Search

This dissertation is concerned with identifying the consequences of string theory at energy scales that can be probed in our lifetimes. We present progress in computing the effective field theories that arise from compactifications of string theory, studying the observable physics that appears in these theories, and testing universal requirements on these theories.

This dissertation consists of five chapters. The first broadly and briefly orients the reader through an introduction to inflationary cosmology, and why we might expect multi-field inflation to take place. The next four chapters correspond to distinct lines of research conducted during my time as a graduate student. Chapter two is based on work conducted with my advisor Sonia Paban, studying the landscape of possible multi-field inflationary models through a random-matrix generated potential [1]. Chapter three is based on work with Diederik Roest and Perseas Christodoulidis, studying universality and prior dependence in multi-field inflation [2]. Chapters four and five are based on work with Sonia Paban and Vikas Aragam, studying inflation in potentials compatible with quantum gravity and rapidly turning trajectories [3, 4].

In this dissertation, we perform a thorough phenomenological study of the string theory landscape. To this end, we compare and contrast the data collected from particle accelerators and detectors against various models of observable particle phenomena. One stark indirect evidence of underlying string theory is the existence of supersymmetric (SUSY) particles, a variety of new particles resulting from a symmetry between the bosons and fermions observed in nature: i.e. every boson should be paired with a fermionic partner and vice versa. The discovery of the Higgs boson at the LHC in 2012, the particle responsible for giving mass to matter particles (e.g. electrons) and the massive gauge bosons, has provided us with strong bounds on the masses of these yet unobserved superpartner particles, which when combined with string theory landscape arguments, can yield strong statistical predictions for observing SUSY in future upgrades to particle accelerators. Various SUSY models are explored in the context of string landscape statistics by which we can rule some models out as realistic extensions to the Standard Model (SM). We also argue how realistic SUSY models requires the Higgs boson mass to be around 125 GeV with superpartners beyond current energy limits of the LHC - just what is observed experimentally. Additionally, we also analyze the prospect of detecting dark matter particles which only gravitate and exhibit at best only weak interactions. The emergence of SUSY also equips us with such Weakly Interacting Massive Particles (WIMPs), whose mass range can then be statistically predicted using string landscape arguments.

Abstract: If the large extra dimensions scenario [1, 2] is correct, the string scale Ms of string theory can be of the order of a few TeV; a theory known as low-scale string theory. In the D-brane [3, 4] formulation of low-scale string theory, string resonances (Regge excitations) can be produced in proton-proton collisions through 2-parton scatterings, and furthermore, in the limit Ms -> infinity, these scattering amplitudes match the tree-level scattering amplitudes derived in QCD. Using the cross-sections and decay widths of the string resonances, we write a Monte Carlo event generator, STRINGS-1.00, for the production and decay of the first and second string resonances, such that colour, quark flavour, baryon number, electric charge and total angular momentum are conserved. STRINGS is also capable of generating QCD tree-level scatterings in proton-proton collisions. We study the discovery potential of the first string resonance at the LHC (luminosity = 140 fb^-1) and the upcoming HL-LHC run (luminosity = 3000 fb^-1) with a centre-of-mass energy of sqrt(s) = 13 TeV. This is done by using STRINGS to generate events for different string scales and studying the significance of the signals over the background in the dijet invariant mass distribution. For luminosities of 140 fb^-1 and 3000 fb^-1, we find the maximum detectable string scales to be 8.4 TeV and 9.1 TeV, respectively. We also use results from an ATLAS paper [5] to set upper limits on the cross-section and lower limits on string scale of the first string resonance. We find with 95% confidence level that string resonances with string scales lower than 7.4 TeV do not exist in ATLAS results with an integrated luminosity of 37 fb^-1.

In this dissertation, we will cover aspects of the phenomenology of supersymmetric models, with an emphasis on models with heavier scalar partners of Standard Model fermions. We first introduce the Standard Model, providing explanations for why it should be extended by Supersymmetry. We also provide an introduction to general aspects of supersymmetric models, making use not only of bottom-up phenomenological constraints, but also top-down theoretical insight from String/M-Theory compactifications. In the body of this dissertation, we study in detail the phenomenology of supersymmetric models with heavier scalars at current and future colliders, as well as in low-energy flavor experiments in both the absence and the presence of CP violation. Finally, we discuss general implications of String theory for dark matter in supersymmetric models, as well as the dynamics of Grand Unified Theories in a broad class of models.

In this Dissertation, we consider three topics in the study of effective field theories derived from orbifold compactifications of the heterotic string. In Chapter 2 we provide a primer for those interested in building models based on orbifold compactifications of the heterotic string. In Chapter 3, we analyze gauge coupling unification in the context of heterotic strings on anisotropic orbifolds. This construction is very much analogous to effective five dimensional orbifold GUT field theories. Our analysis assumes three fundamental scales, the string scale, MS, a compactification scale, MC, and a mass scale for some of the vector-like exotics, MEX; the other exotics are assumed to get mass at MS. In the particular models analyzed, we show that gauge coupling unification is not possible with MEX = MC and in fact we require MEX ≪ MC ~ 3 × 1016 GeV. We find that about 10% of the parameter space has a proton lifetime (from dimension six gauge exchange) 1033 yr ≲ τ(p → π0e+) ≲ 1036 yr, which is potentially observable by the next generation of proton decay experiments. 80% of the parameter space gives proton lifetimes below Super-K bounds. In Chapter 4, we examine the relationship between the string coupling constant, gSTRING, and the grand unified gauge coupling constant, αGUT, in the models of Chapter 3. We find that the requirement that the theory be perturbative provides a non-trivial constraint on these models. Interestingly, there is a correlation between the proton decay rate (due to dimension six operators) and the string coupling constant in this class of models. Finally, we make some comments concerning the extension of these models to the six (and higher) dimensional case. In Chapter 5, we discuss the issues of supersymmetry breaking and moduli stabilization within the context of E8 ⊗ E8 heterotic orbifold constructions and, in particular, we focus on the class of “mini-landscape” models. These theories contain a non-Abelian hidden gauge sector which generates a non-perturbative superpotential leading to supersymmetry breaking and moduli stabilization. We demonstrate this effect in a simple model which contains many of the features of the more general construction. In addition, we argue that once supersymmetry is broken in a restricted sector of the theory, then all moduli are stabilized by supergravity effects. Finally, we obtain the low energy superparticle spectrum resulting from this simple model.

We use the flux attractor equations to study IIB supergravity compactifications with 3-form fluxes. We show that the attractor equations determine not just the values of the complex structure moduli and the axio-dilaton, but also the masses of those moduli and the gravitino. We then show that the flux attractor equations can be recast in terms of derivatives of a single generating function. A simple expression is given for this generating function in terms of the D3 tadpole and gravitino mass, with both quantities considered as functions of the fluxes. For a simple prepotential, we explicitly solve the attractor equations. We also discuss a thermodynamic interpretation of this generating function, and possible implications for the landscape. Having solved the flux attractor equations for 3-form fluxes, we add generalized fluxes to the compactifications and study their effects. We find that when we add only geometric fluxes, the compactifications retain their no-scale structure, and minimize their scalar potential when the appropriate complex flux is imaginary self-dual (ISD). These minima are still described by a set of flux attractor equations, which can be integrated by a generating function. The expressions for the vector moduli are formally identical to the case with 3-form fluxes only, while some of the hypermoduli vii are determined by extremizing the generating function. We work out several orbifold examples where all vector moduli and many hypermoduli are stabilized, with VEVs given explicitly in terms of fluxes.