" This thesis will lead you through a dedicated study of final states with photons using the DELPHI detector at LEP. The LEP collider (Large Electron Positron collider) was located at the CERN laboratory in Geneva. It started operating in 1989 and stopped in the year 2000. LEP was the largest electron-positron circular accelerator ever built, operating at the at the highest energies achieved in e+e- colliding beams. There were four beam intersection regions at LEP, where the detectors were located. The DELPHI detector - DEtector with Lepton, Photon and Hadron Identification - was one of the four experiments operating on the LEP collider (ALEPH, DELPHI, L3 and OPAL). Presently, the DELPHI Collaboration is constituted by about 550 physicists, from 56 universities and institutes from 22 different countries.

The Large Electron Positron collider (LEP) was built after the discovery of the Z0 and W bosons at the SPS at CERN in 1983. The main motivation for building LEP was the study the weak interaction at the electroweak scale. The objectives of LEP were to measure with precision the Z0 boson production and decay rates as well its mass and width, to study W+W- production and decay and to exploit the available energy range in the search for the Higgs boson. LEP enabled as well to perform many different types of searches, such as searches for new particles, as new quarks and leptons, and to search for anomalies with respect to the predictions of the Standard Model of Electroweak Interactions.

LEP had two distinct operating phases, known as LEP 1 and LEP 2. The LEP 1 phase started in 1989 and its main aim was to produce and study the Z0 boson. For that purpose, centre-of-mass energy around M_Z0 (~ 91 GeV) had to be achieved. At LEP 1, the most accurate measurements of the Z0 production and decay rates, its mass and width were performed, as well as of other Standard Model parameters, such as the number of neutrino species and the electroweak mixing angle. These results represented an improvement in precision by two orders of magnitude with respect to the pre-LEP measurements. The LEP 2 phase, started in 1996, consisted on the operation of LEP at centre-of-mass energies above the W+W- production threshold. The strong motivation for LEP 2 was to produce on-shell W+W- boson pairs, measure the W mass and width and the W+W- production and decay rates. However, searching for the Higgs boson was also a very strong motivation for increasing the energy in the centre-of-mass, since no evidence for the Higgs had been found at LEP 1 energies.

At Born level, the reaction e+e- -> gamma gamma (gamma) is exclusively described by Quantum ElectroDynamics (QED), and although at higher orders of perturbation theory, it also has contributions from weak boson interchange, these are nevertheless much smaller than the QED terms, that is, even when comparing just 1-loop level contributions, the QED contribution dominates. Therefore, to measure the total and differential production cross-section for gamma gamma (gamma) at LEP is in fact to test the validity of QED at the electroweak scale. Moreover, if any significant deviation from QED is found, it cannot be explained within the Standard Model framework and there will have to be some New Physics process to account for it. This is why the process e+e- -> gamma gamma (gamma) is known as a benchmark in the search for evidence of Compositness (testing the possibility of the exchange of an excited electron in the t-channel), of contact interactions and other types of models not included in the Standard Model of Electroweak Interactions.

There is a variety of New Physics Models predicting final states in which photons are the only visible particles. In the present thesis, Multi-Photonic events are explored in the search for evidence of a fermiophobic Higgs Boson and of anomalous bosonic couplings of the Higgs boson.

The relevant Multi-Photonic signature in the search for a fermiophobic Higgs Boson consists of final states with two visible photons and missing energy. This signature would be an evidence for the decay of the Higgs boson into two photons when produced in association with a Z0 boson decaying invisibly. In the case of the anomalous couplings of the Higgs boson to the gauge bosons, the most relevant signature is the three photon topology, in which the Higgs boson, produced in association with a photon, decays to a photon pair. In the framework of this model, the two photon plus missing energy signature from HZ0 associated production is also possible.

Searching for New Physics beyond the Standard Model implies that the background processes for the signal signatures are very well known. This is another important motivation for the study Standard gamma gamma (gamma) production. The Standard Model processes yielding Multi-Photonic events are the QED e+e- -> gamma gamma (gamma) process and the production of neutrino-anti-neutrino pairs with the emission of Initial State Radiation (ISR) photons. Neutrino pairs can be produced either by the decay of the Z0 boson or by the t-channel exchange of a W boson. If accompanied by the emission of visible ISR photons, neutrino production channel become visible and can be studied. Moreover, they constitute a major source of background in all searches for New Physics in which final states with photons and missing energy are important signatures.

This thesis is organized in eight chapters. The first two chapters are dedicated to the description of the theoretical frameworks on which the data analyses were based. In chapter 1, the QED e+e- -> gamma gamma (gamma) process is described, while in chapter 2 the explored New Physics models are described. In chapter 3, the concepts used in the statistical treatment of the results of the analyses are introduced. The DELPHI detector is described in detail in chapter 4 with a special focus on the performance of electromagnetic calorimeters which are the most important detectors when studying photons. The photon reconstruction and identification studies performed mostly on simulations of e+e- -> gamma gamma (gamma) events in the DELPHI detector are described in chapter 5. In chapter 6, the real data analysed are introduced for the first time and the selection of the Multi-Photonic samples is described. The two last chapters are dedicated to the specific data analyses and to the interpretations of their results. Chapter 7 focuses on the measurement of the differential and total cross-section of the QED process e+e- -> gamma gamma (gamma) as well as on the derivation of 95% confidence limits on possible deviations from QED. Searches for evidence for New Physics Beyond the Standard Model, both in the framework of a Fermiophobic Higgs boson and of anomalous couplings of the Higgs boson to the gauge bosons, are described in chapter 8."