School of Physics - Theses
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ItemA model of fermion masses without a Higgs mechanism( 2010)The purpose of this thesis is to understand the origin of fermion masses in a model where the electroweak symmetry is not a fundamental gauge symmetry. Electroweak symmetry is instead a global symmetry of a new strongly interacting sector, with the electromagnetic symmetry remaining as a gauged subgroup. The W and Z-bosons are thus composite vector mesons, which obtain their mass from strong dynamics rather than the traditional Higgs mechanism in spontaneously broken gauge theories. As a result, the hierarchy problem is evaded since the compositeness scale is exponentially small compared to the Planck scale MP. Fermions obtain mass by Yukawa coupling to a scalar vacuum expectation value, which breaks electroweak symmetry at the Planck scale. Interestingly, the natural scale of fermion masses is << M_P and the hierarchical pattern of masses and mixings can be solved by Planck-scale physics. After reviewing important background material on flavor physics, extra dimensions and branes, we describe the relationship between warping and compositeness using the holographic principle inspired by the anti de-Sitter/conformal field theory correspondence (AdS/CFT). Despite the non-perturbative nature of the physics underlying composite models, the technology of AdS/CFT is employed to construct a higher-dimensional, weakly-coupled description of fermions interacting with composite electroweak vector bosons in four spacetime dimensions. The techniques of electroweak precision analysis are explained and then applied to the model, using the higher-dimensional model as a calculational tool. It is demonstrated that despite the composite nature of the W/Z-bosons, the model is consistent with electroweak precision data for the first two generations, while the third generation leads to a tension with indirect bounds. Moreover, the model is shown to predict definite deviations from the Standard Model parameters at energies that will be explored by the LHC.