Background estimation, search for heavy resonances and off-shell Higgs boson signal strength measurement in the 4 lepton final state with the ATLAS detector

The discovery of Higgs boson in 2012 by the ATLAS and CMS experiments marked a key milestone in the history of particle physics. It confirmed a long-standing prediction of the Standard Model (SM), the theory that describes our present understanding of elementary particles and their interactions. The Higgs boson was the last particle of the SM which was observed. After its observation the research in the ATLAS experiment focused on the measurement of its properties and also on the effort to observe particles beyond the SM. During Run II (2015-2018) it collected data corresponding to about 140 fb$^{-1}$, having a key role in these studies. Firstly a study for electron identification in the forward region is described. The new ITk will extend pseudorapidity coverage up to |$\eta$| = 4. An identification method for the electrons in the region 2.5 < |$\eta$| < 4.0 was developed. It uses track-cluster matching criteria and a Artificial Neural Network to separate the true electrons from the fakes, which come either from pile-up or jets. Lastly the potential of improving identification by using an Isolation variable is studied. Next the event selection and the background estimation in the 4-lepton channel are described. The description focuses on the Data Driven Estimation of the $ll\mu\mu$ reducible background. The estimation is performed in 4 Control Regions (CRs). Each of them is enriched in a specific background component. The 4 CRs are linked via a fifth region, called Relaxed Region (RR). A simultaneous likelihood fit is performed in the 4 CRs and the results are extrapolated to the RR via Fractions. The results from the RR are subsequently extrapolated to the Signal Region via Transfer Factors (TFs). The TFs are estimated and controlled using a sample containing $Z+\mu$ events. Using the above selection and estimation several physics studies are performed. The search for a high mass $ZZ$ resonance is one of them. In this analysis a cut based selection is used to categorize the events. The signal and the $ZZ$ backgrounds are modeled with analytic functions, while the reducible background is data Driven estimated. The methodology of the Likelihood fit and the extraction of CLs limits is described in detail. Lastly the final limits are presented. At last the measurement of the Higgs boson decay width is described. In order to achieve this measurement, a measurement of the Higgs boson off-shell cross section is required. For this study the MC samples are corrected with the latest k-factors. Αfterward ιn additional to the nominal event selection a Matrix Element based discriminant is used to suppress the $ZZ $ background. The reducible background is estimated in a Data Driven way. Finally the results are combined with these of the $ll\nu\nu$ channel to set limits on the decay width.