Liao, Shell-may2021-02-012021-02-012019https://hdl.handle.net/10539/30456A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy to the Faculty of Science, School of Physics, University of the Witwatersrand, Johannesburg, 2019The discovery of the Higgs boson in 2012 by both the ATLAS and CMS experiments at the Large Hadron Collider (LHC) at CERN marked the beginning of a new age in particle physics. As the last missing piece in the Standard Model (SM), the Higgs boson plays a fundamental role in particle physics. Since its discovery, more emphasis has been placed on attempts to further understand its properties, opening up a new realm of beyond the Standard Model physics (BSM). A model was formulated based on various anomalies observed by both ATLAS and CMS in Run 1 of data taking. The model introduces a heavy scalar particle as well as an intermediate scalar, which can decay into both dark matter and SM particles. Initially, we hypothesize the heavy scalar decaying to the Higgs boson and two dark matter candidates (H!hÂÂ). A search is done in the diphoton channel and driven largely by missing transverse energy (Emiss T ) denoted h(!°°)ÅEmiss T . No significant excess over the expected background is observed in the dark matter search using data corresponding to an integrated luminosity of 36.1 fb¡1. As a result, the search was then redirected towards other final states. In November 2016, an excess was observed with a diphoton invariant mass around 248 GeV associated with moderate Emiss T . In view of this result, we looked at the direct decay of H into °°, where H(!°°)ÅEmiss T . We looked for and found the same excess in Run 1 data. With the implementation of several Emiss T suppression techniques, the diphoton resonance search was performed using Run 2 data and extrapolated to the Z° final state. The analysis was redone following the migration from ATLAS Release 20 to Release 21 and was also documented. For the °° channel most of the excess is found around approximately 246 GeV with a maximum local significance of 3 ¾ and a global significance of 2.5 ¾ (2015+2016+2017 data). In the Z° channel, the excess is found around 252 GeV with a maximum local significance of 4.14 ¾ and global of 3.41 ¾ (2015+2016+2017 data). This document also details the reconstruction and performance of Emiss T in these diphoton final state searches. The stability of the performance of Emiss T is essential for all BSM searches at the LHC which adds a significant amount of value to this documentation. In an effort to aid in the 2019-2020 ATLAS detector upgrade, a comparative study was done to determine the most radiation hard brand of plastic scintillators in order to replace the gap and Minimum Bias Trigger scintillators of the Tile Calorimeter. The scintillators are exposed to harsh radiation environments within the detector, making them susceptible to damage. After experimenting on various scintillator grades, the EJ 200 polyvinyl toluene based scintillator was found to be the most suitable brand as it showed a fairly low decrease in light transmission, and a substantial amount of annealing after irradiation for intermediate to small dosesenThesis