Search for long-lived supersymmetry particles by signature of a high track-multiplicity displaced vertex using the LHC-ATLAS Experiment

Long-lived supersymmetry (SUSY) particles decaying within the tracking volume of the LHC-ATLAS Experiment can be reconstructed as a displaced vertex (DV). The search strategy involves attempting to reassemble the decay point of the long-lived particles (LLPs) by fitting vertices from the trajectories arising from the charged decay products. A search, looking for a signature of a massive high track-multiplicity DV has been conducted using data collected during 2012 by the LHC-ATLAS Experiment at $\sqrt{s}~=~8$ TeV, equaling to an integrated luminosity of 20.3 fb$^{-1}$. A signature of a massive displaced vertex is especially powerful due to the lack of any heavy long-lived standard model particles. Thereby, giving an analysis that is nearly background free. This dissertation describes the new, much more generic, "$DV+\text{jets}$" channel. In this channel events with high momentum jets and at least one displaced vertex are considered. Eliminating the requirement of an associated $\mu$ generated, to date of writing, the most powerful displaced vertex search. Several improvements have been made to the analysis. Extending the map over the tracking volume, used to veto nuclear interaction vertices, has made it feasible to increase the total fiducial volume from a previous $0.02$ mm$^{3}$ to $0.07$ mm$^{3}$ after applied veto. Moreover, a new technique to estimate combinatorial backgrounds of mis-reconstructed vertices has been developed, significantly reducing the total uncertainty on the estimate from $100\%$ to $14.6\%$. In the absence of any signal of new physics, limits on the production cross-sections are set. Interpreted in the context of R-Parity violating (RPV) SUSY, where the lightest SUSY particle decays to purely standard model particles through various RPV couplings. The excluded upper limits cross-section varies depending on lifetime of the long-lived particles and the targeted coupling, with a value of $0.3$ fb in the best case scenario. A model-independent limit on the cross-section is set at $0.14$ fb yielding an significant improvement from earlier result of $5.4$ fb. These limits are the most stringent to date. Limits on RPV SUSY LLP are set over a range of proper decay length 1-1000 mm, focusing on $\tilde{g}(1000~\text{GeV})\rightarrow qq+\tilde{\chi}_{1}^{0}(108~\text{GeV})\rightarrow qq+l$, where the neutralino is long-lived due to the smallness of the RPV coupling $\lambda'$. The upper limits on the cross-sections are set of neutralino decays to light flavour quarks and are ranging from 0.5 to 20 fb as a function of the proper decay lengths.