Search for third generation scalar quarks in events with b-tagged jets with the ATLAS detector

The thesis presents the results of two searches for the direct pair-production of third generation scalar quarks, the stop and the sbottom, in proton-proton collisions at $\sqrt{s}=$13 TeV delivered by the Large Hadron Collider (LHC) and recorded by the ATLAS detector. Third generation squarks are studied in the context of natural supersymmetric extensions of the Standard Model, highlighting their role in the solution of the Higgs hierarchy problem and considering both R-parity conserving and violating decay scenarios. The signal models of interest produce final states characterised by the presence of two bottom quarks, and the identification of the hadronic jets generated by their fragmentation plays a crucial role in the analyses. The performance of b-jet identication algorithms is studied in detail, and a novel approach for the estimate of the associated systematic uncertainties is presented. The first analysis in the thesis is a search for a pair-produced sbottom with two-body decays into Standard Model third generation quarks and quasi-degenerate electroweakinos, while the second targets the pair-production of the stop followed by R-parity violating decays into a bottom quark and a lepton. No evidence of SUSY particles is found, and exclusion limits are set on the relevant signal models using dedicated statistical tools.