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Title: Electroweak precision and intermediate scales in warped extra dimensions
Author: Dillon, Barry
ISNI:       0000 0004 6061 7563
Awarding Body: University of Sussex
Current Institution: University of Sussex
Date of Award: 2017
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In this thesis we study several topics within the subject of extra dimensions and composite Higgs models. We first look at a scenario with a warped extra dimension known as the Randall-Sundrum (RS) model, and put all Standard Model fields in the bulk. We investigate various aspects of the model and argue that the presence of higher dimensional operators in the 5D bulk has a non-negligible effect on the electroweak precision observables, meaning that current electroweak constraints on non-custodial warped models could be weaker than previously thought. Then, using holographic techniques, we study correlations between the top partner masses and the Higgs potential in composite Higgs models. It is known that a light Higgs (~ 125 GeV) generally requires light top partners at around 700-800 GeV. However in these calculations the 5D volume is always fixed such that the 5D cutoff is around ~ MPl. The effect of lowering this 5D cutoff has been studied previously in bulk RS models as a way of reducing constraints from some flavour and electroweak precision observables, these models were dubbed "Little Randall-Sundrum models". Here we consider a similar setup in the context of holographic composite Higgs models and show that reducing the 5D cutoff leads to a lighter Higgs without a lowering of the top partner masses or an increase in fine-tuning. We find that the model is perfectly consistent with a 125 GeV Higgs and top partners above 1 TeV. This reduced 5D cutoff implies an intermediate scale between the electroweak scale and the Planck scale. Lastly we consider a similar warped model with a low 5D cutoff, except this time our goal is to study diphoton signals from Kaluza-Klein gravitons in a warped extra dimension. With a KK graviton of mass 750 GeV and spin-1 states at ~ 2:5 TeV, we show that having a low 5D cutoff increases the diphoton signal and the decay to gluons. With this model we show that we can explain the recently observed diphoton excess in terms of a Kaluza-Klein graviton from a holographic composite Higgs model, while keeping other decay channels within the relevant experimental bounds.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC0170 Atomic physics. Constitution and properties of matter Including molecular physics ; relativity ; quantum theory ; and solid state physics