The theoretical prediction of the value of the cosmological constant is far from being in accord with observations. One way to try to solve this problem is to use a scale invariant theory, however, no standard Lagrangian is truly scale invariant. In this thesis, we try to obtain a scale invariant Lagrangian by adding a suitable counterterm to the Lagrangian for the $ lambda phi sp4$ model. We then apply this technique to different models to see if this method can be applied for very different situations. We also check if flat directions in a scale invariant potential get lifted by renormalization.

We describe brane-world models which partially break supersymmetry from N = 2 to N = 1. The brane models we consider are embedded within a 4-dimensional N = 2 locally supersymmetric bulk-space. We devise the following models: a single-brane action, a single-antibrane action and a two-brane action. In these models the membrane linearly realises one supersymmetry of the bulk and spontaneously breaks the other. We also construct the action for a brane free bulk with a cosmological constant term. This model is different from the previous examples because both supersymmetries are broken. We also attempt to devise a brane-antibrane action. This model has the particular feature of breaking both supersymmetries of the bulk-space. Effects of this investigation on our understanding of the mechanism of partial supersymmetry breaking in brane-world scenarios will be discussed. In particular, the contribution to the cosmological constant via this mechanism will be examined. In order to construct such a model, we use the previous actions and the concepts developed throughout their construction. This investigation constitutes a firm foundation toward the construction of a successful brane-antibrane model.

In the last years, brane world scenarios have been studied extensively, but most of these studies have been done in the case of five-dimensional spacetime. It is therefore of interest to investigate which of the particular features observed are proper to one extra dimension and which are generic to any number of dimensions. In this thesis, I present an overview of models and solutions to Einstein's equations for six-dimensional brane world scenarios. Solutions for a simple setup with cylindrically symmetric bulk centered about a three-brane are derived and classified. There are two main kinds of topology: either solutions are compactified in a spherical topology, closed up by another three-brane, or they have a disc topology, which must be terminated by a four-brane. One of the particular features of codimension-two branes is demonstrated, namely that their tension, or vacuum energy, induces a deficit angle in the bulk. Solutions for different arrangements of codimension-one and codimension-two branes are also reviewed. Although the review focuses on topological and cosmological properties of the solutions, models using a field theoretical approach to the brane-world scenario, i.e. considering the brane as a topological defect arising from higher dimensional fields, are also considered.

At the HERA particle accelerator, 27.5 GeV electrons collide with 920 GeV protons. Photoproduction events, in which the quasi real photon emitted from the electron interacts with the proton with small momentum transfer, are dominant by 90% of the data sample. K0S-K0S candidate pairs, which might couple to glueballs, are selected and studied with the ZEUS detector in the photon-proton center-of-mass range 130 GeV < W_gamma_p < 270 GeV using the entire data luminosity of 0.5 fb^-1. Enhancements attributed to the production of f2(1270)/a02(1320), f'2(1525) and f0(1710) are observed in the K0S-K0S invariant mass spectrum. A coherent fit, which has taken into account the interference effects by SU(3) symmetry argument, provides precise measurement values on mass and width of the states. It gives a five standard deviation enhancement on state f0(1710) in terms of statistical significance. This is by far the best measurement in e-p collision experiments, and is consistent with results from other experiments and the world averages. Furthermore, spin analysis on the glueball candidate f0(1710) are performed as an approach to reveal the glueball content of the meson state. However, due to the complicated background condition, the angular distribution method and the partial wave analysis are undetermined and fail to give a positive result on the spin. / À l'accélérateur de particules HERA, des électrons de 27.5 GeV entrent en collision avec des protons de 920 GeV. Les événements de photoproduction, où le photon émis par l'électron est quasi-réel et interagit avec le proton avec un petit échange de quantité de mouvement, dominent l'échantillon de données. Les paires K0S-K0S, susceptibles de se coupler aux boules de gluons ("glueballs"), sont choisies et étudiées avec le détecteur ZEUS dans un domaine d'énergie du centre de masse photon-proton entre 130 GeV et 270 GeV en utilisant la luminosité complète de 0.5 fb^-1 des données. Des structures observées dans le spectre de masse invariante K0S-K0S sont attribuées aux états f2(1270)/a02(1320), f'2(1525) et f0(1710). Une procédure d'ajustement de fonctions cohérentes, qui tient compte des effets d'interférence de par un argument de symmétrie SU(3), procure des valeurs mesurées précises de masse et de largeur de signal pour ces états. En terme de signification statistique, une déviation par cinq sigmas est obtenue pour l'état f0(1710), ce qui est de loin le meilleur résultat dans des expériences aux collisions e-p et est consistant avec les résultats d'autres expériences et les moyennes mondiales. De plus, une analyse de spin est effectuée sur le candidat "glueball" f0(1710) dans une tentative de déduire le contenu "glueball" de cet état mésonique. Toutesfois, À cause des conditions sévères du bruit de fond, les méthodes de distributions angulaires et d'analyse d'ondes partielles ne peuvent parvenir à donner un résultat positif sur la valeur du spin.

