π⁰ transition form factors at KLOE and KLOE-2

The main subject of this thesis work is the measurement of the π0 transition form factors at KLOE and KLOE-2 experiments. This topic, strongly encouraged by the community of theorists, is of crucial importance in different fields of particle physics, such as the Standard Model prediction of the anomalous magnetic moment of the muon and the search for quark-gluon plasma in heavy-ion collisions. A first part of the thesis work is devoted to the analysis of the KLOE old data set, looking for the rare OZI-suppressed φ → π0e+e− decay. Beside the measurement of the Branching Ratio of the process, the Fφ π0 γ∗ transition form factor has been extracted in a wide range of time-like momentum transfer. This measurement, never carried out so far, is of primary importance to check the consistency of the VMD model, which apparently fails to describe data for π0 transition form factor in ω → π0e+e− decay. The second part of the thesis deals with the possibility to measure the Fπ0 γ γ∗ transition form factor (in the low region of space-like momentum transfer) and the Γ(π0 → γγ) decay width, through γγ interactions at KLOE-2. These measurements, made possible thanks to the installation of new small angle High Energy Tagger (het) detectors, play a crucial role in the improvement of the theoretical calculation of the hadronic Light-by-Light contribution to the anomalous magnetic moment of the muon. The first chapter starts with an essential introduction to the form factor topics, then the physics motivations for the proposed measurements are given. A basic description of DAΦNE accelerator complex and KLOE detector is the subject of the second chapter. In the third chapter, the study of the φ → π0e+e− decay is presented: a description of the Montecarlo generator developed for signal simulation is included, as well as the data analysis and the background subtraction procedures. The feasibility study of the two-photon physics process is discussed in the fourth chapter, together with a detailed description of the het. Detector simulation, assembly and tests are reported, with a special focus on the electronic chain. Finally, the conclusions of this work are drawn.