Search for new heavy resonances in the dilepton final state and calibration of the LAr Phase-1 Upgrade demonstrator at the ATLAS experiment

The Standard Model (SM) of particle physics was tested to a high precision at various experiments in the last decades and no significant deviation has been found so far. The LHC is a perfect machine to search for new phenomena, as its energy reach is considerably higher than that of previous accelerators. The so-called Run 2 of the LHC covers the data taking period between 2015 and 2018, during which proton-proton collisions at a centre-of-mass energy of $13\,\mathrm{TeV}$ were recorded. The full Run 2 integrated luminosity to be used by the ATLAS analyses is $139\,\mathrm{fb}^{-1}$. Many theories beyond the Standard Model predict additional resonances that can decay leptonically. A search for dielectron and dimuon resonances in the $250\,\mathrm{GeV}$ to $6\,\mathrm{TeV}$ mass range, based on the full Run 2 dataset, is presented in this thesis. The background estimation in the analysis is performed by fitting a functional form to the data. A generic signal shape is chosen in order to facilitate reinterpretations of the fiducial cross-section limits. Lower resonance mass limits are set for benchmark models, and reach $4.5\,\mathrm{TeV}$ for the $\mathrm{E}_6$-motivated $Z'_\psi$ boson in the dilepton channel. This search is one of the flagship analyses of the ATLAS experiment. In addition to the dileptonic decay modes, heavy new resonances can decay into other final states such as pairs of SM $W$ and $Z$ bosons, $VH$ pairs with $V \in \lbrace W, Z \rbrace$ and $H$ as the SM Higgs boson, or pairs of a lepton and a neutrino. Combining searches in various decay channels extends the discovery reach by exploiting their complementarity. The combination of the $VV$, $VH$, $\ell\nu$ and dilepton final states with $36.1\,\mathrm{fb}^{-1}$ of $2015-2016$ data is detailed in this document. The increased instantaneous luminosity of the LHC in the next data-taking period (Run 3) requires an upgrade of the ATLAS liquid argon (LAr) calorimeter trigger electronics. The new electronics increase tenfold the granularity of the LAr information provided to the first level of the trigger system and will enable a more advanced selection at the hardware level of the trigger. To test pre-prototypes of the upgraded electronics, an in-situ demonstrator system was installed at the start of Run 2 to collect data with the new read-out in parallel to the current one. The energy and timing calibration of the demonstrator system is presented in this thesis. Further detailed studies, performed with data collected in 2017, include the comparison of the energy and timing reconstruction between the output of the upgraded system and the standard LAr read-out, event-level shower shape information and pulse shape predictions, among other measurements. A good agreement with the expectation is found, demonstrating the readiness for the Run 3 data-taking period.