Search for supersymmetry in final states with two same sign leptons or three leptons and jets with the ATLAS detector at the LHC

Abstract

The Standard Model of particle physics successfully describes the elementary particles and their interactions at low energies, up to 100 GeV. Beyond this scale lies the realm of new physics needed to remedy problems that arise at higher energies, the TeV scale and above. Supersymmetry (SUSY) is the most favored extension of the Standard Model that solves many of its limitations, if predicted SUSY particles exist at the TeV scale. The Large Hadron Collider (LHC) at CERN has opened a new phase of exploration into new physics at the TeV scale after increasing the center-of-mass energy of the proton-proton collisions to 13 TeV. The ATLAS experiment has collected this collision data with over 90% efficiency due to the excellent performance of many of its systems, in particular the data acquisition system. The work described in this dissertation ensures the efficient collection of ATLAS data as well as the analysis of this data to search for SUSY. The search for strongly produced supersymmetric particles decaying into final states with multiple energetic jets and either two leptons (electrons or muons) with the same electric charge or at least three leptons was performed using the proton-proton collision dataset of 36 fb$^{-1}$ at 13 TeV recorded with the ATLAS detector in 2015 and 2016. Due to the low Standard Model background, these final states are particularly adapted to searches for gluinos or third generation squarks in several supersymmetric production topologies determined from a variety of simplified and phenomenological models. The absence of excess over the Standard Model prediction is interpreted in terms of limits on the masses of superpartners derived at 95% confidence level. In the studied decay modes and depending on the decay topology, the existence of gluinos with masses below 1.9 TeV, sbottoms with masses below 700 GeV, and neutralinos with masses below 1.2 TeV are excluded.