Supersymmetry searches in events with at least four leptons using the ATLAS detector at the Large Hadron Collider

This thesis presents a search for supersymmetry using the dataset taken by ATLAS at the Large Hadron Collider with $\sqrt{s}=$8$~$TeV during 2012. Events with four or more leptons are selected and required to satisfy additional kinematic criteria that define optimised signal regions. These criteria are chosen to reject the majority of events produced by Standard Model processes, whilst retaining a large fraction of events produced by a variety of proposed supersymmetry scenarios. The expected number of Standard Model events are estimated using a combination of Monte Carlo and data-driven methods, the predictions of which are tested against data in specifically designed validation regions. No significant deviations from the Standard Model estimations are observed within statistical and systematic uncertainties. Exclusion limits are then set at 95$\%$ confidence level (CL) on a wide range of R-parity conserving and R-parity violating supersymmetry simplified models, as well as models of general gauge mediated supersymmetry. In R-parity violating models, 95$\%~$CL exclusion limits of 1350$~$GeV and 750$~$GeV are set on the masses of gluinos and charginos respectively. Exclusion limits are also set at 95$\%~$CL up to 620$~$GeV on the mass of heavy neutralinos for an R-parity conserving scenario with decays via right-handed sleptons. Results are also presented for the combination of the four lepton analysis with another lepton-rich supersymmetry search.