Search for Dark Matter and Extra Dimensions in Monojet Final States with the ATLAS Experiment

Abstract: The CERN Large Hadron Collider (LHC) is the most powerful particle ac- celerator ever built, in terms of luminosity and center-of-mass energy of the collisions. This thesis presents a search for Dark Matter and Extra Dimen- sions in the proton-proton collisions as recorded by the ATLAS detector.The ATLAS Tile Calorimeter constitutes the central part of the ATLAS hadron calorimetry. In order to accurately reconstruct physics objects such as hadronic jets and missing transverse energy it is important to have an accurate description of the electronic noise. This thesis presents a study of several aspects of the electronic noise, such as stability over time and typical energies. A dedicated study of the electronic noise levels in 40 calorimeter modules for which the Low Voltage Power Supplies were exchanged during the December 2011 LHC shutdown is presented. It is shown that with the new power supplies the electronic noise is lower than before, and its distribution is closer to Gaussian.A search for Dark Matter and Extra Dimensions in a monojet final state with an energetic jet and large missing transverse energy is presented. The analysis is performed using an integrated luminosity of 20.3 fb−1 of collisions at √s = 8 TeV. No significant excess over the Standard Model background is observed, so exclusion limits are set on the pair production of WIMPs and on the production of gravitons in the ADD Extra Dimensions scenario.