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

Abstract

This thesis presents a search for Zγ 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→Zγ, Z→ll, where l = e or μ, 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 √s = 7 TeV and 20.3 fb⁻¹ at a centre-of-mass energy √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 Zγ 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 Zγ to an llγ final state, where l = e or μ, 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 √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 Zγ for such a boson with mass between 250 GeV and 1.5 TeV.