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Abstract

Searches for Dark Matter, one of the biggest unsolved problems of modern physics, at the LHC often comprise the most exotic signatures. At the same time, these pose experimental challenges, but also opportunities: new techniques are developed, which can find useful application also beyond the scope of their original conception. This thesis presents a search for Dark Matter produced in association with an hypothetical Dark Higgs Boson decaying to $VV$, $V=W,Z$, in the fully hadronic final state. The reconstruction of the boosted $V$ bosons is performed with a novel technique, the Track-Assisted Reclustered jets, used in a data analysis for the first time. This is shown to achieve a more robust performance and flexibility with respect to standard methods. The search was performed using 139 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}$ = 13 TeV collected with the ATLAS detector during Run 2. No significant excess is found in the observed data over the Standard Model; the upper limits on the production cross-section of \met+$VV$ in the fully hadronic final state for the Dark Higgs scenario are set at 95\% confidence level.