Trigger for rare kaon decays searches at the CERN NA62 experiment

The work reported in this thesis has been carried out within the Trigger and Data Acquisition (TDAQ) working group of the CERN NA62 experiment, and focused on the development of a trigger strategy for collecting rare kaon decays. The main aim of the NA62 experiment is the study of the ultra rare decay K+ -> pi+ nu nu in order to provide a stringent test of the Standard Model. The theoretical framework of the K+ -> pi+ nu nu decay, the present experimental status of the measurement of the Branching Ratio and the NA62 experimental strategy are described in chapter 1. Chapter 2 provides a description of the NA62 experimental setup. The first part of the work concerned the design, development and assessment of the common Trigger and Data Acquisition system for the majority of detectors in NA62, a high-speed integrated data acquisition and trigger generation system based on digital high resolution time measurements. The architecture of the boards, a detailed description of the developed firmwares and some results obtained during the NA62 TDAQ commissioning phase are presented in chapter 3. The second part of the work focused on Monte Carlo studies of hardware low-level (L0) trigger schemes with the purpose of selecting interesting decays. The dependence of the L0 output trigger rate and of the trigger efficiency upon the variation of several trigger parameters has been studied. The optimization of the L0 trigger scheme for the selection of K+ -> pi+ nu nu decays is described in chapter 4. In addition, a L0 trigger selection for the search for a hypothetical dark photon (U) in pi0 decays is discussed in chapter 5. After a general introduction about the searches for rare and forbidden kaon decays at NA62, the physics motivations and the experimental status are described. Finally, the NA62 expected upper limits on the Branching Ratio of the pi0->gU,U->e+e- decay are computed and compared with present limits.