Reconstruction de muons cosmiques en collision et recherche de gluinos se désintégrant en stop-top dans l'expérience CMS au LHC

The CMS experiment (Compact Muon Solenoid), built on the ring of the LHC (Large Hadron Collider), is recording data from proton-proton collisions for 2 years now. The alignment between all layers of the tracker, the sub-detector of CMS allowing the reconstruction and the measurement of the momentum of charged particles, is made by using tracks of particles created during collisions and tracks created by the passage of cosmic muons through this sub-detector. A first part of the presentation will be dedicated to the reconstruction of the tracks of these cosmic muons during collisions. A new method, called regional cosmic reconstruction, was developed and implemented. The 69 % efficiency and the fake rate around 1 % allow to use these tracks for the alignment. The second part, dedicated to the analysis of collision data, will concern the search of particles predicted by a model of extension of the Standard Model, the Supersymmetry, in a particular scenario, the light Stop scenario. In the case of a strong mixing in the third generation of squarks, the stop, supersymmetric partner of the top quark, could be light. In the analysis developped during this thesis, we were interested in the case where the mstop < mtop. In the MSSM with R-parity conservation, the gluino would be created by pair and would decay in a stop squark and a top quark. Stop would decays in a c quark and a neutralino, the lightest supereymmetric particle of the model, stable and interacting weakly. No excess with regards to Standard Model predictions was observed in 40pb-1 of data recorded by CMS in 2010. The limits obtained at 95 % confidence level allow to exclude masses of stop until 175 GeV for masses of gluinos going up to 350 GeV and low differences of masses between stop and neutralino.