The Search for Supersymmetry in Complex Hadronic States Using the ATLAS Detector

The standard model of particle physics is incomplete. A well motivated solution to several of its issues, including the absence of dark matter, is a theoretical extension called supersymmetry. Supersymmetric particles may be produced in the 13 TeV collisions recorded by the ATLAS experiment at the LHC. The presence of a dark matter candidate particle would be signified by the presence of missing transverse momentum, $\,E_{\mathrm{T}}^{\mathrm{miss}}$, in an event. A quantity designed to find very weakly interacting particles escaping the detector. This thesis first analyses the performance of $\,E_{\mathrm{T}}^{\mathrm{miss}}$ and its significance, $\, \mathcal{S}(E_{\mathrm{T}}^{\mathrm{miss}})$, with the complete ATLAS $\,p\mathrm{-}p$ Run 2 dataset with $\sqrt{s} = 13$ TeV. A direct search for SUSY particles is then presented. This search is aiming to find new physics in events with large jet multiplicities, no leptons and a moderate amount of $\,E_{\mathrm{T}}^{\mathrm{miss}}$. The dominant background in this analysis comes from QCD multi-jet events that have very little real $\,E_{\mathrm{T}}^{\mathrm{miss}}$. Instead, the $\,E_{\mathrm{T}}^{\mathrm{miss}}$ will be from mismeasurement and resolution effects. The $\,E_{\mathrm{T}}^{\mathrm{miss}}$ significance is used to suppress contributions from these events in the signal regions. In addition a new jet definition is employed to improve the reconstruction of jets used in the analysis. No deviation from the standard model is observed. The results are used to calculate exclusion limits in terms of simplified models, the largest excluded mass for the gluino is 2 TeV assuming a massless lightest neutralino. This limit was found using a two step cascade decay of the gluino to SM particles and the lightest neutralino. The final section of the thesis is concerned with finding a method to discriminate between quark and gluon initiated jets in ATLAS. Many new physics searches seek signatures with only quarks in the final state and so gluons can be considered a background. The ability to discriminate between the two types of jets greatly improves the discovery power of these analyses.