Study of the signal lineshape in the analysis for the search of H →ZZ(∗)→4l at CMS

The work presented in this thesis concerns the study of the signal lineshape of the SM Higgs mass decay into four leptons. The 4l final state signal and background processes are described in detail, for a range of mass from 110 to 1000 GeV. The result of the observation of a new particle in this channel with a mass near 125 GeV is presented. In order to estimate the significance of an excess or quantify exclusion, a statistical analysis of the observed data is necessary. For this purpose, a model of the signal lineshape, parametric on the hypothetical Higgs boson mass, is needed. The signal modelization is extracted from the Monte Carlo samples, which do not describe immediately the experimental data with the required precision for this analysis. Therefore, it is necessary to correct the Monte Carlo from discrepancy and possibles systematic errors. A first correction was required to match the distribution of pileup events different in Monte Carlo with respect to the data. A second correction was nec- essary for a discrepancy in the lepton reconstruction, trigger and identification efficiencies, as well as lepton isolation and significance of the impact parameter efficiencies. The correction of the simulated samples through the use of event weight and per-event correction factors and an additional reweighting to cor- rect the line-shape at high-mass is presented. Two different parametrization, one for the low mass region and the other one for the high mass region are illustrated. Finally the statistical analysis for extracting exclusion limits and compute p-values is discussed.