Search for long-lived Supersymmetric particles using displaced vertices with the ATLAS detector at the LHC

The existence of long-lived particles (LLPs) is a common feature in many theories beyond the Standard Model. For example, models with small couplings (i.e. R-parity-violating supersymmetry) and models allowing for decays via highly virtual intermediate states (i.e. Split supersymmetry) predict the presence of LLPs. With lifetimes ranging from picoseconds to nanoseconds, massive LLPs could decay to several electrically charged particles in the inner tracking volume of the ATLAS detector at the Large Hadron Collider, resulting in the reconstruction of a displaced secondary vertex. This thesis presents the first search for high mass, multi-track displaced vertices and multiple energetic jets using the full Run-2 dataset collected by the ATLAS detector, corresponding to an integrated luminosity of 139 fb$^{-1}$ at $\sqrt{s}=13$ TeV. The observed event yields are compatible with those expected from background processes. The results are presented as exclusion limits at 95 % confidence level on supersymmetric scenarios with non-zero R-parity violating couplings. This thesis also presents the ATLAS FastTracKer (FTK), which is a hardware-based track finder envisioned to be integrated into the ATLAS trigger system. The trigger system is responsible for selecting interesting events with a high purity at a rate of only 1 kHz. The FTK was designed to provide tracking information of all charged particles with $p_\text{T}>1$ GeV in the ATLAS inner detector, which is a powerful discriminant for selecting interesting events. The training of the FTK system to achieve a high track-finding efficiency and the adaptability of the system to new detector conditions is presented. Furthermore, the applicability of the system to select candidate events containing long-lived particles is described.