This thesis deals with phenomenological and theoretical aspects of cosmoparticle physics and string theory. There are many open questions in these topics. In connection with cosmology we would like to understand the detailed properties of dark matter, dark energy, generation of primordial perturbations, etc., and in connection with particle physics we would like to understand the detailed properties of models that stabilize the electroweak scale, for instance supersymmetry. At the same time, we also need to understand these issues in a coherent theoretical framework. Such a framework is offered by string theory. In this thesis, I analyze the interplay between Higgs and dark matter physics in an effective field theory extension of the minimally supersymmetric standard model. I study a theory of modified gravity, where the graviton has acquired a mass, and show the explicit implementation of the Vainshtein mechanism, which allows one to put severe constraints on the graviton mass. I address the question of Planck scale corrections to inflation in string theory, and show how such corrections can be tamed. I study perturbations of warped throat regions of IIB string theory compactifications and classify allowed boundary conditions. Using this analysis, I determine the potential felt by an anti-D3-brane in such compactifications, using the explicit harmonic data on the conifold. I also address questions of perturbative quantum corrections in string theory and calculate one-loop corrections to the moduli space metric of Calabi-Yau orientifolds. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows:<strong>  </strong>Paper 3: Manuscript. Paper 5: Manuscript. Paper 6: Manuscript.</p>

Recent measurements show that dark matter makes up at least one fifth of the total energy density of the Universe. The nature of the dark matter is one of the biggest mysteries in current particle physics and cosmology. Big Bang nucleosynthesis limits the amount of baryonic matter that can exist, and shows that the dark matter has to be non-baryonic. Particle physics provides some candidates for non-baryonic matter that could solve the dark-matter problem, weakly interacting massive particles (WIMPs) being the most popular. If these particles were created in the early Universe a substatial relic abundance would exist today. WIMPs in our galactic halo could be gravitationally bound in the Solar System and accumulate inside heavy bodies like the Earth. Supersymmetric extensions to the Standard Model give a viable WIMP dark matter candidate in the form of the lightest neutralino. This thesis describes an indirect search for WIMPs by the neutrino signature from neutralino annihilation at the core of the Earth using the AMANDA detector. As opposed to previous dark matter searches with AMANDA, this work focuses on the hypothesis of a relatively light WIMP particle with mass of 50-250GeV/c2 The AMANDA neutrino telescope is an array of photomultiplier tubes installed in the clear glacier ice at the South Pole which is used as Cherenkov medium. Data taken with AMANDA during the period 2001-2003 is analyzed. The energy threshold of the detector is lowered by the use of a local correlation trigger, and the analysis is taylored to select vertically upgoing low energy events. No excess above the expected atmospheric neutrino background is found. New limits on the flux of muons from WIMP annihilations in the center of the Earth are calculated.

The appearance in string theory of higher-dimensional objects known as D-branes has been a source of much of the interesting developements in the subject during the past ten years. A very interesting phenomenon occurs when several of these D-branes are made to coincide: The abelian gauge theory living on each brane is enhanced to a non-abelian gauge theory living on the stack of coincident branes. This gives rise to interesting effects like the natural appearance of non-commutative geometry. The theory governing the dynamics of these coincident branes is still poorly understood however and only hints of the underlying structure have been seen. This thesis focuses on an attempt to better this understanding by writing down actions for coincident branes using so-called boundary fermions, originating in considerations of open strings, instead of matrices to describe the non-abelian fields. It is shown that by gauge-fixing and by suitably quantizing these boundary fermions the non-abelian action that is known, the Myers action, can be reproduced. Furthermore it is shown that under natural assumptions, unlike the Myers action, the action formulated using boundary fermions also posseses kappa-symmetry, the criterion for being the correct supersymmetric action for coincident D-branes. Another aspect of string theory discussed in this thesis is that of tensionless strings. These are of great interest for example because of their possible relation to higher spin gauge theories via the AdS/CFT-correspondence. The tensionless superstring in a plane wave background, arising as a particular limit of the near-horizon geometry of a stack of D3-branes, is considered and compared to the tensile case.

A relic density of Weakly Interacting Massive Particles (WIMPs) remaining from the Big Bang constitutes a promising solution to the Dark Matter problem. It is possible for such WIMPs to be trapped by and accumulate in gravitational potentials of massive dense objects such as the Sun. A perfect WIMP candidate appears in certain supersymmetric extensions to the Standard Model of particle physics, where the lightest supersymmetric particle is a neutralino which can be stable, massive and weakly interacting. The neutralinos may annihilate pair-wise and in these interactions neutrinos with energies ranging up to the neutralino mass can be indirectly produced. Hence, a possible population of dark matter neutralinos trapped in the Sun can give rise to an observable neutrino flux. The Antarctic Muon And Neutrino Detector Array, AMANDA, is a neutrino telescope that detects Cherenkov light emitted by charged particles created in neutrino interactions in the South Pole glacial ice sheet using an array of light detectors frozen into the deep ice. In this work data taken with the AMANDA-II detector during 2003 are analyzed to measure or put upper bounds on the flux of such neutrinos from the Sun. In the analysis detailed signal and background simulations are compared to measurements. Background rejection filters optimized for various neutralino models have been constructed. No excess above the background expected from neutrinos and muons created in cosmic ray interactions in the atmosphere was found. Instead 90% confidence upper limits have been set on the neutralino annihilation rate in the Sun and the muon flux induced by neutralino signal neutrinos.

