Heavy Higgs Boson Search in the Four Lepton Decay Channel with the ATLAS Detector

The thesis is focused on the heavy Higgs boson search in four lepton decay channel with Run-2 data from the ATLAS detector at the Large Hadron Collider. The analysis published at ICHEP 2016 conference that is based on 14.8 fb$^{-1}$ of $\sqrt{s}$ = 13 TeV data is described in details, while the updated results that include 36.1 fb$^{-1}$ of $\sqrt{s}$ = 13 TeV data are shown as well. The heavy Higgs search is carried out in a model independent way and it is covering di↵erent signal width hypotheses: narrow width approximation that assumes the signal natural width to be negligible comparing to the detector resolution, and the large width assumption including an e↵ect of the interference with the Standard Model backgrounds. The search that benefits from additional kinematic discriminant under the scalar signal assumption is presented as well. This modification allows to increase the analysis sensitivity up to 25%. The thesis also discusses an upgrade of the ATLAS Muons Spectrometer, namely the New Small Wheel project. In particular, simulation of the cavern background that will a↵ect the upgraded detector and commissioning of the Micromegas modules produced at CEA-Saclay for the New Small Wheel are described.