Observation and study of the decay KL→ π0γγ with NA31 experiment

The origins of CP violation is one of the unsolved problems of the Standard Model which describes the interactions of elementary particles. The goal of the NA31 experiment, installed at CERN, was to study CP violation in neutral kaon decays. Meanwhile searches for rare kaon decays among the very large amount of data accumulated on tapes proved to be a very interesting opportunity. This thesis reports the first observation of the decay KL → π0 γγ. The first chapter presents theoretical motivations. The approach of chiral perturbation theory to describe strong interaction and the predictions for the searched decay mode are presented. The contribution of this decay mode to the channel KL → π0e+e- is studied to investigate the possibilities to observe there the direct CP violation. The chapter II describes the detectors, with emphasis on the electromagnetic calorimeter which performances are essential for our analysis. The background being a priori very important, the simulation of such events was treated in details. The results of the simulation are compared with data in the chapter III. The chapter IV discusses the selection criteria defined and optimized with the simulated events. Within all cuts, we have observed for the first time the decay mode KL → π0 γγ: 42 signal events with only 5 estimated background events. The measurement of the decay parameters and their theoretical implications are presented in the last chapter