We find that the spherically averaged density and velocity dispersion profiles are not universal, but rather show subtle but significant deviations from self-similarity. Intriguingly, departures from self-similarity are minimized when cast in terms of the phase-space density profile, $\rho/\sigma^3$, suggesting an intimate scaling between densities and velocity dispersions across the system. The phase-space density profiles follow a power-law with radius, $r^{-1.875}$, identical to that of Bertschinger's similarity solution for self-similar infall onto a point mass in an otherwise unperturbed Einstein-de Sitter universe.

Thesis (doctoral)--Rijksuniversiteit Groningen, 1997. / Includes bibliographical references.

We study the structure and substructure of Lambda-CDM halos using a suite of high-resolution cosmological N-body simulations. Our analysis of the substructure population of dark matter halos focuses on their mass and peak circular velocity functions, as well as their spatial distribution and dynamics. In our analysis, we consider the whole population of subhalos physically associated with the main halo, defined as those that have, at some time, crossed within the virial radius of the main progenitor. We find that this population extends beyond 3 times the virial radius and includes objects on unorthodox orbits, several of which travel at velocities approaching the nominal escape speed from the system. We trace the origin of these unorthodox orbits to the tidal dissociation of bound groups, which results in the ejection of some systems along tidal streams. This process primarily influences low-mass systems leading to clear mass-dependent biases in their spatial distribution and kinematics: the lower the subhalo mass at accretion time the more concentrated and kinematically hotter their descendant population. When quantified in terms of present day subhalo mass these trends disappear, presumably due to the increased effect of dynamical friction and tidal stripping on massive systems. We confirm several of these results using the ultra-high resolution Aquarius simulations, which extend the dynamic range of the subhalo mass function by nearly 3 orders of magnitude. Using these simulations we confirm that the substructure mass function follows a power-law, $dN/dM\propto M^{-1.9}$, and exhibits very little halo-to-halo scatter. This implies that the total mass in substructure within a given halo is bounded to a small fraction of the total halo mass, with the smooth component dominating the halo inner regions. Using the Aquarius simulations we study the structure of galaxy-sized Lambda-CDM halos. We find that the spherically averaged density profiles become increasingly shallow toward the halo center, with no sign of converging to an asymptotic power-law; a radial dependence accurately described by the Einasto profile. In our highest resolution run we resolve scales approaching 100 pc, at which point the maximum asymptotic slope is $\approx -0.89$, confidently ruling out recent claims for cusps as steep as $r^{-1.2}$. We find that the spherically averaged density and velocity dispersion profiles are not universal, but rather show subtle but significant deviations from self-similarity. Intriguingly, departures from self-similarity are minimized when cast in terms of the phase-space density profile, $\rho/\sigma^3$, suggesting an intimate scaling between densities and velocity dispersions across the system. The phase-space density profiles follow a power-law with radius, $r^{-1.875}$, identical to that of Bertschinger's similarity solution for self-similar infall onto a point mass in an otherwise unperturbed Einstein-de Sitter universe.

La matière noire astronomique est une composante essentielle de l'univers. Depuis sa mise en évidence en 1933 par Fritz Zwicky dans l'amas de Coma, sa présence au sein des galaxies et des amas de galaxies a été largement confirmée. Les observations du satellite Planck permettent de fixer le modèle standard cosmologique selon lequel 85% de la matière de l'univers est constituée de matière noire.La nature de cette dernière demeure cependant aujourd'hui inconnue. De nombreux candidats ont été proposés. L'explication la plus plausible fait état de particules massives et interagissant faiblement avec la matière ordinaire. Ces particules de matière noire, dénommées WIMPs pour Weakly Interactive Massive Particles, sont prédites par les extensions du modèle standard des particules, à l'instar des théories supersymétriques ou des théories à dimensions supplémentaires de type Kaluza-Klein.Les particules de matière noire sont traquées activement dans les accélérateurs de particules et dans les détecteurs souterrains. Une stratégie alternative consiste à rechercher les signatures de leur présence dans la Voie Lactée à travers les rayons cosmiques, messagers de l'univers. En effet, on s'attend à ce que les WIMPs présents dans la galaxie s'annihilent en paires particules-antiparticules. Les mécanismes produisant de l'antimatière étant très rares, les antiparticules cosmiques constituent des messagers privilégiés de la présence de matière noire.Ce mémoire de thèse se concentre sur la recherche indirecte de matière noire à travers les flux de positrons et d'antiprotons cosmiques. L'objet de ce travail est de confronter les modèles théoriques de particules de matière noire aux données expérimentales, afin de mettre éventuellement en évidence les hypothétiques WIMPs et d'en déterminer les propriétés.La première partie dresse le bilan des recherches actuelles de matière noire avant de se consacrer aux modes de production et de propagation des rayons