Misura dei Fondi tt, Zbb e Z+jet nella Ricerca del Bosone di Higgs nel Canale di Decadimento H->ZZ->4l all'Esperimento CMS a LHC

After a brief overview of the Standard Model that describes the theoretical framework of this analysis, in Chapter 1 the attention is focused onHiggs boson properties. Theoretical and experimental mass constraints are discussed and the Higgs phenomenology and search at the LHC are presented. In Chapter 2, after an introduction to the LHC, the CMS detector is described. The overall structure of CMS, consisting of several cylindrical layers closed at both ends by detector disks, allows an excellent lepton reconstruction and particle identification. In Chapter 3, the H → ZZ → 4l decay channel analysis is presented. This is the experimental context in which my analysis is focused. The 4-lepton final state signal and background processes are described in detail, for a range of mass from 110 to 600 GeV/c^2. An excess of events is observed above the expected background at a mass near 125 GeV/c^2, signaling the evidence of a new particle. Chapter 4 deals with the reducible and instrumental backgrounds of the H → ZZ → 4l signal. Even if these processes are drastically reduced by the analysis selection cuts, a small contribution is present in the final sample. This requires a detailed study of their composition. My work deals in the reconstruction and selection of the Zbb and tt candidates and the discrimination of these two contributions. Also, I present for the first time a measurement of the Zbb → 4l and tt → 4l cross sections at 7 and 8 TeV. Finally, in Chapter 5 a new data driven procedure to separately measure instrumental and reducible backgrounds that remain after the first steps of the H → ZZ → 4l selection is presented. In order to study the two different contributions, I define a control sample and, fitting distributions relatd to the impact parameter of leptons, I estimate the number of Z+ jets and Zbb + tt events in the signal region of the H → 4l analysis.