<p>It is an intriguing possibility that our world may consist of more than three spatial dimensions, compactified on such a small scale that they so far have escaped detection. In this thesis, a particular realization of this idea -- the scenario of so-called 'universal extra dimensions' (UED) -- is studied in some detail, with a focus on cosmological consequences and appplications.</p><p>The first part investigates whether the size of homogeneous extra dimensions can be stabilized on cosmological time scales. This is necessary in order not to violate the stringent observational bounds on a possible variation of the fundamental constants of nature.</p><p>An important aspect of the UED model is that it can provide a natural explanation for the mysterious dark matter, which contributes nearly thirty times as much as luminous matter like stars, galaxies etc. to the total energy content of the universe. In the second part of this thesis, the observational prospects for such a dark matter candidate are examined. In particular, it is shown how dark matter annihilations taking place in the Milky Way could give rise to exotic contributions to the cosmic ray spectrum in photons and antiprotons, leading to distinct experimental signatures to look for.</p><p>This includes a comparison with similar effects from other dark matter candidates, most notably the neutralino, which appears in supersymmetric extensions of the standard model of particle physics.</p>

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In this thesis we present work done in an analysis of models of brane induced gravity. These are higher dimensional generalizations of Einstein's General relativity where our universe is considered as a brane in a higher dimensional bulk and where the gravitational dynamics on the brane is enhanced. This provides a modification of gravity on the brane as compared to ordinary general relativity, primarily at very large distances. These models are therefore very interesting for adressing the cosmological constant problem. Even though the basic setup is natural to consider from the perspective of effective field theory and also follow from certain string theoretical considerations, the models have been plagued by inconsistencies in the form of unstable modes. In particular, a ghostlike and tachyonic scalar mode appears already at the linear level in a perturbative treatment. In order to gain a deeper insight into the nature of these consistency problems we have revisited the models, performing a more extensive analysis of the generic models than has previously been done. We have worked entirely in a gauge invariant formalism in order not to be obscured by gauge issues. We have also incorporated an effective thickness of the brane in our analysis and performed an explicit analysis of the effect of contributions from the extrinsic geometry. Although our analysis has been carried out at the linear level in a perturbative treatment we are able to get a deeper understanding of the unstable mode and illuminate some of the difficulties of these models that would likely persist even in a full nonlinear analysis.

It is an intriguing possibility that our world may consist of more than three spatial dimensions, compactified on such a small scale that they so far have escaped detection. In this thesis, a particular realization of this idea -- the scenario of so-called 'universal extra dimensions' (UED) -- is studied in some detail, with a focus on cosmological consequences and appplications. The first part investigates whether the size of homogeneous extra dimensions can be stabilized on cosmological time scales. This is necessary in order not to violate the stringent observational bounds on a possible variation of the fundamental constants of nature. An important aspect of the UED model is that it can provide a natural explanation for the mysterious dark matter, which contributes nearly thirty times as much as luminous matter like stars, galaxies etc. to the total energy content of the universe. In the second part of this thesis, the observational prospects for such a dark matter candidate are examined. In particular, it is shown how dark matter annihilations taking place in the Milky Way could give rise to exotic contributions to the cosmic ray spectrum in photons and antiprotons, leading to distinct experimental signatures to look for. This includes a comparison with similar effects from other dark matter candidates, most notably the neutralino, which appears in supersymmetric extensions of the standard model of particle physics.

Inflation, the currently favoured solution to the grievous initial conditions problems of the Big Bang model of the universe, is a very general framework that can be constructed from any number of underlying theories. As inflation is meant to solve a problem of initial conditions, it is generally preferred that it not introduce its own initial conditions problem. The purpose of this thesis is to explore the sensitivity to initial conditions of solutions to two toy models of inflation. The models in and of themselves are not intended to explain inflation, but rather seek to begin to explore, in a controlled way, interesting properties that a full inflationary theory might have. The first model is one with a single scalar inflaton, but two compact extra dimensions. We find this model has two inflationary solutions that can be well understood analytically. These solutions are power laws in time. One is found to be marginally insensitive to its initial conditions, and the other is found to be highly sensitive to its initial conditions. We also find a solution to this model that exhibits 4D quasi-de Sitter space, but is difficult to understand analytically, and its sensitivity to initial conditions is not yet well known. The second model examines an n-scalar field Lagrangian that includes kinetic terms first-order in the derivatives of the fields (similar to certain ferromagnetic Lagrangians). It is found that this model can realize slow-roll inflation with arbitrarily steep potentials. A solution is constructed that can realize an exact de Sitter equation of state without saying anything about the slope of its potential. This solution is found to be marginally insensitive to its initial conditions for a certain range of parameters. Corrections from higher order terms in the Lagrangian are found to introduce a parameter space for which this solution is in fact highly insensitive to its initial conditions. We therefore make progress in understanding higher-dimensional inflation, slow-roll inflation with steep potentials, and the sensitivity of solutions in both those cases to their initial conditions. / Thesis / Master of Science (MSc)

