Searches for new particles in final states with jet(s) and missing transverse momentum using first ATLAS Data

With the start-up of the Large Hadron Collider (LHC) a new era in Particle Physics is dawning. There is good reason to believe that new physics beyond the Standard Model will be discovered at the four experiments analyzing LHC collisions. This thesis contributes to the search for new physics at the ATLAS experiment analyzing events with missing transverse momentum, jets and no isolated leptons. The theoretical interpretation of the results focuses on Supersymmetry (SUSY) as one possible extension of the SM. A typical SUSY signature in the detector includes several jets from the decay cascades of the produced SUSY particles and missing transverse momentum due to the escaping lightest Supersymmetric particles (c.f. Chapter 2). Nevertheless, any other new phenomenon with the same signature would be discovered by the analyses. The layout of this work is as follows. Chapter 2 briefly introduces the theoretical aspects needed for the analysis part. After a short description of the SM, SUSY is presented as one of the possible extensions. The signatures of potential realizations are described in more detail. Besides the Minimal Supersymmetric Standard Model with a neutralino playing the role of the lightest Supersymmetric particle, the Split SUSY Model is introduced. Processes giving rise to a monojet signature in the detector are discussed. The LHC and the ATLAS experiment are covered in Chapter 3. The chapter introduces all detector subsystems emphasizing the calorimeters, which are the most important subdetectors for analyses requiring jets and missing transverse momentum. After the description of the trigger system, the ATLAS analysis chain is briefly introduced. Chapter 4 describes a very early Monte Carlo study, which proposes the use of a monojet analysis in order to spot detector problems. This study was done before any collision data was available. Another possible hazard to physics analyses is studied in Chapter 5. It is crucial to control non-collision background from cosmic rays, beam-gas and beam halo events. Multivariate techniques were used to optimize cuts suppressing this kind of background. Monte Carlo as well as early single beam and cosmic data were used in the study. After the preparational work, the following chapters describe two of the first searches for new physics with the ATLAS detector. For the search in channels with at least two or three jets, missing transverse momentum and no isolated lepton, the QCD background was determined using normalization factors from control regions. The method is described along with the analysis and the final results of the search in Chapter 6. The ongoing joint effort between the ATLAS SUSY and EXOTICS groups concerning searches for new physics in the monojet channel is described in Chapter 7. The QCD back-ground was determined using two different methods. The result of the search is interpreted as limit on the gluino mass in the Split SUSY model. The thesis also reflects the transition of the ATLAS experiment from the preparation to the early running stage.