Searches for electroweak production of supersymmetric particles in compressed mass spectra with the ATLAS detector in LHC Run 2

Despite the success of the Standard Model of particle physics, numerous hints indicate the presence of new phenomena, not contained in this theoretical framework. A promising candidate to introduce physics beyond the Standard Model is Supersymmetry, which predicts the existence of a supersymmetric partner for each of the Standard Model particles. This thesis presents searches for the electroweak production of supersymmetric particles with compressed mass spectra. The searches use 139 fb${}^{-1}$ of proton-proton collision data recorded by the ATLAS detector at a center-of-mass energy $\sqrt{s}$ = 13 TeV in Run 2 of the Large Hadron Collider. Selected events are characterized by missing transverse momentum, two same-flavor, oppositely charged leptons with low transverse momentum, and hadronic activity from initial-state radiation. Dedicated event selections extend the analysis towards lower transverse momenta to enhance the sensitivity reach of the searches. The extension of the muon reconstruction efficiency measurements towards lower transverse momenta allows to probe also the very compressed regime, in which the supersymmetric states differ in mass by several hundreds of MeV to a few GeV. No significant deviations from the Standard Model predictions are found in the data. The results are interpreted in terms of mass exclusion limits for simplified models of higgsino and slepton production, in which the lightest supersymmetric particle is a neutralino with similar mass to a chargino and a heavier neutralino, or to a slepton. In the higgsino model, neutralino masses up to 162 GeV are excluded. Excluded mass splittings between the lightest and heavier neutralino range from 53 GeV down to 2.6 GeV. Lower limits on slepton masses are extended up to 256 GeV, where mass splittings between the slepton and the neutralino from up to 29 GeV down to 590 MeV can be excluded.