Title: A search for supersymmetric phenomena in final states with high jet multiplicity at the ATLAS detector
Description: Proton-proton collisions at the Large Hadron Collider provide insight into fundamental dynamics at unprecedented energy scales. After the discovery of the Higgs boson by the ATLAS and CMS experiments completed the Standard Model picture of particle physics in 2012, the focus turned to investigation of new phenomena beyond the Standard Model. Variations on Supersymmetry, which has strong theoretical underpinnings and a wide potential particle phenomenology, garnered attention in particular. Preliminary results, however, yielded no new particle discoveries and set limits on the possible physical properties of supersymmetric models. This thesis describes a search for supersymmetric particles that could not have been detected by earlier efforts. The study probes collisions with a center of mass energy of 13 TeV detected by ATLAS from 2015 to 2016 that result in events with a large number of jets. This search is sensitive to decays of heavy particles via cascades, which result in at least seven hadronic jets and some missing energy.

Constraints on the properties of reclustered large-radius jets are used to improve the sensitivity. The main Standard Model backgrounds are removed using a template method that extrapolates background behavior from final states with fewer jets. No excess is observed over prediction, so limits are set on supersymmetric particle masses in the context of two different theoretical models. Gluino masses below 1500 and 1600 GeV, respectively, are excluded, a significant extension of the limits set by previous analyses.