The subject of extra dimensions has experienced a renewed interest in recent years. Arkani-Hamed, Dimopoulos, and Dvali (ADD) have pointed out that it is possiblethat there exist extra dimensions that are as large as micrometer sized, if theStandard Model fields are restricted to a four-dimensional brane. In the ADD model, only the graviton is able to probe the extra dimensions. The main motivation for this model is that it could solve the hierarchy problem between the electroweak scale and the Planck scale by lowering the Planck scale to 1 TeV. However, in the ADD model, the radius of the extra dimensions is large, giving a new hierarchy problem between this radius and the electroweak scale. In addition, there are significant constraints on the model coming from astrophysics. To improve on the ADD model, it is possible to consider a model with curved extra dimensions. An interesting scenario is provided by extra dimensions of hyperbolic geometry. In this case, it is possible to bring the Planck scale down to 1 TeV without the need of a large radius. Also, the constraints that are important for the ADD model can be completely avoided. The most efficient probe of large extra dimensions is particle physics. In particular, it is possible to study their phenomenology in high-energy particle accelerators such as the Large Hadron Collider, which will be completed in 2008. The phenomenology of the ADD model has been extensively studied. In this thesis, we consider the phenomenology of a model where the internal space is a hyperbolic disc. We obtain the Kaluza-Klein spectrum approximately and study the Kaluza-Klein modes. The results are cross sections for production of a graviton together with a photon or a hadronic jet, which are the most important reactions for LHC physics.

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.

Consideramos quatro modelos de branas, a saber, o modelo de Binétruy, Deffayet e Langlois com uma brana estática em um bulk dinâmico, o modelo de Chamblin e Reall para paredes de domínio dinâmicas, o modelo de brana estática em um espaço-tempo AdS contendo uma singularidade, e a generalização do último caso, isto é, o modelo de brana dinâmica em um bulk AdS ou dS contendo uma singularidade nua ou protegida. Das equações de Einstein e das condições de Darmois-Israel, estudamos a dinâmica da 4-brana nos casos pertinentes. Usando o formalismo de Euler-Lagrange, derivamos uma equação para o caminho mais curto em 5 e 6 dimensões. Aplicando esta equação, investigamos a possibilidade de se ter atalhos, verificando sua existência nos três últimos modelos. Calculamos os atrasos relevantes entre os sinais de gráviton e do fóton e a razão dos horizontes subentendidos correspondentes. As consequências são discutidas. / We consider four brane world models, namely, the modelo f Binétruy, Deffayet and Langlois with a static brane in a dynamical bulk, the modelo f Chamblin and Reall for dynamic domain dilatonic domain walls, the static brane model in na AdS spacetime containing a shielded singularity, and the generalization of the latter case, i.e., the dynamic brane model in na AdS or dS bulk containing a naked or shielded singularity. From the Einstein equations and the Darmois-Israel conditions we study the 4-brane dynamics in the pertinent cases. Using the Euler-Lagrange formalism we derive na equation for the shortest path in 5 and 6 dimensions. Applying this equation we investigate the possibility of having shortcuts verifying its existence in the three last models. We calculate the relevant delay between gráviton and photon sinals and the ratio of the corresponding subtended horizons. Consequences are discussed.

