With the experiments at the Large Hadron Collider, physicists are in a better position than ever to test the Standard Model of particle physics and what might lie beyond it. In this thesis, we investigate one possibility of physics beyond the Standard Model: supersymmetry. We discuss phenomenological aspects of the theory and describe a series of software that allows us to look for signs of supersymmetry in particle physics experiments. Using this framework, we do a sample analysis in the $m_0$-$m_{1/2}$ plane of the CMSSM, producing a combined likelihood exclusion band based on results of the ATLAS experiment from $35 \> \text{pb}^{-1}$ of $7$ TeV proton-proton collisions. / Avec le Grand collisionneur de hadrons, nous sommes en meilleure position que jamais pour tester le Modèle Standard de la physique des particules, ainsi que ses possibles extensions. Dans cette thèse, nous considérons une de ces possibilités, la supersymétrie, une théorie qui va au-delà du Modèle Standard. Nous présentons des aspects phénoménologiques de cette théorie ainsi que des programmes qui nous permettent d'en chercher les signes dans des collisions de protons. À l'aide de ce cadre de travail, nous faisons une analyse dans le plan $m_0$-$m_{1/2}$ du CMSSM, et produisons une bande d'exclusion à partir des resultats d'ATLAS pour $35 \> \text{pb}^{-1}$ de collisions de protons à $7$ TeV d'énergie.

We compute the integrated Sachs--Wolfe type contribution to the CMB polarization power spectrum from cosmic string wakes. An introduction to topological defects, cosmic strings, CMB polarization, and spin-s fields is given. We then use these tools to compute the angular power spectrum of E- and B-mode polarization due to cosmic string wakes, in the flat sky limit. We find that cross-correlation terms (i.e., EB, ET, BT) vanish, while the EE and BB power spectra are equal in shape and magnitude. This result is in stark contrast with B-mode polarization from Gaussian fluctuations, which vanishes identically. However, we find that the shape of the power spectrum from cosmic string wakes is very similar to the predicted B-mode power spectrum from gravitational lensing, but with a small overall amplitude. As such, the cosmic string wake signal is too small to be picked out from lensing, and background subtraction techniques would be very difficult. We find that the peak amplitude in the dimensionless power spectrum (sqrt(l(l+1)C_l/2pi)) is about 10^(-4) at a peak value of l~400.We briefly study the asymptotic version of the power spectrum and find that C_l is approximately constant until a turn around point set by the angular size of the dominant wakes. We then determine which cosmic string wakes give rise to the dominant contribution to the full, integrated power spectrum and find that the dominant wakes are those which were formed at the time of equal matter and radiation, and which intersect our past light cone at recombination. We conclude that the Fourier space signal (power spectrum) is too weak to detect cosmic string wakes in existing data, and future searches should focus on distinct position space features. / Nous calculons la contribution intégrée au spectre de puissance de la polarisation du fond diffus cosmique (CMB) grâce aux sillages des cordes cosmiques. Une introduction aux défauts topologique, cordes cosmiques, de polarisation du CMB, et aux champs spin–s est donnée. Nous utilisons ensuite ces outils pour calculer le spectre de puissance angulaire de la polarisation des modes E et B grâce aux des sillages des cordes cosmiques, dans la limite du ciel plat. Nous trouvons que les termes de corrélation croisée (EB, ET, BT) sont zero, tandis que les spectres de puissance pour EE et BB ont la même forme et grandeur. Ce résultat contraste le polarisation du mode B causée par les fluctuations Gaussiennes, qui disparaîssent. Cependant, nous trouvons que la forme du spectre de puissance causée par les sillages de cordes cosmiques est très similaire au spectre en mode B prédite par lentille gravitationnelle, mais avec une amplitude globale trés faible. Donc, le signal du sillage de corde cosmique est trop faible pour être measuré au-dessus l'indicatif du lentille. Aussi, les méthodes de soustraction du fond serait très difficile. Nous trouvons que l'amplitude maximum dans le spectre de puissance sqrt(l(l+1)C_l/2pi) est d'environ 10^(-4) pour l~400.Nous étudions brièvement le forme asymptotique du spectre de puissance, et nous trouvons que Cl est quasi–constant en l jusqu'à un tournant correspondant à la taille angulaire du sillage le plus important. Nous déterminons alors quel sillage donne la contribution dominante au spectre de puissance complet. Nous trouvons que les sillages dominants sont ceux qui ont été formés à l'epoque de l'égalité de matière et de rayonnement, et qui croisent notre cône de lumière passé à l'époque de recombinaison. Nous concluons que l'indicatif dans l'espace Fourier est trop faible pour être détecté. Donc, les recherches futures devraient se concentrer sur les caractéristiques l'espace spatial.

