Search for Dark Matter Produced in Association with Heavy Standard Model Particles at 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 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 EmissT+V (hadronic) search, which selects processes with hadronic decays of W and Z bosons in association with large missing transverse energy. The EmissT+tt¯ (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 EmissT+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 EmissT+tt¯ (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