Search for supersymmetry in events containing light leptons, jets and missing transverse momentum in $\sqrt{s} = 8 TeV pp$ collisions with the ATLAS detector

The results of two analyses searching for supersymmetry (SUSY) in data of the ATLAS experiment are presented in this thesis. The data were recorded in proton-proton collisions at the Large Hadron Collider in 2012 at a centre of mass energy of $\sqrt{s} = 8$ TeV and correspond to an integrated luminosity of 20.3 fb$^{-1}$. The first search is performed in signatures containing an opposite-sign electron or muon pair, which is compatible with originating from a $Z$ boson decay, in addition to jets and large missing transverse momentum. The analysis targets the production of squarks and gluinos in R-parity conserving (RPC) models with SUSY breaking via General Gauge Mediation (GGM). The main Standard Model (SM) backgrounds are $t \overline{t}$, $WW$, $W+t$ and $Z \rightarrow \tau\tau$ processes which are entirely estimated from data using different-flavour events. Besides that, the SM production of $Z$ bosons in association with jets and large fake missing momentum from mismeasurements plays a role and is predicted with the data-driven jet smearing method. Backgrounds from events with fake leptons are estimated with the data-driven matrix method. $WZ$/$ZZ$ production as well as smaller background contributions are determined from Monte-Carlo simulations. The search observes an excess of data over the SM prediction with a local significance of $3.0 \sigma$ in the electron channel, $1.7 \sigma$ in the muon channel and $3.0 \sigma$ when the two channels are added together. The results are used to constrain the parameters of the GGM model. The second analysis uses the already published results of an ATLAS search for SUSY in events with one isolated electron or muon, jets and missing transverse momentum to reinterpret them in the context of squark and gluino production in SUSY models with R-parity violating (RPV) $LQ\overline{D}$ operators. In contrast to RPC models, the lightest SUSY particle (LSP) is not stable but decays into SM particles. ``Standard'' analyses often do not consider SUSY models with RPV although they are in principle sensitive to them. The exclusion limits on the squark and gluino mass obtained from the reinterpretation extend up to 1200 GeV. These are the first results by any ATLAS SUSY search which systematically cover a wide range of RPV couplings in the case of prompt LSP decays. However, the analysis is not sensitive to the full parameter space of the $LQ\overline{D}$ model and reveals gaps in the ATLAS SUSY program which have to be closed by dedicated search strategies in the future.