Strings, Unification and dilaton/moduli-induced SUSY-breaking

TL;DR

The work examines whether heterotic string theory can operationalize a realistic unification of the Standard Model with gravity, focusing on gauge coupling unification, string-GUT constructions at level $k=2$, and dilaton/moduli-mediated SUSY breaking. It analyzes the challenges of direct string unification, the selection rules and model-dependent constraints for string-GUTs built with orbifolds, and the structure of soft SUSY-breaking terms under dilaton/moduli breaking, highlighting universal, tachyon-free patterns and connections to finiteness and duality. The contributions include concrete observations about threshold corrections, representation constraints at $k=2$, and the distinctive features of dilaton-dominated soft terms, which inform both theoretical consistency and experimental expectations for SUSY spectra. Overall, the paper clarifies the prospects and obstacles for deriving realistic low-energy phenomenology from string theory and points to duality-based perspectives and non-perturbative effects as important future directions.

Abstract

I discuss several issues concerning the use of string models as unified theories of all interactions. After a short review of gauge coupling unification in the string context, I discuss possible motivations for the construction of $SU(5)$ and $SO(10)$ String-GUTs. I describe the construction of such String-GUTs using different orbifold techniques and emphasize those properties which could be general. Although $SO(10)$ and $SU(5)$ String-GUTs are relatively easy to build, the spectrum bellow the GUT scale is in general bigger than that of the MSSM and includes colour octets and $SU(2)$ triplets. The phenomenological prospects of these theories are discussed. I then turn to discuss soft SUSY-breaking terms obtained under the assumption of dilaton/moduli dominance in SUSY-breaking string schemes. I underline the unique finiteness pr of the soft terms induced by the dilaton sector. These improved finiteness properties seem to be related to the underlying $SU(1,1)$ structure of the dilaton couplings. I conclude with an outlook and some speculations regarding $N=1$ duality.