Pushing SUSY's boundaries: Searches and prospects for strongly-produced supersymmetry at the LHC with the ATLAS detector

In this thesis, a search for new elementary particles predicted by a theory called supersymmetry (SUSY), which attempts to address shortcomings in our current description of particle physics, the Standard Model, is presented. No events incompatible with the Standard Model are observed, however. The results obtained in this search are also used in fits to a larger supersymmetric model, and combined with different analyses to obtain improved limits on simplified models. In addition, prospects for a similar search at the proposed high-luminosity LHC are discussed. Finally, HistFitter is presented, a program developed to perform searches in high-energy physics. Supersymmetry is searched for in a decay channel with 2 to 6 jets, missing energy, and no leptons in the final state. The coupling of squarks and gluinos to the strong force leads to a final state with many jets, in which the lightest supersymmetric particle produced in the cascade decay escapes the detector unseen. The analysis is designed using 15 signal regions, with varying selections on the effective mass of the collision. These regions were optimised for exclusion on simplified models. For each of the signal regions, four control regions are used to estimate the main backgrounds in a semi-data-driven way through a combined likelihood fit: $W$+jets, $Z$+jets, $t\overline{t}$ and QCD multijet events. No significant excess over the Standard Model expectation is observed in any of the signal regions and limits are set in various models. The results are also used, together with all worldwide available limits and measurements, for an interpretation in a 15-dimensional phenomenological MSSM (pMSSM). Using Bayesian sampling, the best-fit point obtained corresponds to a squark mass of 2.3 TeV, a gluino mass of 2.1 TeV and a 130 GeV neutralino with a spin-independent cross-section of $2.4\times 10^{10} \mathrm{pb}$, which is within the reach of future multi-ton scale direct dark matter detection experiments and of the next LHC run. Finally, a statistical combination with the 1- and 2-lepton SUSY analyses is performed and HistFitter, a software tool developed on top of ROOT/RooFit is also discussed.