Durant la passada d`ecada, els models de dimensions extra corbades han estat estudiats com a candidats per a completar el Model Est`andard (ME) de la f´ısica de part´ıcules. En aquesta tesi estudiarem una s`erie d’aspectes dels models amb una dimensi´o extra corbada; en particular, la possibilitat de generalitzar la ben coneguda soluci´o de Randall-Sundrum (RS), la qual es basa en la m`etrica Anti-de Sitter (AdS). Primer, discutim la construcci´o dels models de soft-wall, que s´on una modificaci ´o de RS on la brana infraroja ha sigut substitu¨ıda per una singularitat nua a la m`etrica. Donem receptes per a construir models consistents d’aquests tipus i estudiem com la longitud de la dimensi´o extra pot ser estabilitzada. Tamb´e estudiem l’espectre de les fluctuacions que apareixen en els models de soft-wall, i trobem que podem obtenir des d’un espectre continu a partir d’una certa massa fins a un espectre discret amb un espaiament variable. Discutim les possibles aplicacions dels models de soft-wall, i finalment presentem un model concret on es pot generar una jerarquia ultravioleta/infraroja prou gran sense necessitat de cap ajust fi. Despr´es, retornem a la construcci´o original amb dues branes per tal d’estudiar com la simetria electrod`ebil pot ser trencada en models amb m`etriques generalitzades quan el bos´o de Higgs es propaga a l’engr´os de la dimensi´o extra. Veiem com les cotes sobre l’escala dels modes de Kaluza-Klein (KK), que apareixen a causa dels observables electrod`ebils de precisi´o, poden ser redu¨ıdes quan el Higgs est`a localitzat a prop de la brana infraroja. Apliquem els nostres resultats a una extensi´o m´ınima del ME en 5D, i considerem la geometria AdS i una deformaci´o d’aquesta inspirada pels models de soft-wall. Trobemque la geometria deformada redueix enormement les cotes sobre l’escala de KK, fins al punt en qu`e els estats de KK es poden trobar dins del rang de l’LHC i el problema de la petita jerarquia pot ser eliminat sense requerir la introducci´o de cap simetria custodial. Finalment, estudiem la propagaci´o de tots els fermions del ME al llarg de la dimensi´o extra, la qual cosa fem servir per tractar el problema del sabor en el ME. Trobem expressions generals i expl´ıcites per als observables electrod`ebils oblics i no oblics, aix´ı com per als operadors que violen sabor i la simetria CP. Apliquem aquest resultat al model RS i al model amb geometria deformada, per la qual cosa fem un estudi estad´ıstic a partir d’un conjunt aleatori d’acoblaments de Yukawa en 5D. La comparaci´o de les prediccions amb les dades experimentals actuals mostren una millora de les cotes en el nostre model en comparaci´o amb RS. / Models of warped extra-dimensions have been studied over the last decade as candidates to complete the Standard Model (SM) of particle physics, for they provide a natural mechanism to address its hierarchy problem. In this thesis we study a number of aspects of the five-dimensional warped models, and in particular the possibility of generalizing the well-known Randall-Sundrum (RS) solution, which is based on the Anti-de Sitter metric (AdS). We first discuss on the construction of soft-wall models, which are a modification of RS where the infrared brane is substituted by a naked singularity in the metric. We provide recipes for constructing consistent models of this kind and address the issue of how the length of the extra dimension can be stabilized. We also discuss on the spectrum of fluctuations that arise in soft-wall models, finding that it can range from a continuous spectrum above a mass gap to a discrete spectrum with a variable level spacing. We discuss on the possible applications of soft-wall models, and finally present a concrete model where a large ultraviolet/infrared hierarchy can be generated without any fine-tuning. Next, we return to the original two-brane setup to study how the electroweak symmetry can be broken in warped models with generalized metrics when the Higgs boson propagates in the bulk. We show how the bounds on the Kaluza-Klein (KK) scale that arise from electroweak precision observables can be alleviated when the Higgs is localized towards the infrared brane. We apply our results to a minimal 5D extension of the SM and consider the AdS geometry and a deformation of it inspired by soft-walls. We find that the deformed geometry greatly reduces the bounds on the KK scale, to a point where the KK states can be within the range of the LHC and the little hierarchy problem can be removed without requiring the introduction of any custodial symmetry. Finally, we study the propagation of all SM fermions in the bulk of the extra dimension, which we use to address the flavor puzzle of the SM. We find general explicit expressions for oblique and non-oblique electroweak observables, as well as flavor and CP violating operators. We apply these results to the RS model and the model with deformed geometry, for which we perform a statistical analysis departing from a random set of 5D Yukawa couplings. The comparison of the predictions with the current experimental data exhibits an improvement of the bounds in our model with respect to the RS model.

Malgré le succès expérimental rencontré par le Modèle Standard (MS) de la physique des particules, certains phénomènes (les oscillations de neutrinos) et certaines questions fondamentales (l'origine des hiérarchies entre les différentes échelles caractéristiques) n'y trouvent pas de réponse. Il y a une nécessité d'aller au-delà du MS pour le compléter. Le sujet de cette thèse est d'étudier la phénoménologie des modèles supersymétriques ou extra-dimensionnels et les déviations de leurs prédictions par rapport à celles du MS dans le domaine de la saveur. <br />Tout d'abord, nous avons abordé la question de la conservation du nombre baryonique et leptonique via la conservation de la symétrie de R-parité dans les modèles supersymétriques de basse énergie. Plus précisément, nous nous sommes placés dans le cas où cette symétrie entre particules et `super'-partenaires n'était pas conservée. En utilisant les données expérimentales disponibles pour la désintégration rare du kaon chargé en une paire de neutrino-antineutrino et un pion chargé, nous avons entre autres obtenu une limite supérieure sur la valeur permise de certains couplages caractérisant la violation de la R-parité. Nous avons montré l'intérêt de prendre aussi en compte les contributions conservant la R-parité, négligées jusqu'alors dans les autres études sur le sujet.<br />Dans une seconde partie, nous avons étudié l'impact de l'introduction d'une dimension spatiale supplémentaire, repliée sur elle-même. Les exemples choisis concernent l'électrodynamique quantique et l'invariance de jauge, l'unification des forces forte, faible et électromagnétique ainsi que l'origine de la valeur des masses et des angles de mélanges des neutrinos. Dans ce dernier cas, l'étude a été faite dans le cadre du MSSM à 5dimensions. Typiquement, l'introduction de dimensions supplémentaires réduit le pouvoir prédictif. Cependant, des comportements généraux peuvent toujours être établis. Ainsi, nous avons pu mettre en<br />évidence la possibilité d'un mélange leptonique faible (de type CKM) à haute énergie, pour une région significative et raisonnable de l'espace des paramètres, résultat offrant de nouvelles perspectives pour l'étude de l'origine de la saveur et des liens entre quarks et leptons.