We discuss various aspects of D-branes, ranging from their basic properties as extended objects within string theory to their application, via the AdS/CFT correspondence, to the physics of gauge theories. Our initial emphasis is on the description of time-dependent, yet supersymmetric, brane configurations and to this end we first provide a review of established results on D-branes. We then investigate various supersymmetric brane intersections. Motivated by the recent results on supertubes, we investigate general constraints under which parallel brane-antibrane configurations are supersymmetric. Dual descriptions of these configurations involve systems of intersecting branes in relative motion. In particular, we find new supersymmetric configurations which are not related to a static brane intersection by a boost. In these new configurations, the intersection point moves at the speed of light. We then briefly review the AdS/CFT correspondence between string and gauge theories. Our emphasis here is on the recent development of D-brane configurations that can be used to add fundamental flavour to the gauge theories. We compute the meson spectrum of an N = 2 super Yang-Mills theory with fundamental matter from its dual string theory on AdS5 x S 5 with a D7-brane probe. For scalar and vector mesons with arbitrary R-charge the spectrum is computed in closed form by solving the equations for D7-brane fluctuations; for matter with non-zero mass mq it is discrete and exhibits a mass gap of order mq/ gsN . The spectrum of mesons with large spin J is obtained from semiclassical, rotating open strings attached to the D7-brane. It displays Regge-like behaviour for J ≪gsN , whereas for J ≫gsN it corresponds to that of two non-relativistic quarks bound by a Coulomb potential.

String theory is to date the best candidate for the "theory of everything" which would describe all four fundamental interactions, including gravity, on the same footing. The general description of black holes and other spacetimes with horizons remains a key aspect of quantum gravity that must be addressed by a consistent theory, such as string theory. Also, the resolution of singularities is a fundamental problem in a quantum theory of gravity. At the same time, the recent revolution in observational cosmology creates a pressing need to accommodate fundamental issues such as cosmic acceleration and cosmological horizons within the framework of string theory. Therefore, the focus of this thesis is on new aspects of gravitation from a point of view of string theory. In the first part of this thesis, after an introduction in the subject, we deal with the resolution of singularities in string theory. In particular, we describe one of the intrinsically stringy mechanisms to resolve singularities, namely "Enhancon mechanism". We generalize the basic enhancon solution by constructing solutions without spherical symmetry. / The second part of the thesis is devoted to investigating different aspects of holography. The AdS/CFT correspondence is a concrete realization of the holographic principle. Such correspondence is referred to as duality in the sense that the supergravity (closed string) description of D-branes and the field theory (open string) description are different formulations of the same physics. This way, the infrared (IR) divergences of quantum gravity in bulk are equivalent to ultraviolet (UV) divergences of dual field theory living on the boundary. A novel method to renormalize the stress-energy of gravity and provide a measure of gravitational mass was proposed. We used this method to study locally asymptotically anti-de Sitter spacetimes with nonzero NUT charge and charged black holes configurations in de Sitter spacetime, respectively.

Improving our ability to identify the top quark pair (tt̄) primary vertex (PV) on an event-by-event basis is essential for many analyses in the lepton-plus-jets channel performed by the Collider Detector at Fermilab (CDF) Collaboration. We compare the algorithm currently used by CDF (A1) with another algorithm (A2) using Monte Carlo simulation at high instantaneous luminosities. We confirm that A1 is more efficient than A2 at selecting the tt̄ PV at all PV multiplicities, both with efficiencies larger than 99%. Event selection rejects events with a distance larger than 5 cm along the proton beam between the tt¯ PV and the charged lepton. We find flat distributions for the signal over background significance of this cut for all cut values larger than 1 cm, for all PV multiplicities and for both algorithms. We conclude that any cut value larger than 1 cm is acceptable for both algorithms under the Tevatron's expected instantaneous luminosity improvements.

