Browsing by Subject "String models."
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Item Brane cosmology in string/M-theory and cosmological parameters estimation.(2009-08-25T16:34:05Z) Wu, Qiang, 1977-; Wang, Anzhong.; Physics.; Baylor University. Dept. of Physics.In this dissertation, I mainly focus on two subjects: (I) highly effective and efficient parameter estimation algorithms and their applications to cosmology; and (II) the late cosmic acceleration of the universe in string/M theory. In Part I, after developing two highly successful numerical codes, I apply them to study the holographical dark energy model and LCMD model with curvature. By fitting these models with the most recent observations, I find various tight constraints on the parameters involved in the models. In part II, I develop the general formulas to describe orbifold branes in both string and M theories, and then systematical study the two most important issues: (1) the radion stability and radion mass; and (2) the localization of gravity, the effective 4D Newtonian potential. I find that the radion is stable and its mass is in the order of GeV, which is well above the current observational constraints. The gravity is localized on the TeV brane, and the spectra of the gravitational Kluza-Klein towers are discrete and have a mass gap of TeV. The contributions of high order Yukawa corrections to the Newtonian potential are negligible. Using the large extra dimensions, I also show that the cosmological constant can be lowered to its current observational value. Applying the formulas to cosmology, I study several models in the two theories, and find that a late transient acceleration of the universe is a generic feature of our setups.Item Grand unified theories in higher dimensions: from the heterotic string to Randall-Sundrum.(2007-02-19T20:24:20Z) Dundee, G. Benjamin (George Benjamin).; Cleaver, Gerald B.; Physics.; Baylor University. Dept. of Physics.We explore the phenomenology of effective field theories in two different cases. First, we look at a mirror universe model, using an orbifold compactification of the heterotic string, in the free fermionic approach. An expected non-chiral Pati-Salam mirror universe model is transformed into a chiral model with enhanced hidden sector gauge symmetry and reduced observable sector gauge symmetry: $[SU(4)_C\times SU(2)_L\times SU(2)_R]^{O}\times [SU(4)_C\times SU(2)_L\times SU(2)_R]^{H}$, is necessarily transformed into a chiral $[SU(4)_C\times SU(2)_L]^{O}\times [SO(10)\times SU(2)_R]^{H}$. Second, we look at a non-supersymmetric flipped SU(5) theory built within the Randall-Sundrum framework. We derive the low energy running of the couplings, and show how the effects of the numbers and types of representations used for model building can be constrained by looking at universal contributions from KK modes to the beta functions. We show that the Yukawa couplings are, in general, exponentially small, leading to an exponentially short proton lifetime.Item Towards a systematic investigation of weakly coupled free fermionic heterotic string gauge group statistics.(2009-07-08T18:47:57Z) Robinson, Matthew Brandon, 1981-; Cleaver, Gerald B.; Physics.; Baylor University. Dept. of Physics.We investigate several issues regarding weakly coupled free fermionic heterotic string model building. After reviewing the necessary background, we first address an alternative interpretation of the GSO projection in terms of the Weyl conditions on the root space of the gauge group of a given model. We develop an algorithm to systematically generate and analyze string models very efficiently by linearizing the constraints. Next we investigate free fermionic model building from the approach of quantum computing algorithms. We also consider a few unique and interesting gauge groups which can arise in non-standard ways using this method. Then we develop a variation of the well known “NAHE” basis of (quasi)-realistic models with initial observable sector gauge group SO(10) that offers the possibility of a new class of models with an initial observable sector gauge group of E6. Finally we consider several topics regarding the applications of Optical Unification in string models.