Studio delle proprietà delle risonanze barioniche attraverso reazioni di fotoproduzione pionica

Up to now, a lot of baryonic and mesonic states have been observed. In spite of the existence of such a large number of particles, the mechanisms underlying the formation of these bound states are far from being theoretically understood. The impossibility of using a perturbative approach to Quantum Chromodynamics at low excitation energies requires the development of effective theories (quark constituent models, chiral perturbation theories,...) and approximation methods (lattice QCD) that have still not reached a sufficient degree of reliability. In this framework, the problem of “missing baryonic resonances” is remarkable since all models foresee the existence of a much higher number of excited states than the observed one. Light pseudoscalar meson photoproduction is a promising way for the nuclear resonance investigation. This research is carried out at the MAMI accelerator in Mainz (Germany) in the framework of A2 collaboration. The experiment exploits photon beams of energy up to 1.5 GeV, with high energy resolution and high intensity. In this work original data for the doubly polarized differential cross section of the process γp → pπ0 are presented. This is one of the data sets needed to evaluate the properties of nucleon resonances through a partial wave analysis (electromagnetic multipoles) of the measured angular distributions. In the first part of the thesis a theoretical introduction on this topic will be presented and the usefulness of photoproduction reactions will be discussed. Then, after a description of the experimental apparatus, the off-line analysis of collected data will be described in detail. In the last chapter original results, both for γp → pπ0 and γp → X polarized reactions, will be presented and analysed.