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Title: Symmetries and topological defects of the two Higgs doublet model
Author: Brawn, Gary Derrick
ISNI:       0000 0004 2714 0206
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2011
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The standard model of particle physics is the most precisely verified scientific theory in the history of mankind. However, extended theories are already in place, ready to supersede the standard model should it fail to describe any new physics that may be observed in the next generation of high energy particle accelerators. One such minimal extension is the Two Higgs Doublet Model (2HDM). However, the appearance of additional symmetries to those of the gauge symmetries in the 2HDM can have consequences for the cosmological viability of the model, with the possibility for non-trivial topological defects forming during spontaneous symmetry breaking phase transitions.In this research we perform a systematic study of six accidental Higgs Family and CP symmetries that can occur in the 2HDM potential, by introducing and utilizing our Majorana scalar-field formalism. General sufficient conditions for convexity and stability of the scalar potential are derived and analytical solutions for two non-zero neutral vacuum expectation values of the Higgs doublets for each of the six symmetries are presented, in terms of the parameters of the theory. We identify the topological defects associated with the spontaneous symmetry breaking of each symmetry by means of a homotopy-group analysis. We find the existence of domain walls from the breaking of Z2, CP1 and CP2 discrete symmetries, vortices in models with broken U(1)PQ and CP3 symmetries and a global monopole in the SO(3)HF-broken model. We study the associated topological defect solutions as functions of the potential parameters via gradient flow methods. We also consider the cosmological implications of the topological defects and are able to derive bounds on physical observables of the theory in order to avoid contradictions with the theoretical limits on topological defects. The application of our Majorana scalar-field formalism in studying more general scalar potentials that are not constrained by the U(1)Y hypercharge symmetry is discussed. In particular, the formalism may be used to properly identify seven previously hidden symmetries that may be manifest in a U(1)Y invariant scalar potential for particular choices of the model parameters.
Supervisor: Pilaftsis, Apostolos ; Battye, Richard Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Topological Defects ; Accidental Symmetries ; Two Higgs Doublet Model ; Cosmological Constraints ; Domain Walls