Current estimates put Dark Matter to 26% of the energy-matter content of the universe, but very little is known about it other than its gravitational interactions. Eorts to learn more about Dark Matter include searching for it at high energy particle colliders. The lack of information about the nature of Dark Matter makes this a complicated task, and many searches are performed in dierent channels, and considering dierent theoretical models. In this thesis, I explore two such analyses, performed in the ATLAS collaboration using data from the ATLAS detector at the Large Hadron Collider at CERN: the tW+MET (missing transverse energy) nal state and the tt+MET nal state. I have made a generation-level study of the overlap between the signal regions used, and come to the conclusion that there is some. I have also compared the models used in these analyses, the 2HDM+a and the simplied spin-0 pseudoscalar model. Given the simplications made in my study, however, more sophisticated approaches should be used before anything conclusive can be said.

In this work I used a code of galaxy formation (GALICS) to explore three different points that are relevant to the the problem of galaxy formation. The first deals with the method of simulation itself, the second considers the simulation of infrared galaxies within GALICS and the third investigated the role of galaxy formation in measurements of weak gravitational lensing.<br /><br />I examined the predictability of models such as GALICS, using an exploratory test, based on the response of the model to variations in the parameters controlling star formation. I have also proposed a new description, in the astrophysical context, of merger trees.<br /><br />I helped to implement new physical prescriptions into GALICS to bring the model into better agreement with the available observations of infrared galaxies. <br /><br />I wrote a code (LEMOMAF) to simulate the weak lensing effect, taking advantage of the coevolution of galaxies and dark matter included in GALICS.

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 quest for the first detection of a galaxy cluster in the high energy gamma-ray regime is ongoing, and even though clusters are observed in several other wave-bands, there is still no firm detection in gamma-rays. To complement the observational efforts we estimate the gamma-ray contributions from both annihilating dark matter and cosmic-ray (CR) proton as well as CR electron induced emission. Using high-resolution simulations of galaxy clusters, we find a universal concave shaped CR proton spectrum independent of the simulated galaxy cluster. Specifically, the gamma-ray spectra from decaying neutral pions, which are produced by CR protons, dominate the cluster emission. Furthermore, based on our derived flux and luminosity functions, we identify the galaxy clusters with the brightest galaxy clusters in gamma-rays. While this emission is challenging to detect using the Fermi satellite, major observations with Cherenkov telescopes in the near future may put important constraints on the CR physics in clusters. To extend these predictions, we use a dark matter model that fits the recent electron and positron data from Fermi, PAMELA, and H.E.S.S. with remarkable precision, and make predictions about the expected gamma-ray flux from nearby clusters. In order to remain consistent with the EGRET upper limit on the gamma-ray emission from Virgo, we constrain the minimum mass of substructures for cold dark matter halos. In addition, we find comparable levels of gamma-ray emission from CR interactions and dark matter annihilations without Sommerfeld enhancement. / At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Accepted.

Dark matter, a non-luminous, non-baryonic matter, is thought to constitute 23 % of the matter-energy components in the universe today. Except for its gravitational effects, the existence of dark matter has never been confirmed by any other means and its nature remains unknown. If a hypothetical Weakly Interacting Massive Particle (WIMP) were in thermal equilibrium in the early universe, it could have a relic abundance close to that of dark matter today, which provides a promising particle candidate of dark matter. Minimal Super-Symmetric extensions to the standard model predicts a stable particle with mass in the range 10 GeV/c 2 to 1000 GeV/c 2 , and spin-independent cross-section with ordinary matter nucleon σ x ∠ 1 × 10 -43 cm 2 . The XENON100 experiment deploys a Dual Phase Liquid Xenon Time Projection Chamber (LXeTPC) of 62 kg liquid xenon as its sensitive volume, to detect scintillation ( S1 ) and ionization ( S2 ) signals from WIMP dark matter particles directly scattering off xenon nuclei. The detector is located underground at Laboratori Nazionali del Gran Sasso (LNGS) in central Italy. 1.4 km of rock (3.7 km water equivalent) reduces the cosmic muon background by a factor of 10 6 . The event-by-event 3D positioning capability of TPC allows volume fiducialization. With the self-shielding power of liquid xenon, as well as a 99 kg liquid xenon active veto, the electromagnetic radiation background is greatly suppressed. By utilizing the difference of ( S2/S1 ) between electronic recoil and nuclear recoil, the expected WIMP signature, a small nuclear recoil energy deposition, could be discriminated from electronic recoil background with high efficiency. XENON100 achieved the lowest background rate (∠ 2.2 × 10 -2 events/kg/day/keV) in the dark matter search region (∠ 40 keV) among all direct dark matter detectors. With 11.2 days of data, XENON100 already sets the world's best spin-independent WIMP-nucleon cross-section limit of 2.7 × 10 -44 cm 2 at WIMP mass 50 GeV/c 2 . With 100.9 days of data, XENON100 excludes WIMP-nucleon cross-section above 7.0 × 10 -45 cm 2 for a WIMP mass of 50 GeV/c 2 at 90% confidence level.

In this thesis we investigate two approaches to quantum gravity. The first is the emergence of gravity from a discrete fundamental theory, and the second is the direct quantisation of gravity. For the first we develop tools to determine with relatively high accuracy the speed of propagation of information in collective modes which ultimately should give us some information about the emergent causal structure. We found a way of finding the dependence on the relative interaction strengths of the Hamiltonian and we also managed to calculate this speed in the case where the operators in the Hamitonian were not necessarily bounded. For the second approach, we investigated the phenomenology of Loop Quantum Gravity. We found that ultra light black holes (lighter than the Planck mass) have interesting new properties on top of being non-singular. First their horizon is hidden behind a Plancksized wormhole, second their specific heat capacity is positive and they are quasi-stable, they take an infinite amount of time evaporate. We investigated the dynamics of their collapse and evaporation explicitly seeing that not only was there no singularity, but there is also no information loss problem. Looking at how primordial black holes were in existence, we found that they might account for a significant portion of dark matter. And if they did, their radiation spectrum is such that the black holes in the dark matter halo of our galaxy could be the source for the ultra high energy cosmic rays we observe on earth.