Observational facts indicate that the expansion of the universe is accelerating, rather than decelerating, because 73% of the total energy density of the universe is a "dark energy" with strong negative pressure, ω &lt; −1/3. In this thesis, we introduce a dynamical dark energy model with dilatational symmetry, which contains two scalar fields coupled to gravity. Because of the dilatational symmetry, there is no cosmological constant Λ in the Lagrangian, and, instead, two scalar fields generate the dark energy, evolving slowly in time. At early times, the system is in the slow roll regime, corresponding to Higgs inflation due to the Higgs field. At late times, the dynamical dark energy dominates the universe and eventually behaves just like the cosmological constant, and the universe becomes exponentially expanding with the scale factor a(t) ∝ exp{Ht}. The numerical results from solving the dynamic equations of the system agree well with the observational facts, which indicates that our model gives a good description of the universe. At the end of the thesis, we consider the one-loop corrections to our model, and show that they do not alter the classical results in any significant way. / Les observations actuelles de l'expansion de l'univers indiquent une accéleration decette expansion due à 'l'énergie sombre', qui compte pour 73% de la densité d'énergie totalle de l'univers et qui se comporte comme un fluide avec une pression négative, ω &lt; −1/3. Cette thèse présente un modèle dynamique d'énergie sombre invariant sous une symétrie de dilatation comprenant deux champs scalaires couplés à la gravitation. La constante cosmologique n'est pas présente dans ce modèle, de par la symétrie de dilatation; les champs scalaires génèrent une énergie sombre évoluant dans le temps. Dans l'univers primordial, le système se situe dans le régime d'évolution lente correspondant à l'inflation cosmolgique due au champ de Higgs. L'énergie sombre dynamique agit ultérieurement sur l'évolution de l'univers comme une constante cosmologique; le facteur d'échelle de l'univers accroit de manière exponentielle a(t) ∝ eHt. Les simulations numériques concordent très bien avec les observations actuelles. Les perturbations quantiques de premier ordre sont ensuite calculées, et justifient la validité des résultats obtenus de manière classique.

The Inflationary power spectrum plays an important role in modern cosmology. In this thesis, we studied both the experimental and theoretical aspects of the inflationary spectral index. By exploring the recent WMAP data, we found that the evidence for the running of the spectral index mainly comes from multipoles near l = 40. This fact allows a partial running spectrum to give as good a fit as the WMAP running spectrum. We gave some simple formulae for the inflationary spectral indices based on the Hamilton-Jacobi formulation of inflation. These simple formulae agree with the exact solutions in some special cases. The Hamilton-Jacobi formulation of inflation was also applied to reconstruct inflaton potentials from a given power spectrum. A simple and accurate reconstruction formulation was presented. All analytic potentials giving a constant spectral index are derived, which show that a nearly scale-invariant spectrum can give rise to slow-roll inflation during 60 e-foldings within sub-Planckian inflaton field values and a potential energy V1/4 ~ 1015GeV. Potentials for large running of the spectral index and large tensor-to-scalar ratio were also constructed, which need super-Planckian field values and require that the slow-roll approximation breaks down before reaching 60 e-foldings. We have shown that for the cosmologically interesting scales, a renormalizable potential fits the reconstructed potential for a large running spectrum very well. Our reconstruction formulation also produces a self-consistent tensor spectrum once a scalar spectrum and the tensor-to-scalar ratio are given. Higher order corrections to the slow-roll approximation are also considered. We showed that they can be incorporated straightforwardly into our formulae for spectral indices and the reconstruction formalism.

