Advanced algorithms and software for primary vertex reconstruction and search for flavor-violating supersymmetry with the ATLAS experiment

Abstract

Our best current understanding of the universe is given by the Standard Model of particle physics, a theory that has proven to be remarkably successful in explaining and predicting many different physics phenomena, new particles and their interactions. Despite its great success, the Standard Model is known to be incomplete as it cannot provide explanations for various experimental observations, such as the overwhelming evidence for dark matter, motivating extensive research programs at future particle colliders to better understand physics beyond the Standard Model and thus reshape our most fundamental understanding of the universe. The reconstruction of charged particle trajectories (tracks) and their associated production vertices is a crucial component in the event reconstruction of almost all particle collider experiments. Extensive algorithmic and software improvements will be essential in order to maintain or even improve the performance of current track and vertex reconstruction algorithms under the upcoming computationally intensive challenges of ever increasing energies and luminosities at future particle colliders. This thesis introduces an entirely new, experiment-independent, thread-safe and highly performant vertex reconstruction software suite as well as novel algorithms for primary vertex reconstruction in high-luminosity environments. After its development and dedicated performance optimization, the newly developed software has been fully integrated and validated in the reconstruction software framework of the ATLAS experiment at the Large Hadron Collider (LHC). Due to its outstanding physics and CPU performance, the vertexing software developed in the course of this thesis will be used as the default ATLAS primary vertex reconstruction tool for LHC Run 3 and beyond. Furthermore, using the full ATLAS Run 2 dataset of 139 fb^-1 proton-proton collision data recorded at a center-of-mass energy of 13 TeV at the LHC, a search for flavor-violating supersymmetry, a theoretical extension of the Standard Model that is able to overcome many of its current limitations, is presented. The search targets an asymmetric 0-lepton final state with a top quark, a charm quark and large missing transverse momentum and represents a first of its kind analysis in ATLAS. No significant excess over the Standard Model prediction was found and observed (expected) exclusion limits of up to 880 GeV (1020 GeV) for the supersymmetric top quark partner mass in a flavor-violating supersymmetric model were obtained, excluding a previously largely unexplored region of supersymmetry parameter space.