cosmiques.La thèse se concentre ensuite sur l'interprétation de la fraction positronique mesurée par la collaboration AMS-02. La possibilité d'expliquer les données par la présence de matière noire dans la galaxie est étudiée. Une explication alternative impliquant des pulsars proches produisant des positrons est examinée. Une méthode permettant de traiter la propagation des positrons cosmiques de basse énergie est ensuite développée, et les premières contraintes sur les propriétés de la matière noire sont alors dérivées à partir des mesures du flux de positrons à basse énergie.Ce travail se poursuit avec l'étude de la propagation des antiprotons cosmiques. L'influence des effets de propagation à basse énergie sur la détermination des propriétés de la matière noire est explorée. De nouvelles contraintes sont dérivées à partir des données expérimentales de la collaboration PAMELA. Les incertitudes théoriques sur la détermination du fond astrophysique sont évaluées. L'interprétation des mesures préliminaires du rapport antiprotons sur protons par la collaboration AMS-02 et les conséquences pour la matière noire sont finalement discutées. / The astronomical dark matter is ubiquitous in the universe. Since it was discovered in 1933 by Fritz Zwicky in the Coma cluster, its presence in galaxies and in galaxy clusters has been largely confirmed. The standard cosmological model predicts that about 85% of the matter in the universe is composed of dark matter.Its nature, however, remains unknown today. The dark matter particles must still have the properties of being massive and interact weakly with ordinary matter. This type of particles, the WIMPs (Weakly Interactive Massive Particles) are predicted by the extensions theories of the Standard Model of particles physics, like supersymmetrie or extra-dimensional Kaluza-Klein type theories.The dark matter particles are actively hunted in particle accelerators and in direct detection experiments. An alternative strategy is to look for signatures of the dark matter in the Milky Way through cosmic rays -- the universe messengers. Indeed, we expect that WIMPs that are present in the Galaxy annihilate into particle-antiparticle pairs. As antimatter is rare, cosmic antiparticles are privileged messengers of the presence of dark matter.This thesis focuses on indirect dark matter searches through the study of both cosmic ray positron and antiproton fluxes. The purpose of this PhD is to compare the theoretical models of dark matter particles with experimental data, which will allow us to determine their properties.The first part of the thesis provides an overview of current searches on dark matter and then focuses on the production and propagation of cosmic rays.The second part is devoted to the interpretation of the positron fraction measured by the AMS-02 collaboration. The possibility to explain the data by the presence of dark matter in the Galaxy is studied. An alternative explanation involving nearby pulsars is examined. A method to deal with low energy positrons is developed, leading to the first constraints on dark matter properties.Finally, this work focuses on the propagation of cosmic antiprotons. The influence of low energy propagation effects on the antiproton flux is explored. New constraints on WIMPs are derived from the data of the PAMELA collaboration. The theoretical uncertainties on the astrophysical background are assessed. The interpretation of the preliminary data of the AMS-02 collaboration on the antiproton to proton ratio and the implications for dark matter properties are finally discussed.

Il y a tout juste cent ans que les premières mesures du taux d'ionisation de l'air ont dévoilé que la terre est sans cesse bombardée par une pluie de particules énergétiques provenant du Cosmos. D'un point de vue astrophysique, l'origine de ces particules hautement relativistes, appelés rayons cosmiques (CRs), ainsi que leur mécanisme d'accélération restent très peu connus. Le paradigme actuel suppose une injection sporadique des CRs accélérés par la propagation d'ondes de choc au cours de la mort de certaines étoiles (SNRs).Les mesures récentes des flux de CRs (par les expériences PAMELA et AMS-02 par exemple) inaugurent une nouvelle ère de précision dans la mesure où les incertitudes statistiques sont désormais considérablement réduites. Dans ce mémoire de thèse, nous proposons et approfondissons de nouvelles pistes théoriques de manière à maximiser l'information extraite de ces nouvelles données.Après une introduction générale sur la physique des CRs, nous nous concentrons sur les espèces dites primaires, qui sont produites directement par les SNRs. De la nature discrète des SNRs et de la méconnaissance quasi-complète de leurs positions et de leurs ages résulte une incertitude théorique qui nécessite d'être estimée pour la prédiction des flux observés sur Terre. Jusqu'alors ces prédictions se contentent de calculer la moyenne d'ensemble de ce flux. Dans cette partie nous exposons la théorie statistique que nous avons élaborée, permettant de calculer la probabilité d'une déviation du flux mesuré par rapport à la moyenne d'ensemble. Nous sommes amenés à utiliser une version généralisée du théorème de la limite centrale, avec lequel nous montrons que la loi de probabilité est intimement reliée à la distribution des sources et qu'elle converge vers une loi stable. Cette dernière diffère de la loi gaussienne par sa queue lourde en loi de puissance. Le cadre théorique développé ici peut non seulement être étendu à d'autres observables du rayonnement cosmique, mais aussi enrichi en incluant une description plus complète des