Searches for dark matter and single top-quarks with the ATLAS experiment

Dark matter (DM) remains one of the unrevealed mysteries of the universe. Even though it constitutes ∼ 85% of the matter, considerably little is known about DM, despite its significant influence on the dynamics of galaxies and the expansion of the universe. The search for DM at colliders marks an important pillar in exploring all possible realisations of DM. A search for DM particles with the ATLAS experiment at the LHC is presented in this thesis. The full run-II dataset of sqrt(s) = 13 TeV proton-proton collisions with an integrated luminosity of 139 fb-1 collected from 2015 to 2018 is used. A model with an extended Higgs-sector is probed in the search. In this 2HDM+a model, a second Higgs doublet, a pseudo-scalar DM mediator and a fermionic DM particle are added to the Standard Model of particle physics (SM). DM particles produced in association with a top-quark and a W -boson are searched for. The top-quark decays into a W-boson and a b-quark. This gives a total of two W-bosons in signal events. Both of these can decay into quarks or leptons. Depending on the decays of the W-bosons, the final state of this search is characterised by zero, one or two charged leptons. Three analysis channels are defined according to the charged lepton multiplicity and referred to as 0L, 1L and 2L channel. The search presented in this thesis focusses on the 1L channel. In addition, all final states are statistically combined to provide the most stringent exclusion limits in terms of 2HDM+a model based DM models. The 1L channel exploits that one of the W-bosons can, if decaying hadronically and having a relatively high-pT, be reconstructed in a large-radius jet and be identified using a procedure called W-tagging. This significantly increases the signal-to-background ratio. Selected events must have high missing transverse momentum due to the elusive nature of DM particles and at least one b-tagged small-radius jet. Consistency between the SM prediction and the data is observed and exclusion limits at 95 % CL on the normalisation of BSM signals are derived. The analysis of the full run-II dataset and the statistical combination of all final states constrain large areas of the parameter space. Masses ma up to 400 GeV as well as masses mH± below 300 GeV and beyond 2000 GeV and values of tan β up to 2.2 are excluded. Interesting opportunities to further explore challenging corners of the parameter space arise, e.g. extending the sensitivity at high values of tan β.