Search for Diboson Resonances in the Fully Hadronic Final State Using Jet Substructure Techniques in 8 and 13 TeV Proton-Proton Collisions with the ATLAS Detector

In this thesis, searches for narrow resonances decaying into pairs of vector bosons ($WW$, $WZ$ and $ZZ$) are presented. The searches are performed using 20.3 fb$^{-1}$ and 3.2 fb$^{-1}$ of proton-proton collision data collected with the ATLAS experiment at the LHC at centre-of-mass energies of 8 TeV and 13 TeV, respectively. Events are selected in the fully hadronic final state, characterised by two large-radius jets. Requirements on the jet mass and substructure variables are imposed to select large-radius jets that are compatible with the hadronic decay of highly boosted $W$ or $Z$ bosons. A variety of techniques are compared to optimise the identification of boosted vector bosons. The invariant mass spectrum of the two boson-tagged jets is used as discriminating variable to search for a resonant structure on top of the smoothly falling distribution of the background processes. In the analysis performed at 8 TeV, an excess of events is observed for an invariant dijet mass of 2 TeV. The largest deviation from the background prediction occurs in the $WZ$ channel with a global significance of 2.5$\sigma$. On the contrary, no significant deviations from the background expectations are seen in the analysis using 13 TeV $pp$ collisions. Exclusion limits at 95% confidence level are set on the production cross-section times branching ratio for three benchmark models: a bulk Randall-Sundrum model, an extended gauge model and a heavy vector triplet model. $W'$ bosons as predicted in the extended gauge model are excluded in the mass range from 1.3 to 1.5 TeV at 95% confidence level and $W'$ bosons from the heavy vector triplet model are excluded for masses between 1.38 to 1.6 TeV.