Search for dark sector promptly decaying neutral particles in 140 $\mathrm{fb}^{-1}$ proton-proton collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

Several new physics models predict the existence of dark neutral particles, or dark photons, which decay promptly to collimated pairs of leptons, referred to as lepton jets. In the dark sector model, both couplings between Higgs and dark Higgs via the Higgs portal and couplings between photon and dark photons via the vector portal are favored to be probed in the LHC-ATLAS experiment. In this thesis, we present a search for low-mass dark neutral particles, $\gamma_\mathrm{d}$, in the range between 17 MeV and 0.24 GeV using 140 $\mathrm{fb}^{-1}$ of data collected from 2015 to 2018 during the LHC Run-2 $pp$ collisions at $\sqrt{s}=13$ TeV. By exploring the electron final states using channels of $H \rightarrow (\gamma_\mathrm{d} \rightarrow e^+ e^-)(\gamma_\mathrm{d} \rightarrow e^+ e^-)+X$ in the Falkowski-Ruderman-Volansky-Zupan (FRVZ) model and $H \rightarrow (\gamma_\mathrm{d} \rightarrow e^+ e^-)(\gamma_\mathrm{d} \rightarrow e^+ e^-)$ in the Hidden Abelian Higgs Model (HAHM), we extend the search limits for $\gamma_\mathrm{d}$ mass from the previous 0.25 GeV to 17 MeV. No excess is observed, and we set an exclusion limit of 0.7% BR($H \rightarrow 2\gamma_\mathrm{d} + X$) for the FRVZ model and 0.07% BR($H \rightarrow 2\gamma_\mathrm{d}$) for the HAHM model at $m_{\gamma_\mathrm{d}}=17~\mathrm{MeV}$. Compared to other experiments, this analysis extends the search limits for both the Higgs and vector portals.