Browsing by Subject "string theory"
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Item Higher Derivative D-brane Couplings(2012-10-19) Guo, GuangyuThis dissertation covers two different but related topics: the construction of consistent models in type IIB and heterotic string theories, and the higher derivative couplings for D-brane action, which will enable us to relate some models of type IIB to the heterotic side through duality chain. In the first part, we describe an alternative to the KKLT scenario, in which one can achieve de-Sitter space after fixing all moduli. We fix complex structure moduli and the axio-dilaton by deriving the stability conditions for the critical points of the no-scale scalar potential that governs the dynamics of the complex structure moduli and the axio-dilaton in compactifications of type IIB string theory on Calabi-Yau three-folds. In the second part, we show the existence of a class of flux backgrounds in heterotic string theory. The background metric we will consider is a T2 fibration over a K3 base times four-dimensional Minkowski space. Unbroken space-time supersymmetry determines all background fields except one scalar function which is related to the dilaton. The heterotic Bianchi identity gives the same differential equation for the dilaton, and we will discuss in detail the solvability of this equation for backgrounds preserving an N=2 supersymmetry. In the third part, we obtain the higher derivative D-brane action by using both linearized T-duality and string disc amplitude computation. We evaluate disc amplitude of one R-R field C^(p-3) and two NS-NS fields in the presence of a single Dp-brane in type II string theory. We obtain the action for the higher derivative brane interactions among one R-R field C^(p-3) and two NS-NS B-fields after carefully comparing the supergravity amplitudes with the corresponding string amplitude up to alpha^r2 order. We also show that these higher derivative brane couplings are invariant under both R-R and NS-NS B-field gauge transformations, and compatible with linear T-duality.Item Phenomenology of heterotic and type II orientifold string models(2009-05-15) Mayes, Van EricCryptons are metastable bound states of fractionally-charged particles that arise generically in the hidden sectors of models derived from heterotic string. We study their properties and decay modes in a speci?c ?ipped SU(5) model with long-lived four-particle spin-zero bound states called tetrons. The expected masses and lifetimes of the neutral tetrons make them good candidates for cold dark matter (CDM), and a potential source of the ultra-high energy cosmic rays (UHECRs) which have been observed, whereas the charged tetrons would have decayed in the early Universe. We calculate the spectra of ultra-high-energy cosmic rays (UHECRs) in an ex?plicit top-down model based on the decays of metastable neutral ?crypton? states. For all the decay operators, the total UHECR spectra are compatible with the available data. Also, the fractions of photons are compatible with all the published upper limits, but may be detectable in future experiments. We also construct several intersecting D-brane models on a variety of orientifold backgrounds. In particular, we construct ?ipped SU(5), Pati-Salam, and MSSM-like models. The phenomenological properties of these models are studied. For one model in particular, we ?nd that we may explain the quark masses and mixings, the tau lepton mass, and generate small neutrino masses via the see-saw mechanism.