Effects Of Temperature On Thick Braneworlds And On The Localization Of Fermions

In the framework of higher dimensional space-time, the braneworld theories pro-vide new insights for solving some long-standing problems in physics. In this thesis, we focus on the effects of temperature on thick braneworld and the local-ization of fermions.We begin with an overview of the early extra-dimensional physics, the mod-ern braneworlds picture, and the effects of the temperature on the scalar field theory.We then move on to discuss a flat thick brane model, in which the brane is generated by a real scalar field with the?6self-interaction potential in the five-dimensional (5D) curved spacetime, and to investigate the effects of the variation of the mass parameter (in self-interaction potential) on the brane as well as on the localization of fermions. An interesting result is found:when the critical value of mass parameter is reached, the solution of the background scalar field (as a double kink) is not unique. As a result, the thickness of the brane is not fixed. The massive modes of the fermion are not localized fully on the flat thick brane, but there are some quasi-localized KK resonance modes on the brane. We find that the resonance number, lifetime, and mass is varied monotonically with the mass parameter. But the mass parameter in the?6model can only partly reflect the effects of the temperature.If the thick branes are generated by a complex scalar field with Φ4form self-interaction potential, the mass parameter, with some approximations, can fully reflect the effects of temperature. Here we call the mass parameter as the temperature parameter. Three kinds of thick branes have been discussed, and they are:flat, de Sitter (dS), and anti-de Sitter (AdS) thick branes. For the flat and AdS thick branes cases there is a critical value of the temperature parameter, but for the dS brane case there is not. And in the AdS brane case, the critical value of the temperature parameter can be less or greater than the one in the flat5D spacetime. But in the flat brane case, the critical value of the temperature parameter is less than the one in the flat5D spacetimc. There is a series of fermion KK resonances for the cases of flat and dS branes. And the resonance number, lifetime, and mass are varied monotonically with the temperature parameter. In the case of AdS brane, the spectra of fermion KK modes are discrete, and the mass of fermion KK modes does not varied monotonically with the temperature parameter.