This thesis examines two recent anomalous cosmic ray (CR) positron detections in the context of the ongoing search for the particle nature of dark matter (DM), which composes 85% of the matter content of the Universe. The first CR anomaly, detected by the European INTEGRAL/SPI experiment via 511 keV gamma rays from the center of the Galaxy, suggests an unaccounted-for production of low-energy positrons in the region surrounding the galactic center (GC). We model the production of electron-positron pairs from the decay or interaction of cold dark matter in an Einasto profile. We show that the INTEGRAL signal can be fit by scattering DM in a halo with the shape parameters predicted by many-body simulations, with a significance on par with previous phenomenological fits, but with six fewer degrees of freedom. This can be achieved with annihilating low-mass DM, or with scattering of excited dark matter (XDM), with cross-sections compatible with thermal WIMP production in the early universe.The second CR anomaly is the rising positron fraction from 10 to 200 GeV observed by the PAMELA satellite and confirmed by NASA's Fermi-LAT. Although previous studies had considered Sommerfeld-enhanced DM annihilation as a possible source, they did not consider the full impact of the dark matter substructure predicted to exist by simulations. We show that including this substructure can give a better fit to the PAMELA and Fermi data, but that this is not sufficient to overcome the strict gamma-ray bounds from the Fermi-Large Area Telescope (LAT) diffuse gamma ray data. We finally show that a single, nearby subhalo can explain the excess, while simultaneously avoiding gamma ray and dipole anisotropy constraints, and that it is possible to create a Sommerfeld-enhanced particle physics model that produces the required annihilation cross-section and is compatible with cosmological bounds. / Nous examinons dans cette thèse deux détections récentes de positrons dans le rayonnement cosmique, dans le contexte d'une origine possible sous la forme de matière sombre (MS). Quoique celle-ci englobe 85% de la matière dans l'univers, sa détection jusqu'à présent ne s'est faite que par son intéraction gravitationnelle. La première anomalie, observée par le satellite Européen INTEGRAL via un excès de rayons gamma de 511 keV issus du centre de la Voie Lactée, suggère une production élevée de positrons dans cette région. En modélisant la production de paires d'électrons-positrons par la décomposition ou l'intéraction de MS dans un profil Einasto, nous obtenons un ajustement d'aussi bonne qualité que les meilleures études précedentes purement phénoménologiques, mais avec six degrés de liberté en moins. Ceci peut être réalisé avec l'annihilation de MS d'environ 1 MeV, ou avec la diffusion de MS à plusieurs niveaux d'énergie (XDM) de masse élevée, avec des sections efficaces consistantes avec la production thermique de WIMPs au début de l'Univers. La deuxième anomalie, mesurée par le satellite PAMELA et confirmée par le Large Area Telescope (LAT) de Fermi, est constituée d'une fraction de positrons qui s'élève de 10 à 200GeV et qui ne peut être expliquée par le spectre d'antimatière secondaire attendu. Quoique des études précédentes ont considéré une explication en terme de MS qui s'annihile à l'aide d'un mécanisme de Sommerfeld, nous avons été les premiers à examiner l'impact des milliers de subhalos (SH) de MS qui devraient exister selon les simulations numériques. Nous démontrons que l'inclusion des SH donne un meilleur ajustement aux données de PAMELA et Fermi, mais que ce n'est pas suffisant pour obéir aux limites établies par les observations gamma de Fermi-LAT. Nous montrons finalement qu'un seul SH très proche pourrait expliquer l'anomalie PAMELA, sans enfreindre les contraintes de rayonnement gamma et d'anisotropie dipolaire actuelles et qu'il est possible de créer un modèle de physique des particules qui produit la section efficace nécéssaire et qui est toutefois consistante avec les limites établies par la cosmologie.

