Nuovo metodo per la misura del rapporto tra le sezioni d'urto di produzione gg->ZZ e qq->ZZ nell'esperimento CMS a LHC e sue implicazioni per la ricerca del bosone di Higgs

The work presented in this thesis concerns the study of irreducible backgrounds that the search for a Higgs signal in the HZZ4l channel has to handle. This kind of background is constituted by four-leptons events from non resonant di-boson production (qqZZ4l and ggZZ4l); since these events topology and kinematic are very similar to those of signal events, these processes are considered in the category of irreducible backgrounds. An appropriate knowledge of these processes is therefore needed to minimize the systematic uncertainties. The study of qq→ZZ and gg→ZZ angular distributions at generator and reconstruction level, essential to evaluate if the construction of a kinematic discriminant to separate q q→ZZ from gg->ZZ is possible, is presented. A new technique to measure the ratio of gg over qq induced ZZ production cross sections is presented. After the development of a model which is able to extract, with a fi?t, the fraction of gg → ZZ events, the method is tested on a variety of MC samples, in order to study its goodness on varying the statistic, the luminosity and the energy.