Cosmic Defects and Particle Physics Constraints
Topological defects, and in particular cosmic strings, are natural consequences of phase transitions in the early universe [1, 2, 3]. If detected they could provide windows into physics at very early times and very high energy, giving important information both for particle physics and cosmology. For most cosmological studies the abelian Higgs model is used as a prototypical cosmic string theory. However, in realistic particle physics theories the situation is more complicated and the resulting cosmic strings can have a rich microstructure. Additional features can be acquired at the string core at each subsequent symmetry breaking. This additional microstructure can, in some cases, be used to constrain the underlying particle physics theory to ensure consistency with standard cosmology. For example, if fermions couple to the Higgs field which gives rise to the string then these fermions could become zero modes (zero energy solutions of the Dirac equation) in the string core. The existence of fermion zero modes in the string core can have dramatic consequences for the properties of cosmic strings. For example, the zero modes can be excited and will move up or down the string, depending on whether they are left- or rightmovers. This will result in the string carrying a current . An intially weak current on a string loop will be amplified as the loop contracts and could become sufficiently strong and prevent the string loop from decaying. In this case a stable loop or vorton  is produced. The density of vortons is tightly constrained by cosmology. For example, if vortons are sufficiently stable so that they survive until the present time then we require that the universe is not vorton dominated. However, if vortons only survive a few minutes then they can still have cosmological implications. We then require that the universe be radiation dominated at nucleosynthesis. These requirements have been used in [6, 7] to constrain such models.
KeywordsYukawa Coupling Zero Mode Supersymmetry Breaking Cosmic String Topological Defect
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