Gauge/gravity dualities and the matrix model provide us with exploratory frame-works for the study of quantum gravity (QG) in the presence of a cosmological con-stant that is negative or zero, respectively. Currently, in all the known examples instring theory, the gravity theory contains complicated matter fields and interactions.However, the idea of describing theories of quantum gravity in terms of dual non-gravitational theories, i.e. holography, has proven to go well beyond these examples.Motivated by holography, in the course of this thesis we explore simple models ofquantum gravity with the goal of obtaining insight into non-perturbative QG, in par-ticular the physics of black holes.We consider three-dimensional gravity with a negative cosmological constant. Weuse Ricci flow techniques to study flows between the potential vacua of topologicallymassive gravity and show that for large values of the Chern-Simons coupling anti-deSitter space is the ground state. Moreover, we study spontaneous symmetry-breakingin vacua of theories of gravity coupled to scalars and gauge fields and find examplesof discrete symmetry-breaking phases with exotic thermodynamic properties.We study three-dimensional gravity with a positive cosmological constant as a theoryof quantum cosmology. We compute the full non-perturbative Euclidean partitionfunction of de Sitter gravity by evaluating the gravity path integral on physicallyrelevant Euclidean continuations of the static patch. The partition function divergesin a non-regularizable way and we interpret this as a hint that the theory does notexist at a quantum level. As we show however, the inclusion of the simplest form ofinteraction, the Chern-Simons term, removes the undesirable divergences. / Les dualités gravité-théories de jauge ainsi que le modèle matriciel nous fournissent un échaffaudage pour l'exploration de la théorie quantique de la gravité avec une constante cosmologique respectivement négative ou nulle. De tous les exemples connus à ce jour qui proviennent de la théorie des cordes, la théorie gravitationnelle contient des champs de matière et des intéractions compliqués. L'idée de décrire la gravité par des théories duales non-gravitationnelles, c'est-à-dire l'holographie, peut toutefois aller bien au-delà de ces exemples. En utilisant l'holographie comme point de départ, cette thèse explore des modèles simples de gravité quantique dans le but d'obtenir une meilleure compréhension de la physique non-perturbative de celle-ci et en particulier de la physique des trous noirs.Nous considérons la gravité en trois dimensions avec une constante cosmologique négative. En utilisant des techniques de flot de Ricci, nous étudions les flots entre le potentiel du vide de gravité topologiquement massive, et nous montrons que, pour des grandes valeurs du couplage de Chern-Simons, l'état fondamental est l'espace anti-de Sitter. De plus, nous étudions le bris spontané de la symétrie dans l'état fondamental de théories de la gravité couplées à des champs scalaires et à des champs de jauge. Nous trouvons des exemples de phases de bris de symétries discrètes avec des propriétés thermodynamiques exotiques. Nous étudions finalement la gravité en trois dimensions avec une constante cosmologique positive comme modèlisation d'une théorie quantique de la cosmologie. Nous calculons la fonction de partition Euclidienne entière non-perturbative de la gravité de Sitter en évaluant l'intégrale de parcours sur des prolongements Euclidiens de la zone statique. La fonction de partition diverge de manière non régularizable, ce que nous interprétons comme un indice que cette théorie n'existe pas au niveau quantique. Nous montrons toutefois qu'avec l'inclusion du terme de Chern-Simons, soit la forme d'interaction la plus simple, les divergences indésirables disparaissent.

