Search for Dark Matter produced in association with a Higgs boson decaying to a pair of b quarks and combination of Dark Matter search with 2HDM+$a$ with the ATLAS detector

Dark Matter (DM) searches are of great importance to the LHC program. In this talk, I will first present a DM search using the Higgs boson as a portal. This search targets events that contain large missing transverse momentum and either two $b$-tagged small-radius jets or a single large-radius jet associated with two $b$-tagged sub-jets. No significant excess with respect to the SM prediction is observed. The results are then interpreted to set limits on two benchmark models with a second Higgs doublet and an additional heavy vector boson (2HDM+$Z^\prime$) or a pseudoscalar singlet (2HDM+$a$), both of which provide a viable DM candidate. I will focus on how events are selected and categorized to enhance sensitivity to DM signals, and to help model the major backgrounds. Finally, I will also talk about the latest summary and combination of the searches for DM within 2HDM+$a$.