Exotic Decays of Vector-Like Quarks and Development of a Test Procedure for the ITk Strip Module at the ATLAS Detector for the HL-LHC

Sammanfattning: The naturalness problem of the Standard Model (SM) challenges our understanding of the origin of mass. Several theories have been proposed to tackle the problem, one of them is the Composite Higgs Model (CHM) where the Higgs boson of the SM is not an elementary scalar but instead a composite object bound at a confinement scale of a TeV. The fine-tuning of the Higgs boson mass, arising from the naturalenss problem, is thus reduced by the lower cut-off scale of the SM and an overall symmetry protection in the theory. Consequences of the CHM are new states, both fermions, so called Vector-Like Quarks (VLQ), and scalars with masses low enough to be produced at the LHC.In ATLAS, existing searches for VLQs assume decays into known SM particles. In this thesis, a phenomenological study is presented where the VLQ decays to a Beyond the Standard Model (BSM) scalar or pseduoscalar, quantifying the sensitivity of the current experiments at the LHC to the such decays. The reach is well above 1 TeV in VLQ mass thus directly competing with VLQ decay to known SM states.The phenomenological study is continued with the ATLAS experiment where the VLQ decays into a BSM scalar with a diphoton final state. The data-driven background modeling is covered and expected limits are presented showing good sensitivity to exotic decays of VLQ. The LHC is reaching its limit in energy and luminosity. The next step is an upgrade to the High-Luminosity LHC (HL-LHC), significantly raising the data rates. For the ATLAS experiment to cope with the data rates delivered by the HL-LHC, the detector needs to the upgraded. In this thesis, a detailed description of a test procedure for a trigger and readout scheme for the new inner tracker strip detector modules is presented.