Searches for new resonances decaying to a Higgs and a vector boson with the ATLAS detector at the LHC

Several physics phenomena cannot be explained by the otherwise experimentally thoroughly tested Standard Model of particle physics, implying that it is not a complete theory. This advocates the existence of new elementary particles with masses at the TeV scale, that are contained in a more fundamental theory. Many extensions of the Standard Model predict the existence of additional Higgs bosons or heavy neutral and charged vector bosons. This thesis presents general searches for such particles with the ATLAS detector at the Large Hadron Collider at CERN, based on minimal theoretical assumptions. This allows for the re-interpretation of search results in the framework of many theories beyond the Standard Model, with the focus on the predictions of benchmark scenarios within the Two-Higgs Doublet Model (2HDM) and the Heavy Vector Triplet (HVT) Model. The new particles are searched for in the \(\PV\Ph\) diboson decay channels with a Standard Model vector boson $V=W,Z$, and the Higgs boson $h$, focusing on the semileptonic $\nu\nu bb$, $\ell\nu bb$, and $\ell\ell bb$ ($\ell = e, \mu$) final states with Higgs decays into a pair of $b$-quarks. Results using $pp$ collision data at $\sqrt{s} = 13\,\mathrm{TeV}$, recorded with the ATLAS detector between 2015 and 2018, are presented. No significant excess of events over the background-only expectation is observed and upper limits on the resonance production cross section times branching ratio are set at 95 \% confidence level. Results are presented for the search in the $\ell\nu bb$ final state using $36.1\,\mathrm{fb}^{-1}$ of proton-proton collision data. Cross section times branching ratio values for a HVT $W'$ resonance, \(\sigma_{W' \to Zh} \times \mathcal{B}(h \to b\bar{b},c\bar{c})\), between $0.22\,\mathrm{pb}$ and $1.1\,\mathrm{fb}$ are excluded at 95 \% confidence level for $W'$ resonance masses between $500\,\mathrm{GeV}$ and $5\,\mathrm{TeV}$. The limits correspond to the exclusion of HVT $W'$ resonance masses below $2.82\,\mathrm{TeV}$ in the HVT benchmark models. Limits are also set on the cross section times branching ratio values for a HVT $Z'$ vector boson and a 2HDM $A$ pseudoscalar, using $139\,\mathrm{fb}^{-1}$ of data in the $\nu\nu bb$ final state and in the combination of $\nu\nu bb$ and $\ell\ell bb$ final states. 95 \% confidence level upper limits on cross sections times branching ratios are set between $0.30\,\mathrm{fb}$ and $31\,\mathrm{pb}$, depending on final state, signal model, and production mode, for resonance masses between $300\,\mathrm{GeV}$ and $5\,\mathrm{TeV}$. In the HVT benchmark models, $Z'$ resonances with masses below $3.0\,\mathrm{TeV}$ are excluded at 95 \% confidence level.