The quest for the top squark in semi-leptonic and hadronic final states with the ATLAS Experiment

Two searches for the Supersymmetric partner of the top quark, the stop, are presented in this thesis for scenarios where the R-parity is conserved and the lightest supersymmetric particle is assumed to be the lightest neutralino. They have been performed by analysing the proton-proton LHC collision data recorded by the ATLAS experiment at centre-of-mass energies of 7 and 8 TeV corresponding to an integrated luminosity of 4.7 and 20.1$^{fb-1}$, respectively. A study to define a parameterization for the resolution of the Soft component of the missing transverse momentum is also presented. The signal of the first search is a stop which is lighter or in the same mass range as the top-quark and decays into a b-quark and a chargino, with a 100% decay branching ratio. Events that contain jets, b-jets, one electron or muon, and missing transverse momentum are selected. The discrimination between signal and background is achieved by reconstructing the top-anti-top decay and exploiting the differences in the top mass distribution and in the shape of the $\sqrt{s}_min^{(sub)}$ variable. No significant excess is observed and 95% C.L. exclusion limits are placed. These are obtained by combining statistically the results of this search with the one obtained by a similar search in two lepton final states. In a scenario where the chargino is two times heavier than the neutralino, stop masses between 120 and 167 GeV are excluded for a neutralino with a mass of 55 GeV. The second search focuses on final states with no leptons, jets, b-jets, and large missing transverse momentum. Here the signal discrimination is achieved by using transverse mass variables and by reconstructing the all-hadronic decay of a top-anti-top pair. No significant excess over the SM expectation is observed and 95% C.L. exclusion limits are set. The main model considered is that of a stop decaying directly into a top and a neutralino, with a 100% decay branching ratio. In this case stop masses between 270 and 645 GeV are excluded at 95% C.L. for a neutralino with mass below 30 GeV. Exclusion limits at 95% C.L. on the stop mass are also set for a stop signal that could decay with different branching ratios into top and neutralino or b-quark plus chargino, in models where the mass of the chargino is two times that of the neutralino. For a 50% branching ratio of the stop into top neutralino decay, stop masses between 250 and 550 GeV are excluded for a neutralino with mass below 60 GeV.