The reactions $ gamma gamma$ $ to$ 2$ pi sp+2 pi sp-2 pi sp0$ and $ gamma gamma to pi sp+ pi sp- pi sp0 pi sp0$ have been studied by using the ARGUS detector at the $e sp+e sp-$ storage ring DORIS II at DESY. / In the $2 pi sp+2 pi sp-2 pi sp0$ final state the production of $ omega$-mesons is observed and in particular the reaction $ gamma gamma$ $ to$ $ omega omega$ is seen for the first time. The cross section for $ gamma gamma to omega omega$ is found to have an enhancement at $ sim$1.9GeV/c$ sp2$ of about 12 nb. The topological cross sections for the reactions $ gamma gamma$ $ to$ 2$ pi sp+2 pi sp-2 pi sp0$ and $ gamma gamma$ $ to$ $ omega pi sp+ pi sp- pi sp0$ are also measured. / The production of charged $ rho$-mesons is observed in the $ pi sp+ pi sp- pi sp0 pi sp0$ final state. The cross section for the reaction $ gamma gamma$ $ to$ $ rho sp+ rho sp-$ is measured for the first time. The cross section did not show a threshold enhancement similar to that found in the reaction $ gamma gamma$ $ to$ $ rho sp0 rho sp0$ and is about a factor of four smaller. A spin parity analysis of the $ rho sp+ rho sp-$ system shows that the cross section is dominated by the two amplitudes $J sp{P}$ = 0$ sp+$ and $J sp{P}$ = 2$ sp+$ with helicity 2.

The Standard Model is discussed in general and the predicted (but as yet unobserved) Higgs-boson and top-quark described. Detection of a top-quark at electron-positron colliders and hadron colliders is considered for $m sb t < 80GeV$. The detection of the Higgs-boson is considered at the Z peak in electron-positron annihilation for $m sb H leq 60GeV$, as well as in the continuum for 100 $ leq m sb H leq 200GeV$ (intermediate mass range). For this range of the Higgs-boson mass, consideration is also given to its detection at the Superconducting Supercollider through the tau-lepton decay mode. A dispersive approach is considered for Higgs-boson with large mass ($m sb H geq 500GeV$) in order to estimate non-perturbative self-interaction effects.

In this Ph.D thesis, electronic structure and transport properties of quantum dots are studied using advanced numerical techniques based on fundamental many-body theory. In fact, in such nanostructures, correlation and quantization effects dominate motion of electrons like in real atoms, hence an exact treatment is often necessary to understand and predict their electronic properties. Moreover, experimental realization of quantum dots in the presence of magnetic field gives rise to several new many-body physics that are inaccessible in real atoms, and they provide a crucial testing ground for important concepts of mesoscopic and nanoscopic phenomena. The many-body tools used in this work include exact diagonalization, quantum Monte Carlo, Hartree-Fock, and Keldysh Nonequilibrium Green's function analysis. Each of these methods have their advantages and inconveniences. For studying closed systems with small number of electrons, exact diagonalization is the best choice, since it provides complete information about the total energy, wave functions and correlations for not only the ground state but also for excited states. We used exact diagonalization method to study in detail circular parabolic and ring shaped quantum dots containing up to 8 electrons, and we have found that spatial localization of electrons inside the dot gives rise to interesting new physics. On the other hand, for studying disordered quantum dots exact diagonalization method becomes powerless due to the broken spatial symmetry. Thus, we developed a fixed-phase quantum Monte Carlo algorithm combined with Hartree-Fock solutions, which allowed us to investigate the effect of impurities on electronic properties inside parabolic dots containing up to 13 electrons. To investigate open systems and transport properties, we have expanded exact diagonalization solutions in terms of Keldysh Green's functions which allowed us to discover a new transport blockade regime. Finally, again using Keldysh Green's function formalism, we have studied a double quantum dot system in the presence of Kondo resonance, and we have shown that multi-channel transport gives rise to particularly interesting competition effects between resonant tunneling and Kondo effect.

We study how the resonant decay of moduli fields from the scalar potential of the MSSM could affect large scale curvature perturbations. After introducing the theory of cosmological perturbations and of broad and stochastic resonance, we present the supergravity inputs necessary to study the MSSM moduli in a cosmological context. We find that the resonant amplification of large scale field fluctuations is allowed for a very small range of parameters and is harmless for large scale curvature modes. / Nous étudions l'effet de la désintégration des champs de module du MSSM sur les perturbations gravitationelles adiabatiques à grandes échelles. Après avoir introduit la théorie des perturbations cosmologiques et la théorie de la resonance stochastique, nous présentons les éléments de la supergravité nécessaire à l'étude des champs de module dans un contexte cosmologique. Nous trouvons que l'amplification non-perturbative des fluctuations d'ordre cosmologiques du champ scalaire n'est permise pour que pour un mince interval de paramètres. L'effet de la résonance sur les perturbations gravitationelles à grandes échelles est négligeable.

