Search for Dark Matter in events with a highly energetic jet and missing transverse momentum in proton-proton collisions at $\sqrt{s}$ = 8 TeV with the ATLAS Detector

Any physics analysis at a collider experiment heavily relies on an efficient trigger system to filter out potentially interesting events. To ensure stable operation, a continuous and detailed real-time monitoring is essential. Two such online monitoring features for the Central Trigger of the ATLAS experiment at the Large Hadron Collider (LHC) at the European centre for particle physics, CERN, are developed as part of this thesis and are presented in detail. To prepare the ATLAS experiment for the second run of the LHC starting in 2015, among other systems the Central Trigger hardware will be upgraded to remove resource limitations and allow for the connection of newly installed systems. This thesis reports on the corresponding changes and extensions in the simulation of the Central Trigger, the implementation of which is part of this work. A further part of this thesis presents a search for Dark Matter candidates. Cosmological observations indicate that about 80% of the matter content of the universe consist of a form of non-luminous matter which is traceable only due to its gravitational interaction and for which the Standard Model of particle physics does not provide a viable candidate. A number of experiments searches for evidence of Weakly Interacting Massive Particles (WIMPs), that in a natural way could account for the observed present day abundance of this Dark Matter. In recent years, also the search for WIMP pair production at hadron colliders has gathered momentum. A possible signal signature at a collider is a jet originating from initial state radiation and recoiling against a pair of WIMPs, leading to events with a highly energetic jet and a large amount of missing transverse momentum due to the WIMPs leaving the detector without interacting. The signal is thus expected to manifest itself as an excess above the Standard Model prediction at large missing transverse energy ($E_{\text{T}}^{\text{miss}}$). The search for WIMP candidates presented in this thesis uses such mono-jet events, based on 20$\,$fb$^{−1}$ of proton-proton collision data collected in 2012 with the ATLAS detector at a centre-of-mass energy of $\sqrt{s} = 8\,$TeV. The main Standard Model backgrounds are estimated in a semi-data driven way. The event selection is optimised with respect to the sensitivity for a WIMP signal and the search is performed in eight signal regions of increasing $E_{\text{T}}^{\text{miss}}$. No significant excess is observed and model independent limits both at 90% and 95% confidence level (CL) are set on the cross section for new physics. In addition, 90% CL limits are derived on the suppression scale of an effective field theory (EFT) for various operators and compared to the results from other search experiments. The collider limits for all considered effective operators are stronger than the bounds from other experiments at low WIMP masses in the case of spin- independent interactions, and over a large mass range for spin-dependent interactions. In the light of concerns about the applicability of an EFT at LHC energies, the results are furthermore interpreted in terms of a simplified model with an s-channel vector mediator. A simulation based sensitivity study on the prospects of the Dark Matter search with mono-jet events at a centre-of-mass energy of 14$\,$TeV is presented and expected limits at 95% CL as well as discovery potentials are given. It is found that already with the first few fb$^{−1}$ of $\sqrt{s} = 14\,$TeV data the expected limits can improve by a factor of 2. The discovery potential ultimately reaches up to suppression scales of $2.6\,$TeV, while for $\sqrt{s} = 8\,$TeV it is of the order of $700\,$GeV.