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The Large Hadron Collider (LHC) is currently the most powerful particle accelerator ever built. It accelerates protons and collides them at the center of mass energy of √s = 13 TeV. At one of the collision points of LHC, a general-purpose particle detector — ATLAS — is installed to measure the outgoing particles produced in the collisions allowing the study of interactions between the elementary particles. The work presented in this thesis uses the collisions data produced by LHC
and the data collected by ATLAS duringthe period of 2015-2018 which amounts to the integrated luminosity of 139 fb-1. This thesis focuses on searchesof spin-0 particlesin two areas of Beyond Standard Model physics: supersymmetry and extended Higgs sector. Supersymmetry offers a solution to the hierarchy problem by introducing partners to every Standard Model particle. Stop — the superpartner of the top quark — is particularly interesting due to its abilityto cancel the dominant top contribution in the Higgsboson mass loop correction, and therefore becomesthe target of a searchin this thesis. Many Beyond Standard Model scenarios extend the Higgs sector for they are motivated by neutrino oscillations, dark matter, and baryogenesis. One of the consequences of these scenarios is the prediction of extra Higgs-like scalar particleswhich may decay into the 125 GeV Higgs boson. This is the signature targeted by the second ATLAS data analysis documented in this thesis.
The thesis also includes a performance study of the Tile Calorimeter. The Tile Calorimeter is part of the ATLAS calorimeter system whose main task is to measure the energy of hadrons. The study is conducted on theTile Calorimeter using muons from W boson decay originating from proton-proton collisions. Each calorimeter cell response is measured in data and compared with detector simulation to verify that the energyscale in simulation matches that in the real detector.