Evidence for a Standard Model Higgs boson like particle decaying into four leptons with the CMS detector

This thesis reports the discovery of the new boson recently observed at a mass near 125 GeVin the CMS experiment at CERN. The measurements of the properties of the new boson arereviewed. The results are obtained from a comprehensive search for the standard model Higgsboson in the H → ZZ decay channel, where both Z bosons decay to electron or muon leptonpairs. The search covers Higgs boson mass hypotheses in the range 110 < m H < 1000 GeV.The analysis uses proton-proton collision data recorded by the CMS detector at the LHC,corresponding to integrated luminosities of 5.1 fb−1 at s = 7 TeV and 12.2 fb−1 at s = 8 TeV.The new boson is observed with a local significance above the expected background of 4.5standard deviations. The signal strength µ, relative to the expectation for the standard modelHiggs boson, is measured to be µ = 0.80+0.35 at 126 GeV. A precise measurement of its mass has−0.28been performed and gives 126.2 ± 0.6 (stat) ±0.2 (syst) GeV. The hypothesis 0+ of the standardmodel for the spin J = 0 and parity P = +1 quantum numbers is found to be consistent withthe observation. The data disfavour the pseudoscalar hypothesis 0− with a CLs value of 2.4%.No other significant excess is found, and upper limits at 95% confidence level exclude theranges 113–116 GeV and 129–720 GeV while the expected exclusion range for the standardmodel Higgs boson is 118–670 GeV.A special emphases throughout the thesis has been put on lepton isolation. Lepton isolationbeing one of the key observables for the discovery is highly susceptible to pile-up conditions ofthe LHC machine. This thesis establishes a robust method to marginalize the effect of pile-upon isolation. The method is now used across different analysis in CMS. A special attentionhas also been put on measurements of the efficiencies of lepton identification, isolationand impact parameter requirements directly from data using leptonic decays of Z boson.The measurements were used to produce final per lepton scale factors when calculating thesignificance of excess of four lepton events.