A generator-level study of dilepton ttbar events to search for light charged Higgs bosons at the LHC is presented with the aim of finding a new variable to discriminate between charged Higgs signals and Standard Model signatures. The spin effects in leptonic decays of H± and W bosons are studied and a new angular distribution is identified as a discriminating variable. Studies are performed for different charged Higgs masses at two different center ofmass energies. Monte Carlo samples of Z+jets production are also used to test the analysis.

In partial fulfillment of the requirements for the "Habilitation a Diriger des Recherches" at Pierre et Marie Curie University in Paris

This thesis presents the results of a signature-based search for new physics using the exclusive dijet plus missing transverse energy data sample from 2 fb^-1 of p-pbar collisions at \sqrt{s} = 1.96 TeV collected with the Collider Detector at Fermilab (CDF). A study is made of the production of events with two high energy jets and large missing transverse energy (missing E_T, or MET) in a kinematic regime requiring the scalar sum of the E_T of the two jets (referred to here as H_T) to be greater than 125 GeV and the event MET to be above 80 GeV. A second kinematic region is also examined, with the MET cut increased to 100 GeV and the H_T cut increased to 225 GeV. The number of events observed in the data is within 0.43 standard deviations of the expected number of background events in the low kinematic region, and within 0.34 standard deviations in the high kinematic region. Based on these results, 95% C.L. lower mass limits for scalar leptoquarks are extracted: 190 GeV/c^2 for 1st generation, 190 GeV/c^2 for 2nd generation, and 178 GeV/c^2 for 3rd generation production. The results are also interpreted in terms of cross-section limits on generic minimal supersymmetric (MSSM) models.

Superspace is a fundamental tool in the study of supersymmetry, one that while often used is seldom defined with a proper amount of mathematical rigor. This paper examines superspace and presents three different constructions of it; the original by Abdus Salam and J. Strathdee as well as two modern methods by Alice Rogers and Buchbinder-Kuzenko.Though the structures arrived at are the same the two modern constructions differ in methods, elucidating different important aspects of super-space. Rogers focuses on the underlying structure through the study of supermanifolds, and Buchbinder-Kuzenko the direct correlation with the Poincare superalgebra, and the parametrisation in terms of exponents.

This thesis presents the results of a signature-based search for new physics using the exclusive dijet plus missing transverse energy data sample from 2 fb^-1 of p-pbar collisions at \sqrt{s} = 1.96 TeV collected with the Collider Detector at Fermilab (CDF). A study is made of the production of events with two high energy jets and large missing transverse energy (missing E_T, or MET) in a kinematic regime requiring the scalar sum of the E_T of the two jets (referred to here as H_T) to be greater than 125 GeV and the event MET to be above 80 GeV. A second kinematic region is also examined, with the MET cut increased to 100 GeV and the H_T cut increased to 225 GeV. The number of events observed in the data is within 0.43 standard deviations of the expected number of background events in the low kinematic region, and within 0.34 standard deviations in the high kinematic region. Based on these results, 95% C.L. lower mass limits for scalar leptoquarks are extracted: 190 GeV/c^2 for 1st generation, 190 GeV/c^2 for 2nd generation, and 178 GeV/c^2 for 3rd generation production. The results are also interpreted in terms of cross-section limits on generic minimal supersymmetric (MSSM) models.

Dark matter has been shown to be extremely abundant in our universe. It comprises about 23 percent of the energy density of the entire universe, which is more than five times greater than the regular matter we already know about. Dark matter cannot be explained within the Standard Model of particle physics. However, models which extend the Standard Model, such as supersymmetry, can explain dark matter. This dissertation investigates the signals of some supersymmetry models in the context of collider physics. If dark matter particles or other supersymmetry particles are produced at some collider experiment, such as the Large Hadron Collider, it is important to know how we can find and measure the signatures and properties of these particles. This dissertation provides some measurement techniques for that exact purpose. These measurement techniques are also very general, making them useful for examining other models of particle physics as well. Lastly, if the supersymmetry model can be understood well enough from collider data, the connection back to cosmology can be made. Namely, it is possible to determine (from LHC data and using a standard cosmological calculation) the abundance of dark matter in the universe. Comparing this collider value with the value already measured will be a crucial step in understanding dark matter. This dissertation provides simulated results of this dark matter abundance calculation for a number of supersymmetry model points.