Welcome to Twin Particles: From Novel ZZ Estimate to Searches for Supersymmetric Sleptons and Higgsinos using the ATLAS Run-2 Data and at the High-Luminosity LHC

After the Higgs boson discovery by the ATLAS and CMS collaborations at CERN, one of the main goals of the Large Hadron Collider programme is to find Beyond the Standard Model particles. This thesis focuses on the search for supersymmetric partners of the leptons (sleptons) and presents the prospects of a search for the supersymmetric partners of the electroweak gauge bosons and the Higgs boson (higgsinos). The detectable final state for both signatures consists of two electrons or muons with opposite electric charge. The search for the sleptons is performed using the full Run-2 data collected by the ATLAS detector, corresponding to an integrated luminosity of $139~\text{fb}^{-1}$ at $\sqrt{s} = 13~\text{TeV}$. In the regions sensitive to the slepton signal, the selected events are found to be consistent with the Standard Model predictions. The higgsino search is developed in the context of the next major upgrade of the LHC, the High-Luminosity LHC (HL-LHC). The thesis assesses the sensitivity reach on the search for higgsinos by the end of the HL-LHC era, by which the ATLAS detector is expected to have recorded an integrated luminosity of $3000~\text{fb}^{-1}$ at $\sqrt{s} = 14~\text{TeV}$. The results obtained show that using the full HL-LHC dataset, higgsinos with masses of about $200~\text{GeV}$ could be discovered, and masses up to $350~\text{GeV}$ could be excluded. The thesis also discusses the possibility of estimating the Standard Model $ZZ$ process using $Z\gamma$ events from data in Beyond the Standard Model searches, showing that theoretical uncertainties of about $4\%$ can be achieved. In addition to the LHC data analyses, the thesis reports on the design of a cooling setup with the aim of testing the silicon modules that will be placed in the upgraded ATLAS Inner Detector.