Search for Supersymmetric Partners of the Top Quark with Leptonic Signatures

Supersymmetry (SUSY) is one of the most elegant theories that can provide solutions to the problems in the Standard Model. Natural SUSY models favor light scalar top quarks (stops) and higgsinos. This dissertation presents two searches for the direct productions of stops using collision data at a center of mass energy of $\sqrt{s}=13~\mathrm{TeV}$ collected with the ATLAS detector at the Large Hadron Collider. The first search is performed using data collected in 2015-2016, corresponding to an integrated luminosity of 36 $\mathrm{fb}^{-1}$. The lightest supersymmetric particle (LSP) is assumed to be a higgsino-like particle, and a large mass-splitting between the stop and the LSP is considered. This is the first search where the three decay modes of the stop decaying into the higgsino-like LSP are taken into account. The second search targets a small mass difference between the stop and the LSP, referred to as ``compressed signal,'' using data collected in 2015-2018, corresponding to an integrated luminosity of 139 $\mathrm{fb}^{-1}$. In order to increase the acceptance of the compressed signal, a novel identification technique for low momentum $b$-hadrons (soft $b$-tagging) has been developed. The soft $b$-tagging is an algorithm based on the secondary vertex reconstruction. It shows an improvement by a factor of 10 in the identification efficiency for low momentum $b$-hadrons compared to the standard $b$-hadron identification algorithm in ATLAS. In both searches, no significant excess over the Standard Model expectation is observed. Exclusion limits at 95% confidence level are derived for comprehensive decay scenarios. For the large mass-splitting in the higgsino-like LSP scenario, stop masses up to 800 $\mathrm{GeV}$ are excluded for the LSP mass of 150 $\mathrm{GeV}$. In the compressed case with the stop four-body decay, the exclusion limit is extended by 240 $\mathrm{GeV}$ for the stop mass from the previous results, and stop masses up to 640 $\mathrm{GeV}$ for the LSP with a mass of 580 $\mathrm{GeV}$ are excluded.