corrélations entre les sources. De plus, la méthode que nous avons développée peut être appliquée à d'autres problèmes de physique/astrophysique impliquant des distributions à queue lourde.Deuxièmement nous nous penchons sur les CRs dits secondaires (comme le bore), qui sont produits par les collisions des espèces primaires avec le milieu interstellaire. Plus précisément nous nous concentrons sur le rapport du flux du bore sur celui du carbone qui est traditionnellement utilisé pour comprendre la propagation des CRs. Ainsi, tout porte à croire que les mesures extrêmement précises de ce rapport nous donneraient de fortes contraintes sur les scénarios de propagation. Malheureusement il n'en est rien et nous montrons que le calcul théorique dépend fortement de certaines hypothèses telles que le lieu de production des secondaires et le choix du jeux de sections efficaces d’interaction. Nous estimons à au moins 20 % les incertitudes sur les paramètres de propagation dérivés jusqu'à maintenant. Grâce aux nouvelles données de l'expérience AMS-02, nous présentons les points de départ de notre nouvelle analyse pour laquelle nous utilisons le code semi-analytique USINE.Finalement, dans une troisième partie, nous utilisons ces données de précision pour réactualiser les analyses portant sur la recherche indirecte de matière noire. En effet, les CRs d'antimatière seraient -au même titre que le bore- des particules secondaires. La prédiction de leur fond astrophysique repose sur une connaissance précise de la propagation des CRs et de leurs interactions dans la Galaxy. Nous les traitons ici sous les hypothèses habituelles et réévaluons les flux de positrons et d'antiprotons à la lumière des nouvelles données d'AMS-02. Nous discutons ensuite les conséquences pour la matière noire et les possibles explications astrophysiques d'éventuels excès observés. / A hundred years ago, pioneering observations of air ionization revealed that the Earth is showered with particles coming from the Galaxy and beyond. Because of their high energies, these particles coined cosmic-rays are still a crucial tool in the field of particle physics, complementary to man-made accelerators. From an astrophysical point of view, the origin of cosmic-rays and the mechanisms which accelerate them are still very poorly known. The present paradigm involves sporadic production associated with the expanding shock waves from dying stars (SNRs).Recent experiments (notably PAMELA and, more recently, AMS-02) are ushering us into a new era of measurements of cosmic-ray fluxes with greatly reduced statistical uncertainties. In this dissertation, we propose and investigate new theoretical refinements of our predictions to fully benefit from these advances.After a general introduction on cosmic-ray physics, we first focus on the so-called primary species which are directly produced by SNRs. In this context of precision measurements, the discreteness of the sources in space and time, together with a substantial ignorance of their precise epochs and locations (with the possible exception of the most recent and close ones) may lead to significant uncertainties in the predictions of the fluxes at the Earth. So far, the conventional approach just relied on average trends. Here, we elaborate a statistical theory in order to compute the probability for the actual flux to depart from its ensemble average. Using the generalized version of the central limit theorem, we demonstrate that the probability distribution function of the flux is intimately related to the source distribution and follows a stable law with a heavier tail than the Gaussian distribution. Our theoretical framework can not only be extended to other cosmic-ray observables, such as the lepton flux, but also can be enriched to include a more comprehensive description of the correlations between the sources. Moreover the method which we have developed may be applied to a variety of problems in physics/astrophysics involving heavy tail distributions.Secondly, we concentrate on secondary CRs, like the boron nuclei, which are thought to be produced only by the collisions of cosmic-rays on the interstellar medium. More precisely, the ratio of the boron to carbon fluxes is a traditional tool used to understand and gauge the propagation of cosmic-rays in the Galaxy. Hence a very precise measurement of this ratio should imply stringent constraints on the propagation scenario. However we show that its theoretical derivation strongly depends on where these secondary species are produced as well as on the chosen set of nuclear cross-sections. Hence we assess at the 20% level the theoretical uncertainties on the so far derived propagation parameters. As new data from AMS-02 were freshly released, we present the starting points of a comprehensive new analysis for which we use the semi-analytical code USINE.Finally these high precision measurements offer new opportunities for a number of astroparticle problems, such as indirect dark matter searches which is the main thrust of the third part of the thesis. Antimatter cosmic rays are thought to be secondary species and their relatively low fluxes make them a channel of choice to look for rare processes such as dark matter annihilation. Nonetheless, the predictions of the expected backgrounds rely on a precise modeling of cosmic-ray propagation and interactions in the Galaxy. We treat them under commonly used simplified assumptions and discuss two studies where we re-evaluate the anti-proton and the positron fluxes in the light of the new AMS-02 data. Then we discuss the implications for dark matter and astrophysical explanations.

