Search for Flavour-Changing Neutral Current Top-Quark Decays and Optimization of the Hadronic Tile Calorimeter Performance at the ATLAS Experiment

The main topic of the thesis is the search for new physics signal in the LHC proton-proton collisions data collected by the ATLAS experiment. A flavour-changing neutral-current (FCNC) top-quark decays $t\to qZ$ (q = u, c) are searched as a new physics signal using the top-quark$-$top-antiquark pair production events, with one top quark decaying through the $t\to qZ$ FCNC channel, and the other through the dominant Standard Model mode $t\to bW$. The search is performed in the data collected in 2012 at a center of mass energy of $\sqrt{s} = 8$ TeV and in 2015-2016 at $\sqrt{s} = 13$ TeV, corresponding to an integrated luminosity of 20.3 fb$^{-1}$ and 36.1 fb$^{-1}$, respectively. No evidence of a new physics signal is found. The 95% CL limits on the $t\to qZ$ branching ratio are set at $BR(t\to uZ) < 1.7\times 10^{-4}$ and $BR(t\to cZ) < 2.4\times 10^{-4}$, constituting the most stringent experimental limits to date of finishing of the dissertation work. The sensitivity of the ATLAS experiment in the search for $t\to qZ$ FCNC decays is studied in the context of the high luminosity phase of the Large Hadron Collider with a center-of-mass energy of 14 TeV and an integrated luminosity of 3000 fb$^{-1}$ . An improvement by a factor of four is expected over the current results with $\sqrt{s} = 13$ TeV data. The branching ratio limits that are obtained are at the level of 4 to $5\times 10^{-5}$ depending on the considered scenarios for the systematic uncertainties. The ATLAS detector performance studies are also performed and described in the thesis, which are important in the search for new physics. Namely, the high granularity upgrade of the ATLAS Hadronic Tile Calorimeter in the context of high luminosity LHC upgrade, and the electron energy calibration improvements for the ATLAS EM calorimeter crack region ($1.4 < |\eta| < 1.6$) are investigated.