Probing Natural Supersymmetry With Initial State Radiation: The Search For Stops And Higgsinos At ATLAS

Natural supersymmetry has earned a unique focus in the search for new physics, in part because of its solution to the hierarchy problem. Minimizing fine tuning in this solution often puts the superpartners of the Higgs boson (higgsino) and top quark (stop) well within LHC reach. In the most natural scenario, the higgsino and stop masses are expected to be close to the masses of their Standard Model partners. Additionally, in the case of higgsinos with multiple mass eigenstates, these masses should be close to each other. Such small mass splittings are known as a compressed spectrum. In a decay chain this presents several experimental challenges, since the final state objects are very soft and there is little missing energy. In order to build a sensitive analysis around these difficulties, a variety of new techniques are utilized, including improvements with ISR-assisted topologies using the Recursive Jigsaw Reconstruction (RJR) method. In this thesis, these novel strategies and the idea of naturalness are used as a lens to interpret the status of stop and higgsino searches with ATLAS at the conclusion of the Large Hadron Collider Run II.