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Standard Model (SM) and many extensions of SM predict that the electroweak (EW) symmetry was restored in the early universe when the temperature was around 160 GeV. However, recent studies showed that the interactions between some new scalars and SU(2)_L Higgs doublet(s) can cause the EW symmetry to remain broken at temperatures well above the EW scale in certain renormalizable extensions of SM. In this study, we found that new fermions from renormalizable models can also induce this EW symmetry non-restoration effect, provided that they have the appropriate temperature-dependent masses. These masses can arise naturally from the interactions between the new fermions and scalar fields. After introducing these models, I will explain how the important higher-order corrections are handled in the calculations of effective potentials. There are also theoretical and experimental constraints that must be taken into account. Within the allowed parameter space, I will examine the characteristics of the novel thermal histories predicted by these models. Certain cases predict that the EW phase transitions are strongly first-order and occur at temperatures much higher than the EW scale. I will discuss the prospect of detecting the stochastic gravitational-wave background from these cosmological phase transitions at future gravitational wave observatories, such as BBO and DECIGO.
Advisor: Peisi Huang