Searches for supersymmetry in final states containing b-tagged jets with the ATLAS detector

Supersymmetry is one of the most theoretically studied subjects in modern particle physics; so far no direct experimental proof of its existence has been observed. The Large Hadron Collider (LHC) is a machine designed to create high energy particle collisions, which are analysed by multiple experiments, probing the substructure and fundamental properties of matter. The experiment ATLAS is used in this thesis in searches for the signatures supersymmetric particles as they decay. A detailed overview of two signature searches for third generation supersymmetry in events with b-tagged jets is the main focus of this thesis. A data-driven technique for estimating the Multi-jet background in zero lepton final state signatures is additionally presented.
The firrst search used a dataset with an integrated luminosity of 3.2 fb−1 collected in 2015 during Run-II of the LHC at a centre-of-mass energy of 13 TeV. The analysis was optimised for a simplified signal model in which only the supersymmetric partners of the bottom quark are pair-produced in LHC collisions. No significant excess above the Standard Model background was observed, setting 95% CL limits on the masses of the scalar bottom quark and lightest neutralino. Many sophisticated techniques for estimating the Standard Model backgrounds were employed using Monte-Carlo simulation and data-driven techniques, which are applicable to many hadron collider analyses. The prospects of future discovery of scalar bottom quarks were studied, resolving a 5σ discovery potential above 1 TeV at the High Luminosity LHC for low mass neutralinos.
The second search was performed at the end of Run-I of the LHC using a dataset corresponding to an integrated luminosity of 20.3 fb−1 at 8 TeV. The analysis focused on a unique search for the pair-production of scalar top and bottom quarks decaying asymmetrically to neutralinos and charginos in a more complex and arguably more realistic model scenario. The models targeted were inspired by a natural pMSSM scenario with low mass supersymmetry partner of the third generation quarks. Again, no significant deviation from the Standard Model background was observed setting 95% CL limits on the masses of the third generation supersymmetric quarks. The results of the analysis were interpreted in the context of a full pMSSM scan together with many other ATLAS analyses to provide the theoretical community with a more meaningful summary of the exclusion limit on supersymmetric particles set by ATLAS.