We explore the "Roulette Inflation" model, a variety of two-field Kahler moduli inflation scenario with a nonstandard kinetic term derived from Type IIB string theory in the large-volume compactification. We provide an overview of the model and the cosmological tools used to compute observables. Our calculations show that entropy perturbations can account for up to 90% of Ps(k) at COBE scales. We further show that the effective single-field result over-estimates the scalar spectral index; a full calculation predicts a slightly red-shifted spectrum 0.92 < ns < 0.98. Calculation of nongaussianity from the superhorizon evolution of adiabatic modes shows that for most trajectories examined, Roulette inflation predicts small, positive fNL, which would be unmeasurable by the upcoming 9-year WMAP data and the Planck mission. Should they be detectable, momentum-dependent bispectra would provide a good discriminator between models and inflationary trajectories within models. / Nous etudions le modele d'inflation de type "Roulette", un modele base sur la relaxation de modules de Kahler sur une variete Calabi-Yau, ayant un terme cinetique non-trivial et provenant de la theorie des cordes de Type IIB, dans le contexte de la compactification a grand volume. Nous demontrons que l'influence des perturbations d'entropie peuvent compter pour jusqu'a 90% de Ps(k) a l'echelle de COBE. Nous demontrons que le resultat effectif provenant de la theorie a un seul champ surestime ns; un calcul complet predit un spectre decale vers le rouge 0.92 < ns < 0.98. Nous calculons finalement la non-gaussianite des perturbations adiabatiques resultant de leur evolution a l'exterieur du rayon de Hubble, et arrivons a des predictions de fNL ≪ 1, ce qui ne serait pas mesurable par les observations des prochaines annees. Toutefois, s'il s'avere possible de detecter un bispectre dependant du nombre d'onde k, ceci servirait d'excellent moyen de diff´erentier divers modeles d'inflation.

We give a brief review of the bosonic and Type II string models, concentrating on matters of symmetry. In particular, we discuss how modular invariance leads to the GSO projections in the RNS superstring formalism, and contrast the GSO conditions arising from an embedding in flat target space with those arising from an embedding in a toroidally compactified target space. These results are then used to demonstrate the stability of a compact spacetime, the AdS soliton, against the formation of tachyonic winding modes in the low curvature limit. This constitutes further evidence for a positive energy conjecture for asymptotically, locally AdS spacetimes, with the AdS soliton as the minimum energy solution within the context of superstring theory.

A measurement of the inclusive inelastic J/ y photoproduction cross section using the ZEUS detector at HERA is described. The cross section of the reaction e+ p &rarr; e+J/ y X was determined for events with an inelasticity, z, less than 0.9, where z is the ratio of the energy of the outgoing J/ y to the energy of the incoming photon in the proton's rest frame. The measurement was performed in the photon-proton centre of mass energy range 110 < Wgp < 160 GeV using the decay channel J/ y &rarr; e+e--. The measurements compare favourably with recent NLO calculations using the colour singlet model.

The advent of the ep collider HERA at the DESY research center has opened a wide kinematical window for the study of the Deep Inelastic Scattering (DIS) processes e+p&rarr; e+X. Recent measurements of the structure function F2 of the proton have confirmed the fact that the Bjorken scaling which arises from the naive Quark-Parton Model is broken in the limit where the scaling variable, x-Bjorken, is small. It also revealed that F2 grows exponentially with x-Bjorken, at low x. The standard picture of parton evolution, based on the DGLAP equations, reproduces this steep rise if it is used with an appropriate set of F2 parametrizations. However these parametrizations include "ad-hoc" assumptions on the non-perturbative behaviour of the structure functions and depend on a large set of phenomenological parameters. / The steep rise can also be reproduced by using a different perturbative treatment for the parton evolution. This treatment, based on the BFKL evolution equations, differs from the standard DGLAP picture by the lack of ordering in the transverse energy of the emitted partons. Unlike DGLAP, the BFKL picture reproduces the steep rise of F2 without assuming any particular shape of the structure function in the non-perturbative domain. / In order to gain an insight on parton dynamics in DIS at low x and discriminate between these two pictures, jet observables are used as they are expected to be closely related to the hard scattering and depend only slightly on the hadronization effects. Two jet observables are studied here in more details: the azimuthal correlation between the two leading-order jets and the cross section of the forward jet production. The measured cross sections are corrected for detector effects and compared to several DIS Monte Carlo models and next-to-leading order simulations over a wide kinematic range. The results are evaluated in the light of the BFKL and the DGLAP pictures. The experimental results are compared to other models of parton evolution as well, like the Colour Dipole Model (CDM), the Linked Dipole Chain (LDC) and the resolved photon model in DIS.

We study the effect of the thermal background on the mechanism of electroweak baryogenesis. Firstly, we model the effect of collisions by solving the Schrodinger equation in presence of a potential representing the bubble of phase transition of the Higgs field and a series of delta functions simulating the collisions amongst the particles. We validate this technique in chapter 2 by comparing it to the results obtained by application of the Born approximation. Later, we apply this "delta functions recurency technique" to the case of relativistic particles by solving the Dirac equation in the precense of similar potentials. In the second part of this thesis, we attempt to model the interaction by a totally distinct formalism that incorporates the effect of the exchange of momentum during a collision between the particles of the thermal background. To this end, we begin by describing a preexisting method for predicting the evolution of the density matrix of a system subjected to an interaction. Then we expend it in order to model the effect of the thermal background on baryogenesis. Lastly, we apply the "density matrix formalism" to the case of relativistic particles.

We derive fully covariant expressions for all two-point scattering amplitudes of two massless closed strings from a Dirichlet p-brane. This construction relies on the observation that there is a simple relation between these D-brane amplitudes in type II superstring theory and four-point scattering amplitudes for type I open superstrings. From the two-point amplitudes, we derive the long range background fields for the D-branes, and verify that as expected they correspond to those of extremally charged p-brane solutions of the low energy effective action.