The interface of Cosmology and High Energy physics is a forefront area of research which is constantly undergoing development. This thesis makes various contributions to this endeavor. String-inspired cosmology is the subject of the first part of the thesis, where we propose both a new inflationary and a new alternative cosmological model. The second part of the thesis concentrates on the problems of integrating cosmology with particle physics beyond the Standard Model. Inspired by new opportunities due to stringy degrees of freedom, we propose a non-inflationary resolution of the entropy and horizon problems. In this string-inspired scenario, 'our' dimensions expand while the extra dimensions first expand and then contract, before eventually stabilizing. The equation of state of the bulk matter (which consists of branes) is negative. Hence, there is a net gain in the total energy of the universe during the pre-stabilization phase. At the end of this phase, the energy stored in the branes is converted into radiation. The result is a large and dense 3-dimensional universe. Making use of similar ideas, we propose a not-fine-tuned model of brane inflation. In this scenario the brane separation, playing the role of the inflaton, is the same as the overall volume modulus. The bulk matter provides an initial expansion phase which drives the inflaton up its potential, so that the conditions for inflation are realized. The specific choice of the inflationary potential nicely fits the cosmological observations. Another aspect of this research concentrates on the cosmological moduli problem: namely, the existence of weakly coupled particles those decay is late enough to interfere with Big Bang Nucleosynthesis. As a solution, we suggest parametric and tachyonic resonances to shorten the decay time. Even heavy moduli are dangerous for cosmology if they cause the overproduction of gravitinos. We find that tachyonic decay channels help to transfer most of the energy of thes / L'interface entre la Cosmologie et la Physique des hautes énergies est un sujet de recherche d'avant-plan en constant développement. La cosmologie inspirée par la théorie des cordes est le sujet de la première partie de cette thèse, dans laquelle nous proposons d'une part un nouveau mécanisme pour l'inflation et d'autre part une nouvelle alternative de modèle cosmologique. Dans la seconde partie nous nous concentrons sur les problèmes reliés à l'intégration de la cosmologie dans un modèle de physique des particules au-delà du Modèle Standard. Motivés par les nouvelles possibilités venant des degrés de liberté de la théorie des cordes, nous proposons une résolution non-inflationiste aux problèmes d'entropie et d'horizon. Selon notre scenario fondé sur la théorie des cordes, les trois dimensions spatiales habituelles ainsi que les dimensions supplémentaires s'étendent, mais ces dernières se contractent eventuellement avant de se stabiliser. L'équation d'état de la matière du bulk, qui consiste de branes, est négative. Il y a donc un net gain dans l'énégie totale de l'univers durant la phase de pré-stabilisation. A la fin de cette phase, l'énergie stockée dans les branes est convertie en radiation. Le résultat est un univers tri-dimensionel large et dense. En utilisant des idées similaires, nous proposons un modèle d'inflation qui ne requiert pas d'ajustements fins. Dans ce scénario, la séparation entre les branes, qui joue le rôle de l'inflaton, est la même que le module du volume global. La matière du bulk fournit une phase d'expansion initiale qui pousse l'inflaton vers le haut de son potentiel, réalisant ainsi les conditions pour l'inflation. Le choix spécifique du potentiel de l'inflaton est en accord avec observations cosmologiques. Un autre aspect de ma these adresse le problème cosmologique des champs de module: c'est-à-dire l'existence de particules faiblement couplées dont la désintégration a li

A Kalman filter track fit system for reconstructing charged particle track parameters from solenoidal tracking chamber data has been constructed. A supporting simulation of particle transport and detector response in an all stereo wire drift chamber such as that proposed for the B Factory project has been written based on the Gismo package. Basic fitting operation has been verified, however the effects of particle interactions with detector material have yet to be incorporated into the filter. The filter system has been written usings an object oriented language in the hopes of facilitating its application to various future applications at the B Factory and elsewhere.

