Development and improvement of track reconstruction software and search for disappearing tracks with the ATLAS experiment

High-energy particle physics concerns itself with the most fundamental level at which the laws of nature can be understood. Using particle collisions, it is possible to probe and measure the properties of particles and their interactions. The Standard Model of particle physics is a set of theories describing three of the four fundamental interactions, and is the baseline with which observations are interpreted. Even though the Standard Model allows precise calculations of particle phenomena, it is thought to be incomplete, as certain observations like dark matter or neutrino oscillations remain unexplained. The analysis of particle collisions requires the measurement of particles produced in these collisions. One major part of these measurements is the reconstruction of the trajectories of charged particles, called tracks. Dedicated sensitive elements are used to obtain measurements of the particle along its trajectory, in order to ultimately reconstruct the track using software algorithms. Track reconstruction is a complex application, which grows in complexity with event activity. Therefore, future increases of the instantaneous luminosity of the LHC pose a challenge and require advancements in both the computational and physics peformance of track reconstruction algorithms. One part of this thesis presents work toward the development and improvement of track reconstruction in the context of the ATLAS experiment, and an experiment-independent software toolkit called ACTS. Additionally, the effort to use ACTS components in the ATLAS software is described. Specific contributions that were made to both domains are shown, which address the aforementioned challenge. Prominent examples are the description of current and future ATLAS tracker geometries, concurrent handling of misalignments, and the improvement of data structures used for reconstruction. Another part of this thesis describes a concrete application of these reconstructed particle tracks, in the form of an analysis of data from proton-proton collisions at $\sqrt{s} = 13$ TeV recorded by ATLAS, searching for an extension of the Standard Model. In this theoretical scenario, particles travel through the innermost part of the tracker, the Pixel detector, before decaying, resulting in so-called disappearing tracks. The analysis described here uses recent developments in the dedicated techniques used to reconstruct these short tracks to evaluate their expected sensitivity. An increase in expected sensitivity in the form of a higher expected mass limit is found to result from inclusion of new shorter tracks than were used in previous analyses. Potential for further developments is discussed in light of the LHC upgrade, and future colliders.