Search for Dark Matter Produced in Association with Heavy Standard Model Particles at $\sqrt{s} = 13$ TeV with the ATLAS Detector at the LHC

Dark Matter composes a significant part of the Universe, while its physical nature remains unknown. This thesis presents two searches for Dark Matter produced in association with heavy Standard Model particles using $pp$ collision data at a center-of-mass energy of $\sqrt{s} = 13$ TeV recorded by the ATLAS detector at the Large Hadron Collider. Moreover, jet mass scale calibrations for variable-radius calorimeter jets are performed to improve the reconstruction performance of heavy particles in boosted event topologies. A data sample corresponding to an integrated luminosity of 36.1 fb$^{-1}$ is analyzed in the $E_{T}^{miss}+V$ (hadronic) search, which selects processes with hadronic decays of $W$ and $Z$ bosons in association with large missing transverse energy. The $E_{T}^{miss}+t\bar{t}$ (fully-hadronic) search is performed using 126.7 fb$^{-1}$ of collision data and targets events containing fully-hadronically decaying top quark pairs and medium missing transverse energy. No significant excess over the Standard Model prediction is observed in both analyses. The results of the $E_{T}^{miss}+V$ (hadronic) search are interpreted in terms of constraints on the parameter space of spin-1 vector mediator simplified model and mediator masses of up to 650 GeV are excluded for Dark Matter masses of up to 250 GeV at 95% confidence level with a dark sector coupling of 1.0 and a coupling to Standard Model particles of 0.25. The results of the $E_{T}^{miss}+t\bar{t}$ (fully-hadronic) search are interpreted in the framework of spin-0 mediator simplified models with unitary couplings. For scalar and pseudo-scalar mediators, masses below 190 GeV and 240 GeV are excluded assuming a Dark Matter mass of 1 GeV, respectively. The implications of these results are discussed and compared to results from current direct detection experiments.