La ricerca di nuova fisica a LEP mediante lo studio di eventi con soli fotoni nello stato finale

A wide search for new physics can be performed at LEP carefully analysing the events having two or more photons in the final state. As a first example, the direct production of a couple of photons, each one having the LEP beam energy, is at tree level a pure QED process, with a negligible electroweak loop contribution. Possible deviation from the QED predictions could arise in the angular differential cross section, as a result of an alteration of the Coulomb potential at distances shorter than about 10^-16 cm. The highest sensitivity to this deviation is at the highest center-of-mass energy (183 GeV) data collected at LEP in 1997. At energies around Z resonance, new physics beyond the Standard Model could be detected observing some Z decays into neutral final states, such as the direct decay into two photon, forbidden from angular momentum conservation and Bose statistic, or very low rate predicted decays into three photons or into a neutral light meson plus a photon, with subsequent meson decay into two or more photons. In particulary, the Z decay into three photons is predicted to be enhanced in many Compositeness models. Finally, a big amount of missing energy in the final state with two photons could be the signal of the production of the next-to-lightest supersymmetric particles, such as the neutralinos, with subsequent decay into a photon and the lightest supersymmtric particle (LSP), which in the most popular SUSY models is predicted to be the lightest neutralino or the gravitino. This thesis reports the analysis performed on the full data sample collected by the DELPHI detector from 1990 to 1997, at center-of-mass energies ranging from 89 to 183 GeV. No evidence of deviations from the Standard Model predictions has been observed. New and improved 95% CL limits have been set on the anomalous parameters describing the QED deviations in terms of effective lagrangians, on the branching ratios of rare or forbidden Z decays into aneutral states, and on the cross sections for neutralino pair production as a function of their mass for two different SUSY scenarios.