Search for Charginos and Sleptons in ATLAS and Identification of Pile-up with the Tile Calorimeter

The standard model of particle physics (SM) describes the elementary particles and their interactions. Supersymmetry (SUSY), a symmetry beyond those included in the standard model could resolve some of the SM shortcomings. It can provide a candidate for Dark Matter and a solution to the hierarchy problem. The Large Hadron Collider (LHC) has the potential to produce the particles predicted by SUSY. This thesis presents two searches for SUSY particles in proton-proton collision data recorded by the ATLAS experiment. The first search described in this thesis looks for direct production of chargino and slepton pairs in a final state characterized by the presence of two leptons and missing transverse energy. The second search looks for production of chargino pairs via vector boson fusion (VBF) in a final state containing of two leptons, two jets and missing transverse energy. This is the first attempt in ATLAS to search for supersymmetric particles produced via VBF. A possible observation of such process would prove that the exchanged neutralino is a Majorana particle. These analyses are done using $\mathscr{L}=20.3$ fb$^{-1}$ proton-proton collisions at $\sqrt{s}=8$ TeV collected in 2012. No significant excess over background is observed. New exclusion limits at 95% confidence level on chargino, neutralino and slepton masses and cross section for chargino pair production via VBF are set. The energy measurements of the particles created in LHC collisions are performed by the ATLAS calorimeters. Energy deposits from different collisions in the same read-out window and in the same calorimeter channel (pile-up) can spoil the energy measurements by the calorimeter. It is shown that the quality factor computed offline for each collision and for each channel in the Tile Calorimeter (TileCal) can be used to identify channels that need a special treatment to account for large energy depositions from pile-up. Efficient criteria to detect pile-up in TileCal are proposed.