Search for a supersymmetric partner of the top quark in the one-lepton channel with the ATLAS detector

This thesis presents a search for evidence of a scalar top quark (top squark, or stop), predicted by supersymmetry (SUSY), which is a hypothetical extension of the Standard Model of particle physics (SM). The search is using the full dataset recorded in 2012 using the ATLAS detector, one of the four main detectors at the Large Hadron Collider. The search is developed using simplified models containing only a small number of SUSY particles. Besides the top squarks, only the lightest neutralinos and charginos are considered, which are spin-1/2 particles predicted by SUSY. The neutralino is electrically neutral and assumed to be stable, leaving the detector without interaction. In each simplified model, top squarks are produced in pairs and are allowed to decay (a) into a top quark and a neutralino; (b) into a bottom quark and a chargino; or (c) into a bottom quark, a W boson, and a neutralino (three-body decay). This work extends earlier searches for pair-produced top squarks: mixed scenarios are studied, where decay modes (a) and (b) can occur simultaneously; decay mode (c) is studied for the first time using data recorded with the ATLAS detector. Using datasets of simulated events, selection criteria for proton-proton collision events are established, defining regions of phase-space enriched in hypothetical supersymmet- ric events and with few expected Standard Model events. Among other requirements, the selected events need to have exactly one isolated electron or muon, and high missing transverse momentum. Several dedicated variables are presented and their performance is demonstrated. The most important SM background contributions stem from top-antitop pair production events and W boson events with additional jet production. Modified event selection requirements are defined in order to estimate the normalisation of these background contributions from data, significantly reducing the uncertainty of this back- ground estimate. No significant excess of recorded events over the expected number of SM events is observed, and for certain mass assumptions of the supersymmetric particles, the signal hypothesis can be ruled out at 95% confidence level. Stop pair production followed by three-body decays is ruled out for stop masses below 300 GeV assuming a stop- neutralino mass difference of 150 GeV. Mixed stop decays at an equal branching ratio of the considered decay modes are excluded up to a stop mass of 550 GeV for a neutralino mass of 150 GeV. The search reach is expected to improve significantly when the analysis is updated using the ATLAS data to be recorded in 2015.