Search for supersymmetric particles in final states with multiple heavy-flavour jets with the ATLAS detector

<!--HTML-->This dissertation presents a search for supersymmetry in final states containing multiple $b$-quarks and large missing transverse energy using proton-proton collisions at a center-of-mass energy of $\sqrt{s}=13$ TeV as provided by the Large Hadron Collider (LHC) and recorded by the ATLAS experiment. The collisions used in the analysis correspond to the data taking period from 2015 to 2018, also known as LHC Run 2, amounting to a total integrated luminosity of 139~fb$^{-1}$. <br>The search targets a model for higgsino pair production in the context of R-parity conserving supersymmetry coupled with gauge-mediated symmetry-breaking mechanisms, where the higgsino is expected to be light due to naturalness arguments and decay to a Higgs boson and a gravitino, which is the lightest supersymmetric particle (LSP). The analysis is optimized focusing on the dominant decay channel of the Higgs boson into bottom-antibottom quark pairs. Thus, the characteristic final states contain high multiplicity of $b$-jets. The gravitinos do not interact with the detector, and therefore the final states are expected to exhibit large values of missing transverse energy. This is the only analysis in ATLAS optimized to probe the high-mass regime for the targeted model with large higgsino decay branching ratio into a Higgs boson. <br>No significant excess over the Standard Model prediction is observed in the analysis. The results are interpreted in the context of simplified models in order to translate them into upper limits on the higgsino pair production cross section. The search excludes at 95% confidence level (CL) all higgsino mass hypotheses between 200 and 950 GeV for scenarios with 100% decay branching ratio into a Higgs boson and a gravitino. Additionally, 95% CL exclusion limits for mixed branching ratio scenarios are derived, for which all mass hypotheses between 250 and 800 GeV are excluded for models with a branching ratio of 50% into a Higgs boson. The lowest value excluded for the decay branching ratio into a Higgs boson is 15% for a higgsino mass of 400 GeV.