The azimuthal distributions of charged particles produced in Au+Au collisions at 11.5 A&middot; GeV/c have been analyzed relative to the reaction plane orientation using the E877 experimental setup at the AGS. Details of the calibration procedures, data reduction and analysis methods are presented. With the event-by-event reconstruction of the reaction plane, a Fourier expansion is used to describe the anisotropy in particle distribution. Directed and elliptic flows are quantified by the dipole and quadrupole Fourier coefficients. A method allowing the decoupling of these two effects is introduced. Elliptic flow signals of protons, deuterons, pi +, pi--, K+, K-- have been studied as a function of particle rapidity and transverse momentum for different centralities of the collision. Transition from in-plane to out-of-plane flow as a function of particle rapidity is observed for protons and deuterons. The dependence of elliptic flow on rapidity is suggested to be a good probe for the stud of the transient pressure in the collision. The first and second order azimuthal anisotropies of charged pions and kaons have been measured. A weak in-plane elliptic flow of charged pions is detected for the first time at the AGS. The experimental results have been compared with the predictions of the RQMD event generator run in cascade and mean field modes.

In this thesis, we add a new vectorial particle to the standard model, a new particle that has the special property of having a vacuum state breaking Lorentz invariance and, therefore, CPT invariance. At the level of energy of the world we live in, this particle is expected to be in this vacuum state and we therefore should see around us some phenomenon that do not respect Lorentz invariance that comes from the interaction of this vectorial particle with ordinary matter. This thesis is presented in two parts. In the first part, we write every possible term involving our new vectorial field with ordinary matter while, in the second part, we calculate the effects of some of those terms and look at any experimental evidence for the calculated effects.

This manusript based thesis explores the consequences of string gas cosmology for the moduli problem in string theory. We compile three published papers which set up the formalism and technique of massless string gas cosmology as developed by the author in collaboration with Robert Brandenberger, and explores the consequences of such a framework for late time cosmology. We find that when consistently coupled to the massless background of string theory (given a fixed dilaton), a fluid of massless strings can stabilize all radial and shape moduli of a toroidal compactification of bosonic or heterotic string theory. This stabilization mechanism is consistent with observational bounds coming from fifth force experiments and reproduces all of the desirable features of late time cosmology. We identify the open issues associated with the string gas framework as it stands, and propose directions for future work. We also investigate the possibility that string gas cosmology might offer a dynamical solution to the moduli problem. / Cette these basee par manusript explore les consequences de la cosmologie de gaz des cordes pour le probleme de modules dans la theorie des cordes. Nous compilons trois papiers publies qui installent le formalisme et la technique de la cosmologie de gaz des cordes sans masse comme developpee par l'auteur en collaboration avec Robert Brandenberger, et explorons les consequences d'un tel cadre pour la cosmologie de temps tardives. Nous constatons qu'une fois uniformement couple au fond sans masse de la theorie de corde (donne un dilaton fixe), un fluide des cordes sans masse peut stabiliser tous les modules de radial et de forme d'un compactification toroidal de theorie bosonic ou heterotique de corde. Ce mecanisme de stabilisation est conforme aux limites d'observation venant des experiences de force cinquiemes et reproduit tous les dispositifs souhaitables de cosmologie de temps tardives. Nous identifions les issues ouvertes liees au cadre de gaz de corde sans modification, et proposons des directions pour les travaux futurs. Nous etudions egalement la possibilite que la cosmologie de gaz de corde pourrait offrir une solution dynamique au probleme de modules.