La découverte récente du boson de Higgs complète le Modèle Standard de la physique des particules, mais aucun signal de nouvelle physique n’a été observé en dépit des nombreuses recherches effectuées par les expériences du Large Hadron Collider (LHC). Cependant le problème de hiérarchie et la présence de matière noire sont des motivations importantes pour considérer des théories qui prédisent de nouveaux états à l’échelle électro-faible, de fait, de nombreux travaux ont été initiés sur l’interprétation des résultats négatifs et leurs implications pour de tels scénarios. Les modèles simplifiés sont devenus une norme pour l’interprétation des recherches de supersymétrie (SUSY) au LHC, et plus récemment pour les recherches de matière noire. Le succès de cette approche vient d’un petit nombre de paramètres liés aux observables au LHC de façon claire, ce qui permet une optimisation efficace des stratégies de recherche. De plus, les modèles complets peuvent être projetés sur de tels modèles simplifiés ce qui permet une compréhension intuitive des contraintes sur l’espace des paramètres et un test rapide des contraintes du LHC. Puisque les relations entre les paramètres de modèles généraux et les modèles simplifiés ne sont pas en général directes, des outils numériques sophistiqués sont nécessaires pour faciliter cette projection.Cette thèse explore de nombreux aspects de l’interpretation des recherches du LHC par les modèles simplifiés, et de la façon dont ils sont utilisés pour faire le lien entre les observations expérimentales et les descriptions théoriques. En particulier le code SModelS est présenté, il permet la décomposition automatique de modèles généraux en modèles simplifiés inspirés par la SUSY, et de les tester aux contraintes expérimentales incluses dans une base de données. Sous certaines hypothèses SModelS peut être utilisé pour contraindre une grande classe de modèles comprenant un candidat à la matière noire. Ces hypothèses sont discutées en détail et des études de modèles supersymétriques (non-minimaux) utilisant SModelS pour l’évaluation rapide des contraintes expérimentales sont présentées. Ces études soulignent les avantages ainsi que les limitations de l’utilisation de modèles simplifiés. Finalement, concernant les modèles simplifiés pour la recherche de matière noire, des scénarios avec un médiateur de spin-2 sont étudiés en détail. / The recent discovery of the Higgs boson completes the standard model of particle physics, but no compelling signal for physics beyond the standard model has been observed despite the numerous searches performed by experiments at the Large Hadron Collider (LHC). Nevertheless, the hierarchy problem and the observation of dark matter are compelling arguments to study theories predicting new states at the weak scale, and a main effort has been directed towards understanding the negative search results and their implications for such weak scale new physics scenarios. Simplified models have become a standard in the interpretation of LHC searches for supersymmetry (SUSY), aiming at maximal model independence. More recently a similar approach was adopted for the interpretation of dark matter searches. The success of this approach is due to the fact that the small set of free parameters relates to the observables in LHC searches in a clear way, allowing an efficient optimisation of search strategies. Moreover, generic models can be projected on a simplified model description giving an intuitive understanding of the constraints on the parameter space, and providing a fast test against LHC constraints. As the relation between generic model parameters and simplified models is generally not straightforward, sophisticated computational tools are required to facilitate such a projection.This thesis explores the various aspects of simplified model interpretations of LHC searches and how they can be used to understand the results and bridge the gap between theoretical descriptions and experimental observations. In particular the software tool SModelS is presented, a tool that automates the mapping of generic models onto SUSY-like simplified model components, and that allows direct tests against corresponding experimental limits in the included database. Under certain assumptions SModelS can be used to constrain a wide class of new physics models with a dark matter candidate. These assumptions and some explicit tests are discussed in detail, followed by studies of (non-minimal) supersymmetric models using SModelS for the fast evaluation of constraints from SUSY searches. These studies highlight the capacity as well as the limitations of using simplified model results to study generic models. Finally regarding simplified models for dark matter searches, scenarios with a spin-2 mediator are studied in detail.