Search for exotic Higgs boson decays to four leptons with the ATLAS detector

Searches for exotic Higgs boson decays are well motivated by various theoretical models, as well as the constraints from Higgs boson measurement results. The focus is on exotic Higgs boson decays to a pair of beyond-the-Standard-Model spin-0 particles $a$. This dissertation conducts two analyses for exotic Higgs bosons decaying into leptons, which are $H\rightarrow2a\rightarrow4\mu$ and $H\rightarrow2a\rightarrow4\tau$, and target the mass ranges of $1<m_a<15\text{ GeV}$ and $15<m_a<60\text{ GeV}$. Various data-driven techniques are developed and used to estimate the background that cannot be well modeled with the standard simulation in ATLAS. The searches use proton-proton collision data collected with the ATLAS detector at the LHC with an integrated luminosity of $139\text{ fb}^{-1}$ and a center of mass energy $\sqrt{s}=13\text{ TeV}$. No significant excess has been observed. Therefore, this dissertation summarizes the $95\%$ confidence-level upper limits of the branching ratio of exotic Higgs decays to a pair of beyond-the-Standard-Model light scalars in the lepton final states.