Electron reconstruction and performance studies, search for a heavy Higgs boson decaying to four-leptons using the ATLAS detector, irradiations at the Birmingham Irradiation Facility for the HL-LHC

This thesis presents electromagnetic (EM) calorimeter performance studies using electrons and photons, a search for a heavy Higgs state using the ATLAS detector at the Large Hadron Collider (LHC) and irradiation work at the Birmingham Irradiation Facility. A new electron reconstruction algorithm using the ATLAS detector is described, and the performance of these electrons is compared to the current ATLAS algorithm. Multivariate techniques are used to derive calibration weights for the energy response of the EM calorimeter. This thesis also presents cross-section times branching ratio limits in the search for a heavy Higgs state in the four-lepton channel, using 20.7 fb−1 of $\sqrt{s}$ = 8 TeV ATLAS data. The observation of the first π 0 → γγ peak during Run-II using 13 TeV ATLAS data is presented, and J/ψ → e+e−events at 13 TeV are studied, focusing on background subtraction techniques to compare the EM calorimeter shower variables in data with Monte Carlo simulations. Finally, irradiations of materials and components to be used at the High Luminosity LHC (HL-LHC) are performed at the Birmingham Irradiation Facility. This thesis describes the commissioning and running of the facility, the irradiation procedure and results of irradiations.