Predictions from String Theory.

Abstract

String theory is the leading candidate for an underlying theory of nature, as it provides a framework through which to address critical questions left unanswered by the Standard Model and Supersymmetry. A number of predictions of string constructions can be empirically tested at the Large Hadron Collider (LHC) and dark matter experiments. In this work I aim to make generic predictions of string theory, while combining bottom-up approaches to fill in the gaps in our understanding of string theory to make predictions for current and upcoming experiments.

First I study moduli masses and claim that moduli dominated the energy density of the universe prior to big bang nucleosynthesis. We argue that in any string theory with stabilized moduli there will be at least one modulus field whose mass is of order the gravitino mass. Cosmology then generically requires the gravitino mass be greater than 30 TeV and the early cosmological history of the Universe be non-thermal. We are then led to believe that the best-motivated channel for early LHC discovery is gluino pair- production events decaying into a high multiplicity of third generation quarks. We analyze signals and background at the LHC for 7 TeV center of mass energy for 1 fb-1 integrated luminosity, suggesting a reach for gluinos for masses about 650 GeV.

Second, I seek to construct a Grand Unified Theory (GUT) within different branches of string theory. One promising GUT, developed outside of string theory, is Flipped-SU(5), which I show has serious phenomenological difficulties. I demonstrate both that Flipped-SU(5) requires an R-symmetry to solve the mu-problem, and that no R-symmetries exist in F-theory. Thus Flipped-SU(5) cannot serve as a GUT within F-theory. Similarly, I seek to construct a GUT within M-theory. My study is based upon the discrete symmetry proposed by Witten that forbids the mu-term and solves the doublet-triplet splitting problem, but does not address how the symmetry might be broken. I find that the symmetry must be broken by moduli stabilization. The inclusion of such symmetry-breaking yields a modified solution to the mu and doublet-triplet splitting problems, along with testable predictions regarding dark matter properties.