[en] The central subject of this thesis is the search for new physics in which the top quark plays a crucial role. The LHC is a proton collider, designed for a nominal operation with ps = 14 TeV. Until december 2011 it worked with an energy of 7 TeV in the center of mass. The first 5 fb^-1 of collected data allowed us to constrain some parameters of the new physics processes presented in this thesis. The document is divided into three parts: The first one describes briefly the LHC accelerator before presenting the ATLAS experiment and operation of the detector. The Tower Builder Boards for the electromagnetic level one trigger are especially studied. The second part is a phenomenological analysis of physics beyond the Standard Model with four top quarks in the final state. This analysis is presented for several theoretical models. This study was performed mainly at the generator level for ps = 14 TeV but we also comments on prospects at 7 TeV. It can be applied to both ATLAS and CMS experiments. The third part is an experimental study, performed with the ATLAS detector, searching for two exotic processes with two same signs leptons in the final state (T(5/3) and four tops production). A limit on the mass of the partner top T(5/3) has been obtained as well as a limit on the cross section of exotic events with four tops. We have got an expected limit of 718 GeV on the T(5/3) mass of the top partner for an observed limit of 713 GeV with a coupling constant of λ=1 and an expected limit of 715 GeV for an observed limit of 710 GeV with a coupling constant of λ=3. For the exotic event involving four tops, an expected limit of 0.1 pb has been obtained for the production cross section

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[en] Of the many interesting final states that may be produced at the LHC, four top production is maybe one of the most spectacular. In this contribution, the sensitivity of this final state to several classes of physics beyond the Standard Model is discussed. The focus is on models where this topology is produced through a heavy resonance. The possibility to reconstruct the top and anti-top quarks in these events is explored. (authors)

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[en] In an exactly solvable model of separable potentials a two-channel scattering problem is solved in l=0 state. The location and the width of a resonance are then defined in three different ways and the resonance parameters so extracted are then compared. This problem is of interest vis-a-vis the Δ (3,3) resonance parameters

[en] Possible ambiguities in the reconstruction of πN scattering amplitudes from experimental data and the elastic unitarity condition below the threshold of inelastic processes are discussed. Discussion is restricted to partial waves with l = 0 and 1 and it is shown that certain twofold ambiguities can occur if only two experimental quantities are measured (the cross section plus polarization or one of the polarization parameters). It is also shown that these ambiguities can be removed by a further measurement, unless the polarization happens to be zero or the experiment is performed at an unfortunately chosen angle, for which both sets of amplitudes predict the same value for the corresponding experimental quantity

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[en] A second-order differential equation for the fast wave propagating in a hot, two-ion species plasma is obtained. This second-order approximation is obtained unambiguously and allows the wave amplitude to be identified with one of the electric field components. The approximation is based on replacing the coupling to the ion-Bernstein wave by a localized perturbation of the fast wave. For the case of perpendicular propagation, the second-order equation reduces to Budden's equation giving the well-known transmission coefficient for both two-ion hybrid and second-harmonic resonance. The equation includes the effect of simultaneous minority fundamental and majority second-harmonic cyclotron damping. The solutions of the second-order equation as a function of n/sub parallel/ give absorption transmission and reflection coefficients that agree well with the results based on models giving higher-order differential equations and solved by means of much more complex numerical codes

[en] Sulphur-containing molecules are responsible for the poisoning of catalysts used in many chemical processes such as fuel processing for hydrogen production and for fuel cells. An option which would constitute a breakthrough in this field would be to develop sulphur tolerant catalysts. Ceria (CeO₂) is an important ceramic material exploited in a wide range of applications such as solid oxide fuel cells. Then it is important to understand its surface catalytic properties. The adsorption of S, H₂S and other S-containing compounds on different surfaces of ceria are investigated at the ab initio quantum mechanical level, by using the periodic CRYSTAL06 code. In this extended abstract, we focus on the S adsorption on the stoichiometric (111), (110) and (100) surfaces of ceria. The equilibrium lattice parameters of CeO₂, surface stabilities, and S adsorption energies have been evaluated. The calculations have been performed at the Hartree-Fock (HF), density functional theory (DFT) and hybrid levels. A good agreement between calculated, and, other theoretical and experimental various properties has been found with hybrid approximations. The role of f orbitals of Ce is commented

[en] The French 600 MWe Advanced Sodium Technological Reactor for Industrial Demonstration (ASTRID) project reached in 2015 the end of its conceptual design phase. The core design studies are being conducted by the CEA with support from AREVA and EDF. Innovative design choices for the core have been made to comply with the GEN IV reactor objectives, marking a break with the former Phenix and SuperPhenix Sodium Fast Reactors. One of the biggest challenges of the last five years was to propose a consistent design for the reflectors and neutron shielding sub-assemblies surrounding the fuel core in order to fulfill ASTRID requirements of minimising the secondary sodium activity level. Heavy iterative studies on both core and sub-assemblies were necessary to propose and evaluate different solutions following a strict value analysis process considering neutron shielding performances, life duration, maturity levels, washing and manufacturing capability, and qualification needs. Evaluated options were reflectors sub-assemblies made of steel or MgO rods, and radial neutron shielding sub-assemblies made of B₄C or borated steel, with different configurations in the design and in the core layout. This paper presents the iterative engineering studies, conducted by CEA and performed by AREVA-NP, concerning the radial shielding sub-assemblies for ASTRID core, from the selection of possible solutions to a final consistent conceptual design. (author)