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