Search for Supersymmetry in Monojet Final States with the ATLAS Experiment

Abstract: The Large Hadron Collider is the most powerful particle accelerator built to date. The LHC is a proton–proton and heavy ion collider, in 2015 it operated at an un- precedented center of mass energy of sqrt(s) = 13 TeV. This thesis presents the results of the search for physics beyond the Standard Model of particle physics in a compressed supersymmetric squark–neutralino model. The present work uses an experimental sig- nature with a single high energetic jet and large missing transverse energy, so called monojet signature. The search is carried out using an integrated luminosity of 3.2 inverse femtobarn recorded by the ATLAS experiment in 2015. No significant excess compared to the Standard Model prediction has been observed thus a 95% CL limit has been set on the production of squark pairs with the subsequent decay ~q -> q + chi01 (q = u, d, c, s). Squark masses up to 608 GeV are excluded for m(~q) - m(chi01) = 5 GeV significantly improving earlier results.The Tile Calorimeter is the ATLAS hadronic calorimeter covering the central region of the detector. It is designed to measure hadrons, jets, tau particles and missing energy. In order to accurately be able to properly reconstruct these physical objects a careful description of the electronic noise is required. This thesis presents the work done in updating, monitoring and studying the noise calibration constants used in the processing of data and the identification of hadronic jets. These studies showed an unexpected variation over time of the cell noise and further investigation led to discover that the tile noise filter was not behaving as expected in some situations in approximately 5% of the detector cells.