The existence of a charged Higgs boson is hypothesised in different extensions of new physics beyond the Standard Model of particle physics. This thesis presents a search for charged Higgs bosons decaying to top and bottom quarks that was carried out using data collected by the DØ experiment at the Fermi National Accelerator Laboratory in Batavia, IL. The search investigated the existence of charged Higgs bosons in the mass range 180 ≤ MH+ ≤ 300 GeV/c2, and in the context of different two-Higgs-doublet model extensions to the Standard Model. Nearly 1 fb−1 of data from proton-antiproton collisions at the centre-of-mass energy √s=1.96 TeV were analysed. No evidence for the existence of a charged Higgs boson was found. Upper limits on production cross-sections were derived for Types I, II, and III two- Higgs-doublet models. Excluded regions in the (MH+, tan β) plane for Type I were calculated. / L'existence d'un boson chargè de Higgs est suggérée hypothétiquement dans différentes extensions de la nouvelle physique allant au-delà du Modèle Standard de la physique des particules. Cette thèse présente une recherche de bosons chargés de Higgs se désintégrant en quarks top et bottom dans les données de l'expérience DØ au Fermi National Accelerator Laboratory à Batavia, IL. La fouille investigue l'existence de bosons chargés de Higgs ayant une masse 180 ≤ MH+ ≤ 300 GeV/c2, et dans le contexte de différentes extensions du Modèle Standard à doublets de deux Higgs. Près de 1 fb−1 de données provenant de collisions proton-antiproton à une énergie de centre de masse √s=1.96 TeV ont été analysées. Aucune évidence de l'existence de bosons chargés n'a été trouvée. Des limites supérieures sur la section efficace de production ont été dérivées pour les Types I, II et III des modèles à doublets de deux Higgs. Des régions d'exclusion dans le plan (MH+, tan β) pour le Type I ont été calculées.

This thesis presents a first study of various aspects of $K sp0$ and $ Lambda$ production in neutral current deep inelastic scattering of 27.5 GeV positrons and 820 GeV protons. The data were accumulated with the ZEUS detector at HERA in the 1994 run period. The average multiplicities for the neutral strange particle production as functions of pseudorapidity $ eta$ and transverse momentum $p sb{t}$ have been determined in the kinematic range 10 $GeV sp20.04.$ The particle and energy flow with respect to the struck quark direction, as well as the dependence of the average production rate as function of the four-momentum transfer $Q sp2$ and the hadronic centre of mass energy W of the ep scattering process have been studied. The K$ sp0$ and $ Lambda$ multiplicities in the kinematic range 10 $GeV sp20.04$ are compared with the results from data from the 1993 run period with 26.7 GeV electrons and 820 GeV protons. The production mechanisms of $K sp0$ in events with and without large rapidity gaps with respect to the proton direction are compared in order to probe differences in diffractive and non-diffractive scattering processes.

Quantum Chromodynamics is the underlying theory of hadrons and their interactions. In deriving results from this theory one relies on perturbative calculations. Sometimes indirect methods have been explored to circumvent direct calculation of pure gluonic amplitudes. For example, it has been shown that supersymmetric extension of QCD along with supersymmetric Ward identities can be used to establish relations between amplitudes with the same total number of particles but a different number of gluons. Such relations are used here to connect pure gluonic and pure fermionic amplitudes in the case of 4-pt and 6-pt functions. These relations offer an indirect way of calculating tree level pure gluonic amplitudes since these amplitudes are identical in supersymmetric and non-supersymmetric QCD. The aforementioned relations however, provide no insight into the relation between Feynman diagram of the amplitudes involved. In this regard, we investigate the relation between individual Feynman diagrams in the Wess-Zumino model. / Another calculational difficulty arises when one is concerned with high energy scattering in QCD. In the high energy regime, because the effective coupling constant is relatively large, it is necessary to sum up an infinite number of diagrams. This is made even more difficult due to the cancellations in certain color channels that occurs at any perturbative order. The new non-abelian cut diagram technique provides considerable assistance by giving the result with the cancellations already built into its rules. Sixth-order calculations are carried out to show the efficiency of this technique. Finally, we consider the question of diagram with fermion loops that need regularization because of their UV divergence. We find that regularization leads to an enhancement in their high energy behavior.

In this thesis we investigate three approaches to generating new solutions to low energy string theory. The first one consists in gauging WZW models. We construct a gauged extension of a model first introduced by Witten and Nappi. Then we look at Garfinkle's wave generating technique and apply it on a known five-dimensional black string to generate a new family of solutions. We investigate their properties with a special emphasis on singularities. Finally, in the last part, we study O(d, d + p) transformations and duality and we explore the effect of gauge transformations on those transformations. With the latter, we are able to answer a question raised by Burgess, Myers and Quevedo on this issue.