This thesis addresses the challenge of understanding the typical star formation histories of red sequence galaxies, using linestrength indices, mass-to-light ratios and redshift evolution as complementary constraints on their stellar age distribution. We first construct simple parametric models of the star formation history that bracket a range of scenarios, and fit these models to the linestrength indices of low-redshift cluster red-sequence galaxies. For giant galaxies, we con firm the downsizing trend, i.e. the stellar populations are younger, on average, for lower σ galaxies. We find, however, that this trend flattens or reverses at σ < 70km/s. We then compare predicted stellar mass-to-light ratios with dynamical mass-to-light ratios derived from the Fundamental Plane, or by the SAURON group. For galaxies with σ ~ 70 km/s, models with a late 'frosting' of young stars and models with exponential star formation histories have stellar mass-to-light ratios that are larger than observed dynamical mass-to-light ratios by factors of 1.7 and 1.4, respectively, and so are rejected. The single stellar population (SSP) model is consistent with the Fundamental Plane, and requires a modest amount of dark matter (between 20% to 30%) to account for the difference between stellar and dynamical mass-to-light ratios. A model in which star formation was 'quenched' at intermediate ages is also consistent with the observations, although in this case less dark matter is required for low mass galaxies. We also find that the contribution of stellar populations to the 'tilt' of the Fundamental Plane is highly dependent on the assumed star-formation history: for the SSP model, the tilt of the FP is driven primarily by stellar-population effects. For a quenched model, two-thirds of the tilt is due to stellar populations and only one third is due to dark matter or non-homology. Comparing to high redshift cluster data, we find again the SSP and quenched models, as well as a model where strangulation begins at intermediate ages after a period of constant star formation, are preferred. They predict the recent faint-end build up of the red sequence, along with observed dwarf-to-giant ratios. Only the SSP model appears to predict the observed M/L evolution, but only if selection effects are carefully modeled.

This thesis addresses the challenge of understanding the typical star formation histories of red sequence galaxies, using linestrength indices, mass-to-light ratios and redshift evolution as complementary constraints on their stellar age distribution. We first construct simple parametric models of the star formation history that bracket a range of scenarios, and fit these models to the linestrength indices of low-redshift cluster red-sequence galaxies. For giant galaxies, we con firm the downsizing trend, i.e. the stellar populations are younger, on average, for lower σ galaxies. We find, however, that this trend flattens or reverses at σ < 70km/s. We then compare predicted stellar mass-to-light ratios with dynamical mass-to-light ratios derived from the Fundamental Plane, or by the SAURON group. For galaxies with σ ~ 70 km/s, models with a late 'frosting' of young stars and models with exponential star formation histories have stellar mass-to-light ratios that are larger than observed dynamical mass-to-light ratios by factors of 1.7 and 1.4, respectively, and so are rejected. The single stellar population (SSP) model is consistent with the Fundamental Plane, and requires a modest amount of dark matter (between 20% to 30%) to account for the difference between stellar and dynamical mass-to-light ratios. A model in which star formation was 'quenched' at intermediate ages is also consistent with the observations, although in this case less dark matter is required for low mass galaxies. We also find that the contribution of stellar populations to the 'tilt' of the Fundamental Plane is highly dependent on the assumed star-formation history: for the SSP model, the tilt of the FP is driven primarily by stellar-population effects. For a quenched model, two-thirds of the tilt is due to stellar populations and only one third is due to dark matter or non-homology. Comparing to high redshift cluster data, we find again the SSP and quenched models, as well as a model where strangulation begins at intermediate ages after a period of constant star formation, are preferred. They predict the recent faint-end build up of the red sequence, along with observed dwarf-to-giant ratios. Only the SSP model appears to predict the observed M/L evolution, but only if selection effects are carefully modeled.

The low-energy supersymmetry phenomenology for specific classes of string compactifications is investigated given that the low-energy physics may provide a clue as to the structure of the fundamental theory at high energy scales. The one-parameter model (OPM), a highly constrained subset of minimal Supergravity where all the soft-supersymmetry breaking terms may be fixed in terms of the gaugino mass, is studied, in addition to a three-family Pati-Salam model constructed from intersecting D6-branes. Furthermore, the phenomenology of gravity mediated supersymmetry breaking F-theory SU(5) and SO(10) models, as well as F-SU(5) models with vector- like particles, are examined. We determine the viable parameter space that satisfies all the latest experimental constraints, including the most recent WMAP relic neutralino abundance observations, and find it to be consistent with the CDMS II and other concurrent direct-detection experiments. Moreover, we compute the gamma-ray flux and cross-sections of neutralino annihilations into gamma-rays and compare to the published Fermi-LAT satellite telescope measurements. In F-theory SU(5) and SO(10) models, we predict the exact small deviation of the gaugino mass relation at two-loop level near the electroweak scale, which can be tested at the colliders. More- over, in F-SU(5), we predict the precise deviations from the mSUGRA gaugino mass relations due to the presence of the vector-like particles, also testable at the colliders. The compilation of all these results form a comprehensive collection of predictions with which to evaluate these string models alongside anticipated experimental dis- coveries in the coming decade.