Studies of Long-Lived Supersymmetric Top Quarks and Online Monitoring of the ATLAS First Level Calorimeter Trigger

Abstract: The Large Hadron Collider (LHC) has recently started colliding protons  at  7 TeV centre-of mass energy. Extensive searches for hitherto unobserved particles and physics processes will be made as part of an ambitious scientific programme which aims to answer some of the fundamental questions in modern physics. The ATLAS detector, designed to discover a wide range of physics processes, will play a crucial part in these searches.The First Level Calorimeter Trigger is one of the first steps in the ATLAS data acquisition chain. Excellent performance at this stage is required to fully make use of the ATLAS potential for discovery of new physics processes. Online monitoring of the First Level Calorimeter Trigger is therefore of great importance to ensure good quality of data and correct performance of the trigger.  As part of the overall online trigger monitoring this thesis describes the monitoring of the Jet/Energy-sum Processor, responsible for defining the first jet candidates and summation of the total energy deposited in the ATLAS calorimeters.Supersymmetry is one of the most studied models of physics beyond the Standard Model. Some supersymmetric models give rise to long-lived stop squarks or gluinos which, if produced at the LHC,  immediately will form exotic hadrons, called R-hadrons. The discovery or non-discovery of R-hadrons can provide important information about physics beyond the Standard Model and searches for these particles are therefore a part of the ATLAS scientific programme. This thesis investigate the capability of the ATLAS detector to discover stop R-hadrons using different track configurations and ß measured by the calorimeters. A technique for measuring the R-hadron mass using time-of-flight information from the ATLAS  muon detectors is demonstrated as well. Included in this thesis is also a phenomenological study of the possibility for a long-lived stop squark to arise in supersymmetric models where the neutralino is the lightest supersymmetric particle. The conditions for the stop to be long-lived as well as the implications it has on the  dark matter density and the supersymmetric mass  spectra  are studied in detail.