Search for Supersymmetry in Multi-Leptonic Final States with the ATLAS Detector at the LHC and Construction of New Precision Muon Tracking Detectors for the High-Luminosity LHC Upgrade

Supersymmetry (SUSY) is one of the best-studied extensions of the Standard Model (SM) of particle physics answering several open questions of the SM. SUSY postulates a superpartner differing in spin by one half for each of the Standard Model particles. This thesis presents two searches for Supersymmetry using 139 fb−1 of proton-proton collision data recorded by the ATLAS experiment at the Large Hadron Collider (LHC) at √𝑠 = 13 TeV. The first search focuses on final states with at least four charged leptons. Due to the low SM background, four-lepton final states promises good sensitivity to several supersymmetric models. The first type of models assumes R-parity violation via lepton number violating interactions, which allows the lightest supersymmetric particle (LSP) to decay into two charged leptons and a neutrino. The second type of SUSY models assumes gauge mediated SUSY breaking. In this models pair produced higgsinos decay into a gravitino LSP and a 𝑍 or Higgs boson. No significant deviations from the SM predictions have been observed. Exclusion limits on the masses of the SUSY particles and the branching ratios of the higgsino decaying into 𝑍 bosons have been set. The second search presented in this thesis targets the supersymmetric partners of the electron and muon. Especially light smuons are of particular interest, because such SUSY scenarios are capable of providing an explanation for the observed deviation of the measured value of the anomalous of the anomalous magnetic moment of the muon to the SM prediction. The search concentrates on a region of the parameter space with differences in mass of the slepton and the LSP between 20 and 60 GeV, which is not covered by previous searches. The LHC will be upgraded to the High Luminosity LHC, increasing the instanteneous luminosity by a factor of 7, in order to collect much more data to increase the sensitivity for physics beyond the SM. A major upgrade of the ATLAS detector is required to cope with the increased data rate. This thesis presents the construction of new small-diameter Monitored Drift Tube (sMDT) chambers that together with integrated resistive plate chambers (RPC) will replace the existing Monitored Drift Tube (MDT) chambers in the small sectors of the inner layer of the central detector region of the ATLAS muon spectrometer. Several measurements of the chamber geometry are performed during the production of each chamber, since precise knowledge of positions of sense wires of a sMDT chamber is crucial to achieve the target 10% momentum resolution for muons at 𝑝T ≈ 1 TeV