Search for production of charginos and neutralinos with a small mass gap to the lightest supersymmetric particle with ATLAS at √s = 13 TeV

The supersymmetry search performed in this thesis focuses on the production of charginos and neutralinos decaying to the lightest supersymmetric particle. The research is performed with the ATLAS experiment at CERN in Geneva. The supersymmetric models are not yet excluded in the case of a small mass gap between the produced chargino and neutralino. These models are also well-motivated through global fits, which take into account data from multiple experiments. The produced supersymmetric particles decay into a final state of three leptons and missing energy. The models with a small mass gap are arduous to detect since they result in low transverse momentum leptons and low missing energy. Reconstruction and triggering become difficult in this scenario because there are many different background processes with similar signatures in the detector. For this reason new identification techniques are used in this thesis to reconstruct leptons that can be classified to result from the signal process. The search at low missing energy also closes uncovered phase space as it is important to care for unintended and untargeted signatures. The sought for supersymmetric models have not been found, making it possible to exclude them. The models enclosed in the region where the mass of the lightest supersymmetric particle is 250 GeV and the mass of the produced chargino or neutralino is 300 GeV have been excluded. Although the analysis was not able to find evidence or a discovery of supersymmetry, it was able to exclude a large part of before uncovered phase space.