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

Abstract : This thesis reports the discovery of the new boson recently observed at a mass near 125 GeV in the CMS experiment at CERN. The measurements of the properties of the new boson are reviewed. The results are obtained from a comprehensive search for the standard model Higgs boson in the H → ZZ decay channel, where both Z bosons decay to electron or muon lepton pairs. The search covers Higgs boson mass hypotheses in the range 110 < mH < 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 sqrt(s) = 7 TeV and 12.2 fb^−1 at sqrt(s) = 8 TeV. The new boson is observed with a local significance above the expected background of 4.5 standard deviations. The signal strength μ, relative to the expectation for the standard model Higgs boson, is measured to be μ = 0.80+0.35-0.28 at 126 GeV. A precise measurement of its mass has been performed and gives 126.2 ± 0.6 (stat) ±0.2 (syst) GeV. The hypothesis 0+ of the standard model for the spin J = 0 and parity P = +1 quantum numbers is found to be consistent with the 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 the ranges 113-116 GeV and 129-720 GeV while the expected exclusion range for the standard model Higgs boson is 118-670 GeV. A special emphases throughout the thesis has been put on lepton isolation. Lepton isolation being one of the key observables for the discovery is highly susceptible to pile-up conditions of the LHC machine. This thesis establishes a robust method to marginalize the effect of pile-up on isolation. The method is now used across different analysis in CMS. A special attention has also been put on measurements of the efficiencies of lepton identification, isolation and 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 the significance of excess of four lepton events.
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Submitted on : Wednesday, May 29, 2013 - 11:30:40 AM
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Roko Plestina. Evidence for a Standard Model Higgs boson like particle decaying into four leptons with the CMS detector. High Energy Physics - Experiment [hep-ex]. Ecole Polytechnique X, 2013. English. ⟨pastel-00827458⟩

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