Recherche du neutralino avec les détecteurs ATLAS et PICASSO

At the eve of the Large Hadron Collider (LHC) start-up - the most energetic particle collider ever built - the scientific community is actively preparing the analysis in order to be ready for possible key discoveries in particle physics, like the discovery of supersymmetric particles. In a complementary approach, the direct dark matter search experiments are now becoming sensitive enough to start probing the theoretically allowed parameter space if the dark matter is formed of a supersymmetric particle called the neutralino. A part of the thesis is thus centered on the LHC based ATLAS detector. The neutralino is a good dark matter candidate under the assumption that it is the lightest supersymmetric particle (LSP). Using WMAP (Wilkinson Microwave Anisotropy Probe) results on cosmic microwave background anisotropies, limits on the dark matter cosmological density can be set. These boundaries can then be converted in limits on the supersymmetric parameters, thus constraining the possible models of neutralino. In this section of the thesis, the theoretically permitted supersymmetric parameter space compatible with WMAP results is explored and the fraction of this space reachable in ATLAS at LHC is determined. It is also possible that supersymmetry is indeed a property of Nature without the neutralino being stable - this would mean that the neutralino could exist without being a dark matter candidate. Using a simulation of the ATLAS detector, it is possible to study some supersymmetric particle decays after their production at LHC in order to detect the neutralino. In this section of the thesis, the top quark decay into a selectron followed by the selectron decay into a neutralino is studied in order to optimize the neutralino detection by identifying the analysis cuts to use. Another part of the thesis focuses on the direct detection of the neutralino as dark matter. In this part, the neutralino-nucleon interaction cross sections are calculated for different types of interaction and the extraction method of cross-section limits from experimental data is shown. Neutralinos can, among other methods, be detected by their interaction with nuclei in superheated droplet detectors: this is the objective of the PICASSO experiment. The last section of the thesis explains how these detectors work. A Monte Carlo simulation code was written. It allows the characterization of the detectors, i.e. the comprehension of how they work and the prediction of their response. Experimental measurements were also done in order to understand the response.