Searches for R-Parity Violating Supersymmetry in Multilepton Final States with the ATLAS Detector

This thesis presents two searches for signs of R-parity-violating supersymmetry (SUSY) through decays of the lightest supersymmetric particle (LSP) into two charged leptons and one neutrino. The searches are performed with the ATLAS experiment at the Large Hadron Collider (LHC) using a data set of $pp$ collisions at $\sqrt{s} = 8\TeV$ recorded during the 2012 LHC run, corresponding to an integrated luminosity of $20.3\ifb$. As a prerequisite for these studies, which rely heavily on efficient reconstruction of charged leptons, a tag-and-probe method based on $Z\to\mu\mu$ decays is developed to measure the muon reconstruction efficiency of the ATLAS experiment with an accuracy of $0.1\%$ and validate the predictions made by the detector simulation. If the decay of the LSP occurs with a lifetime of less than about $1\mm/c$, the standard reconstruction of leptons within ATLAS is efficient. A search for anomalous events with at least four charged leptons is presented. Since processes with four or more charged leptons are rare in the Standard Model, a very low level of background is achieved. A special effort is made to provide sensitivity to decays of an LSP that is very light compared to the initially produced supersymmetric particles. No sign of a signal is observed, and strong exclusion limits in the parameter space defined by the supersymmetric particle masses are derived. A second search targets scenarios where the LSP lifetime is much greater than $1\mm/c$, using the signature of a high-mass displaced vertex with two associated charged leptons. The reconstruction of such displaced vertices requires a non-standard event reconstruction. Signal vertices do not occur naturally in the Standard Model, leading to a negligible level of background. A novel technique is developed to estimate the dominant residual background from lepton tracks that randomly cross inside the tracking volume. No signal vertices are observed, in agreement with the background prediction, and upper limits on the number of supersymmetric particle decays in the data set and the production cross-sections within simplified SUSY models are set as a function of the LSP lifetime. The results of the four-lepton search are also extrapolated as a function of the LSP lifetime and included in the study to enhance sensitivity at short LSP lifetimes.