Higgs Properties: Implications at the LHC and on Cosmology

Abstract

The aim of this thesis is to study the properties of the 125 GeV Higgs-like resonance discovered at the Large Hadron Collider (LHC) in 2012, and to elucidate its role in electroweak (EW) symmetry breaking. The first step is to study the spin and charge-parity (JCP) assignments for this resonance, which are alternate to the Standard Model (SM) Higgs. In particular, we use unitarity arguments to eliminate the possibility that the new resonance is a spin-two impostor. Furthermore, it was found that such an impostor leads to large deviations from the observed oblique precision parameters. This resonance must then be a scalar, a pseudoscalar, or a mixture of these two cases. A non-linearly realised electroweak symmetry may lead to CP-violating top-Yukawa couplings. Collider data was used to put indirect constraints on the modulus, yt and CP- phase, ξ, which parameterise such couplings. We then studied the LHC potential to probe the tth coupling directly through the pp → thj channel, focusing on the scalar (|ξ| = 0), pseudoscalar (|ξ| = 0.5π) and maximally mixed (|ξ| = 0.25π) scenarios. It was found that large QCD backgrounds in h → bb decays make it difficult to observe thj production. Instead, we demonstrated that higher signal significance is expected in h → γγ decays, where signal reconstruction is significantly improved due to cleaner signatures. The lepton forward-backward asymmetry was found to be a good CP-observable. As it measures the polarisation of the produced t-quark, it allows different ξ’s to be distinguished. The last part of this thesis examines the phase transition (PT) of a non-linearly realised EW gauge symmetry. Electroweak baryogenesis may then generate the observed matter anti-matter asymmetry without augmenting the SM particle content. This is realised by extra sources of CP-violation and first order PT due to the anomalous top- Higgs and cubic Higgs interactions of the non-standard gauge structure.