Search for the Standard Model Higgs Boson in the H$\rightarrow$ ZZ$\rightarrow$> ee(mu mu) nu nu channel in the CMS experiment at LHC

The Standard Model (SM) is a framework which describes the fundamental particles and their interactions at the most fundamental level. The Large Hadron Collider (LHC) is a superconducting proton-proton collider designed to test the SM at high energies, to search for the SM Higgs boson ($H$) and also to search for any possible physics beyond the SM. In this Thesis, a search has been presented for the SM Higgs boson in H → ZZ → `+`−ν`ν(` = electron or muon) decay mode using the data collected by the Compact Muon Solenoid experiment from proton-proton collisions in the LHC at European Center for Nuclear Research (CERN), Geneva, Switzerland. The analysed data is collected over two years, 2011 and 2012, which correspond to an integrated luminosity of 5.0 $fb$$^{−1}$ at center of mass energy of 7 TeV and 19.7 $fb$$^{−1}$ at center of mass energy of 8 TeV, i.e. the full data recorded by the CMS experiment till the LHC stopped for its first long shutdown in 2013. This decay mode is mostly sensitive in high Higgs mass ($M$$_{H}$) range from 200 GeV to 1000 GeV. Two most dominant production modes of the Higgs boson, gluon-gluon fusion and vector boson fusion (VBF), are considered. Main backgrounds to this analysis come from $Z$ + jets, $t$$\bar{t}$ + jets, $WW$ , $WZ$ and $ZZ$. Selection criteria is based on the properties of the signal final state, which is high transverse missing energy ($E$$_{T}^{miss}$) arising from two T undetected neutrinos and two same flavor leptons (electrons or muons) consistent with Z boson decay. The selection procedure is divided into preselection, event categorization and final selection. Preselection includes the kinematical selection of leptons and topological selection based on minimum azimuthal separation of jets and ($E$$_{T}^{miss}$). To reduce top quark T related backgrounds, events having jets arising from b-quarks are rejected. To enhance the sensitivity of the search, the events are categorized on the basis of jet multiplicity. A separate selection is applied to identify VBF type events and then rest of the events are divided into exclusive 0-jet and inclusive 1-jet (≥ 1-jet) categories. Finally, Higgs mass dependent selection is applied based on ($E$$_{T}^{miss}$) and transverse mass of the Higgs ($M$$_{T}$). All the major backgrounds like $Z$ + $jets$, $t\bar{t}$+ jets and $WW$ are estimated using data-driven techniques. All sources of the systematic uncertainties are also considered. No significant excess is observed over the SM background prediction over the entire Higgs mass range. A statistical analysis is carried out to set upper limits on the Higgs boson production cross-section. The baseline approach is the shape analysis based on final discriminant (MT for non-VBF category and ($E$$_{T}^{miss}$) for VBF category) in which the full T distribution of the final discriminant is used to impose limits. A cross-check has also been made by imposing a selection requirement on final discriminant (instead of using the full distribution), commonly referred to as the cut-and-count analysis. In order to explore the possible improvements in the analysis sensitivity and to cross-check the baseline analysis using an independent technique, a multivariate analysis is introduced based on the Boosted Decision Tree (BDT) technique. Training of BDT is performed for each Higgs mass hypothesis and is done separately for $e$+$e$−ν ν ̄ and μ+μ−ν ν ̄ decay modes. The optimization of BDT parameters is done keeping the best expected limit on ratio of measured Higgs boson production cross-section to the SM cross-section. Back- ground estimation techniques are same as used in baseline analysis and all systematic sources are also considered. A similar statistical analysis is carried out and exclusion limits are imposed on $M$$_{H}$.