Search for resonances decaying to a Z boson and a photon in the ATLAS Detector using data from the Large Hadron Collider

This thesis presents a search for $Z\gamma$ resonances in the context of both the Standard Model Higgs boson and exotic high mass particles. The search for evidence of a Standard Model Higgs boson undergoing the decay $H \to Z\gamma, Z \to \ell\ell$, where $\ell = e$ or $\mu$, is performed using proton-proton collision data recorded by the ATLAS detector at the LHC corresponding to 4.5 fb$^{-1}$ at a centre-of-mass energy $\sqrt{s} = 7$ TeV and 20.3 fb$^{-1}$ at a centre-of-mass energy $\sqrt{s} = 8 $ TeV. No significant excess over the Standard Model prediction is observed and so exclusion limits on the production cross-section of an SM Higgs boson decaying to $Z\gamma$ are set at 95\% confidence level: production of the Higgs boson at $9.0 \times \text{SM}$ predictions is excluded. Refinements to the analysis are presented, making use of an alternative event classification based on the mass of the reconstructed $Z$ boson to increase the sensitivity and set improved exclusion limits: production of the Higgs boson at $8.0 \times \text{SM}$ predictions is excluded. A search for a new Higgs-like boson, $X$, with high mass and decaying through $Z\gamma$ to an $\ell\ell\gamma$ final state, where $\ell = e$ or $\mu$, is performed using proton-proton collision data recorded by the ATLAS detector at the LHC corresponding to 3.2 fb$^{-1}$ at a centre-of-mass energy $\sqrt{s} = 13$ TeV. No significant excesses above the SM background are observed and so exclusion limits at 95\% confidence level are set on the production cross section times decay branching ratio to $Z\gamma$ for such a boson with mass between 250 GeV and 1.5 TeV.