Searching for Supersymmetry in Boosted Fully-Hadronic Final States with B-Jets and Calibrating B-Jet Identification Using Spatially Matched Muons at the ATLAS Detector

This thesis presents two analyses that involve the identification of hadronic jets produced by the fragmentation of $b$-quarks or ``$b$-jets''. The identification exploits the hard fragmentation of $b$-quarks and relatively long lifetime of $B$ hadrons. The first analysis is the calibration of the efficiency of the $MV2c10$ $b$-jet identification algorithm using a sample of $b$-jets with muons from $B$ hadron decays and a kinematic property of these jets knows as ``$p_{\text{T}}^{\text{rel}}$'', which is constructed from the muons. This analysis was performed using 68 $fb^{-1}$ of data collected at $\sqrt{s} = 13$ $Te\kern -0.1em V$ with the ATLAS detector during Run 2 of the Large Hadron Collider. Improvements in the methods of this calibration have led to a reduction by more than an order of magnitude in the uncertainty of earlier measurements of $b$-jet tagging efficiency using this approach. This work included substantial contributions to the software and computing framework used to the study the identification of jets produced by heavy flavor decay; these contributions are described herein. The second analysis is a search for electroweakly-produced supersymmetric partners of the gauge bosons or ``gauginos''. This search uses 140 $fb^{-1}$ of data collected at $\sqrt{s} = 13$ $Te\kern -0.1em V$ with the ATLAS detector during Run 2 of the Large Hadron Collider. My work focused on developing a new signal region that targeted final states specifically containing two $b$-jets and two light-quark jets. As part of this optimization, I examined different supersymmetric scenarios and explored alternative techniques for estimating the Standard Model backgrounds. Based on preliminary results, in the wino-bino scenario, this search is expected to provide sensivity to charginos with masses up to $\sim1$ $Te\kern -0.1em V$. For a scenario in the general-gauge-mediation model (where higgsino becomes the next-lightest SUSY particle), this search will provide sensitivity for higgsinos with masses up to 600 $Ge\kern -0.1em V$ (discovery) or 800 $Ge\kern -0.1em V$ (exclusion) for most of the possible $Z/h$ branching ratios. Finally, the sensitivity to the scenarios where higgsino is next-lightest and bino is the lightest SUSY particle has been studied, on which no explicit exclusion has been reported yet by the searches in LHC.