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A bstract The Two Higgs Doublet Model predicts the emergence of 3 distinct domain wall solutions arising from the breaking of 3 accidental global symmetries, Z 2 , CP1 and CP2, at the electroweak scale for specific choices of the model parameters. We present numerical kink solutions to the field equations in all three cases along with dynamical simulations of the models in (2+1) and (3+1) dimensions. For each kink solution we define an associated topological current. In all three cases simulations produce a network of domain walls which deviates from power law scaling in Minkowski and FRW simulations. This deviation is attributed to a winding of the electroweak group parameters around the domain walls in our simulations. We observe a local violation of the neutral vacuum condition on the domain walls in our simulations. This violation is attributed to relative electroweak transformations across the domain walls which is a general feature emerging from random initial conditions.

A bstract We construct and simulate the dynamics of gauged vortons — circular loops of cosmic string supported by the angular momentum of trapped charge and current and provide additional details on the fully stable vorton that we have previously presented. We find that their existence and dynamical properties can be accurately predicted by an analysis based on infinite, straight superconducting strings if an additional constraint on their phase frequency is satisfied. We show a good quantitative agreement with the thin string approximation (TSA) and provide evidence that curvature corrections are inversely proportional to the vorton radius. This is verified with an energy minimisation algorithm that produces vorton solutions and subsequent axial and full three dimensional evolution codes. We find that we can predict the frequencies of each mode of oscillation, determine which modes are unstable and calculate the growth rate of the unstable modes to a high degree of accuracy.

We perform a systematic study of generic accidental Higgs-family and CP symmetries that could occur in the two-Higgs-doublet-model potential, based on a Majorana scalar-field formalism which realizes a subgroup of . We derive the general conditions of convexity and stability of the scalar potential and present analytical solutions for two non-zero neutral vacuum expectation values of the Higgs doublets for a typical set of six symmetries, in terms of the gauge-invariant parameters of the theory. By means of a homotopy-group analysis, we identify the topological defects associated with the spontaneous symmetry breaking of each symmetry, as well as the massless Goldstone bosons emerging from the breaking of the continuous symmetries. We find the existence of domain walls from the breaking of Z 2 , 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. The spatial profile of the topological defect solutions is studied in detail, as functions of the potential parameters of the two-Higgs doublet model. 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 same formalism may be used to properly identify seven additional symmetries that may take place in a U(1) Y -invariant scalar potential.

We compute new solutions of the Skyrme model with massive pions. Concentrating on baryon numbers which are multiples of four, we find low-energy Skyrmion solutions that are composed of charge four subunits, as in the α-particle model of nuclei. We summarize our current understanding of these solutions, and discuss their relationship to configurations in the α-particle model.

The computation of long range linear self-interaction forces in string and higher dimensional brane models requires the evaluation of the gradients of regularised values of divergent self-interaction potentials. It is shown that the appropriately regularised gradient in directions orthogonal to the brane surface will always be obtainable simply by multiplying the regularised potential components by just half the trace of the second fundamental tensor, except in the hypermembrane case for which the method fails. Whatever the dimension of the background this result is valid provided the codimension is two (the hyperstring case) or more, so it can be used for investigating brane-world scenarios with more than one extra space dimension.

We present superconducting vortex solutions in the two-Higgs doublet model which has a gauged \\(U(1)\\) Higgs-family symmetry. We write down an ansatz for the solution and study its basic properties, for the case of both global and gauged symmetries. We demonstrate its prima-facie stability using 3D numerical simulations of a global version of the theory, observing the flow of current along the string. We discuss how generic such a phenomena might be and the possible consequences if such a model is found in nature.

We discuss the problems faced in trying to deduce the evolution of cosmological perturbations in brane-world models. There are two natural ways to formulate the problem: one which makes the equations of motion simple and the other which makes the boundary condition simple. Unfortunately the problem is difficult to solve, even numerically, in either formalism. We present a more phenomenological approach which, while it does not solve the problem for any given model, illustrates some generic features one might expect to see in the tensor part of the cosmic microwave background power spectrum. We find that the observed scale invariance of the cosmic microwave background provides bounds on brane world models.

Using numerical simulations of the full nonlinear equations of motion, we investigate topological solitons of the Skyrme-Faddeev system, which is a modified O(3) sigma model in three space dimensions, in which the solitons are stabilized by the Hopf charge. We find that for solitons up to charge five the solutions have the structure of closed strings, which become increasingly twisted as the charge increases. However, for higher charge the solutions are more exotic and comprise linked loops and knots. We discuss the structure and formation of these solitons and demonstrate that the key property responsible for producing such a rich variety of solitons is that of string reconnection.

We construct and analyse two-dimensional, current-carrying ring solutions, known as kinky vortons, in the \\(Z_2\\)-symmetric global two-Higgs-doublet model (2HDM). We demonstrate the existence of multiple dynamically stable configurations that persist under non-axially symmetric perturbations. These solutions are described with high accuracy by the thin string approximation and elastic string formalism, which correctly capture both their equilibrium radii and dynamical oscillation frequencies. Kinky vortons in the \\(Z_2\\)-symmetric theory establish the viability of vorton solutions in a phenomenologically motivated extension of the Standard Model, and should provide a computationally tractable proxy for vortons in the \\(U(1)\\)-symmetric 2HDM. In addition, we identify a composite domain wall configuration in which localized condensates are supported on secondary domain walls existing on a \\(Z_2\\) wall, suggesting a mechanism by which kinky-vorton-like defects could arise in a three dimensional setting.

We present superconducting vortex solutions in the two-Higgs doublet model which has a gauged \\(U(1)\\) Higgs-family symmetry. We write down an ansatz for the solution and study its basic properties, for the case of both global and gauged symmetries. We demonstrate its prima-facie stability using 3D numerical simulations of a global version of the theory, observing the flow of current along the string. We discuss how generic such a phenomena might be and the possible consequences if such a model is found in nature.