Mesures de production de la particule neutre et étrange $K^{0}_{S}$ dans l'expérience HARP

The hadron production experiment HARP (PS214) carried out a programme of measurements of secondary hadrons at the European Organization for Nuclear Research (CERN) proton synchrotron (PS) during the years 2001 and 2002. It serves several purposes. On one side there is a need to acquire knowledge of pion yields for an optimal design of the proposed neutrino factories and superbeams. On the other one, we have to improve substantially the calculation of the atmospheric neutrino flux, used by past and forthcoming experiments designed to measure neutrino oscillations. HARP data will also provide a general input for simulation of particles traversing matter, as well as neutrino beam predictions for the K2K and MiniBooNE experiments. The subject of this thesis is to provide strange particle production, K0S, measurements, from HARP data. This study is of interest because, for neutrino beams, the charged kaon contamination induces an electron-neutrino type background in the muon-neutrino beam. The knowledge of such a background, in the HARP momentum range, and for different target materials, is essential in order to optimize the parent proton beam momentum, the target material as well as the focusing system. The written report is divided in five parts. In the first part we describe the physics motivation through a review of the Standard Model theory and its limits. The second part is devoted to a detailed description of performance requirements for the HARP experiment and the kind of data HARP should provide. We also explain why K0S study is of interest. The third part will be devoted to a more detailed description of the HARP time projection chamber, a detector which construction I was deeply involved in. The fourth part contains the description of the software that was written to find the primary vertex and that I have developed and tested against simulated and physics data. In the last part we finally give preliminary results on the invariant mass search for the K0S produced in the target.