We first present a brief overview of the quark-gluon plasma (QGP) and its possible production in a heavy ion collision. Equilibrium quantum field theory is then outlined as a tool to study QGP. With this, we investigate the consequence of a finite charge density plasma through the breaking of Furry's theorem. In particular, we calculate the dilepton differential production rate for one of the simplest medium-induced processes: 2g → ll¯. We show that this effect is sub-dominant to the leading tree-level qq¯ → ll¯.

The evidence for an accelerating expansion of our Universe at the present epoch have led us to look for an energy component which could generate it. That may consist in quintessence in the form of a system of scalar fields {φa} rolling down a potential V({φ a}). This master's thesis presents the equations relevant to the cosmological evolution of such a system, along with the different conditions required for a viable cosmology. Also, via the exploration of two distinct models, two fundamental problems of quintessence theory are outlined. The impossibility of escaping the fine-tuning of initial conditions is first presented in link to the Liouville's theorem, and second the difficulty of avoiding the coincidence of the recent rise of quintessence even in the presence of attractors is shown.

Inspired by the Randall-Sundrum (RS) solution to the weak-scale hierarchy problem we present a new five-dimensional brane-world model. While many cosmological implications of RS-type scenarios have been considered, the question of inflation is still relatively unexplored. In this work we investigate the dynamics induced by fluctuations in the extra dimension and find that the so-called radion can be used for inflation. A class of models are found which simultaneously allow for radius stabilization by utilizing a bulk scalar field with a potential in the bulk and on the branes. The coupled radion-scalar field system is analyzed from the five-dimensional point of view and is seen to consistently reduce to a four-dimensional Friedmann-Robertson-Walker-like cosmology. An analysis of the constraints imposed during inflation fixes the radion mass and we speculate on the possibility that the new physics at work here may be distinguishable from typical four-dimensional models.

Photoproduction events in ep collisions with photon virtuality Q2, the negative squared four-momentum of the exchanged photon, less than 1 GeV2, and at least two jets of high transverse energy have been studied in the photon and proton center-of-mass range Wgp of 134 < Wgp < 277 GeV with the ZEUS detector at HERA. The data sample was collected during the 1996 and 1997 running periods with an integrated luminosity of 38.7 pb-1 allowing for a precise measurement of differential cross sections in a kinematic regime unreachable for experiments at current e+e- colliders. For the determination of these differential cross sections, both jets were required to have a transverse energy larger than 11 GeV with at least one having more than 14 GeV. Differential hadron cross sections were extracted for these dijet events using separate resolved and direct enhanced subsamples. The cross sections have been compared to NLO QCD calculations using several different parametrisations of the photon structure functions within these calculations. The achieved sensitivity of the data at the photon structure functions allows to test the validity of the used parametrisations of the photon structure functions. This test shows the need for improvements of those parametrisations in the kinematical range under investigation. The extracted data at low xOBSg constrains the parton densities in the photon which can be exploited in future parametrisations of the photonic parton densities.

At the HERA particle accelerator, 27.5 GeV electrons collide with 920 GeV protons. In the photoproduction mode, the quasi-real exchange photon emitted by the electron interacts with the proton at small momentum transfer. Such events are selected and investigated with the ZEUS detector in the photon proton center-of-mass range 130 GeV < Wgamma p < 270 GeV using the 55.1 pb-1 gated luminosity of the year 2000 data. The two typical photoproduction processes are studied separately: in the direct case photons can interact directly with the proton, while in the resolved case the photon is resolved into its hadronic components. An inclusive data sample of two parton jet events with each a minimum transverse energy of 6 GeV and a third jet with a minimum transverse energy of 2 GeV are reconstructed and selected using the inclusive kT clustering algorithm. The third jet travelling in the electron propagation direction can be assumed to be the photon remnant jet. Jet transverse energy density distributions, which are also known as 'jet shapes' are calculated on the jet sample. The photon remnant jets are found to be broader than the parton jets.

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&macr; 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.