Searching for Supersymmetry with the ATLAS Detector at the Large Hadron Collider

On Monday 23rd November 2009, the ATLAS experiment at the Large Hadron Collider began taking data at $\sqrt{s}=900$ GeV. On the penultimate day of March the following year, after a brief shutdown, ATLAS resumed data taking but at $\sqrt{s}=7$ TeV. These $\sqrt{s}=7$ TeV collisions continued until the end of October 2010. This thesis presents the very first low-$p_T$ electron candidates from the complete 9$\mu b^{-1} \sqrt{s}=900$ GeV dataset, and higher-$p_T$ candidates from the first 1 nb$^{-1}$ of the $\sqrt{s}=7$ TeV dataset. These candidates are presented in the context of electron reconstruction and identification and illustrate how various properties of these electrons compare with expectations based on Monte Carlo simulations. An observation is made of the $Z$ candidates present in the first ~220 nb$^{-1}$ of $\sqrt{s}=7$ TeV collision data, these being amongst the first $Z$ bosons ever produced by a proton-proton collider. A detailed study is then presented of the full ~35pb$^{-1}$ 2010 $\sqrt{s}=7$ TeV dataset in the context of a search for supersymmetry in final states with two leptons (electrons or muons) and high missing transverse energy. No significant deviations from standard model expectations are observed. A "flavour subtraction" analysis is then performed on this data to search for supersymmetry with reduced dependence on experimental and theoretical uncertainties, and increased sensitivity towards particular event topologies. Limits are set at 95$%$ confidence on the number of excess identical-flavour lepton-pairs, over those of different-flavour, multiplied by detector acceptances and efficiencies, from supersymmetry. Two distinct Monte Carlo studies are also documented, studies carried out in the year preceding the successful running of the Large Hadron Collider. Supersymmetric models which violate R-parity through non-zero $\lambda^{''}_{112}$ are investigated. In this study, which uses the full ATLAS detector simulation, it is shown that given a well understood detector, R-parity violating supersymmetric $\overline{\chi}^{0}_{1}\to qqq$ decays may be detectable using jet substructure. This thesis also investigates how a Large Hadron Collider experiment could be used to measure the mixings and masses of the six sleptons in models in which lepton flavour is violated, given significant integrated luminosity. This analysis uses a generic detector simulation package to reasonably model the effects of a detector on the observable particles.