Search for higgsinos with compressed mass spectra in final states with low-momentum leptons using $140~\textrm{fb}^{-1}$ of proton-proton collisions at $\sqrt{s}=13~\textrm{TeV}$

Although the Standard Model (SM) has attained substantial success in describing particle physics phenomena, it has confronted major challenges such as its inability to explain dark matter and to address the hierarchy problem. Hence, the ultimate goal of modern particle physics is to discover the physics beyond the SM. Supersymmetry (SUSY), a yet-to-be-found symmetry between fermions and bosons, is one of the leading candidates of physics beyond the SM. Under the framework of Minimal Supersymmetric Standard Model (MSSM) with $R$-parity conservation, scenarios with light higgsinos are viable candidates of new physics, as they are consistent with constraints from collider experiments and dark matter observations. Experiments at the Large Hadron Collider (LHC) have the potential to search for such scenarios, however, the sensitivities are limited by experimental difficulties owing to the compressed mass spectra of the higgsinos. The dissertation presents the search for higgsinos using $140~\textrm{fb}^{-1}$ of proton-proton collisions at center-of-mass energy of $\sqrt{s}=13~\textrm{TeV}$ at the LHC, targeting final states with low-momentum leptons. The sensitivity to higgsinos in the target mass regions has been considerably improved by the development and application of low-$p_{\textrm{T}}$ lepton track identification algorithms and the introduction of neural-network-based event selection techniques. No significant excess in data has been observed in the unblinded dataset, and exclusion limits have been set on the target higgsino scenarios. In the context of the simplified higgsino model with $\Delta m(\tilde{\chi}_{2}^{0},\tilde{\chi}_{1}^{0})=2\Delta m(\tilde{\chi}_{1}^{\pm},\tilde{\chi}_{1}^{0})$, higgsino mass of up to 130 GeV has been excluded for $\Delta m(\tilde{\chi}_{1}^{\pm},\tilde{\chi}_{1}^{0})=1\textrm{-}2~\textrm{GeV}$. This mass splitting region has been covered for the first time at the LHC, extending the limit by about 40 GeV in higgsino mass, where the original limit was set by the LEP2 experiment.