String Theory and the Path to Unification: A Review of Recent Developments

TL;DR

This review surveys recent approaches in string theory aimed at resolving the discrepancy between string unification scales and those predicted by the MSSM, highlighting various models and corrections that contribute to unification.

Contribution

It provides a comprehensive overview of recent developments and methods in string phenomenology addressing gauge coupling unification scale discrepancies.

Findings

Multiple approaches can reconcile unification scales

Heavy string thresholds significantly impact unification predictions

Non-supersymmetric and strong coupling models offer alternative solutions

Abstract

This is a pedagogical review article surveying the various approaches towards understanding gauge coupling unification within string theory. As is well known, one of the major problems confronting string phenomenology has been an apparent discrepancy between the scale of gauge coupling unification predicted within string theory, and the unification scale expected within the framework of the Minimal Supersymmetric Standard Model (MSSM). In this article, I provide an overview of the different approaches that have been taken in recent years towards reconciling these two scales, and outline some of the major recent developments in each. These approaches include 1) string GUT models; 2) higher affine levels and non-standard hypercharge normalizations; 3) heavy string threshold corrections; 4) light supersymmetric thresholds; 5) effects from intermediate-scale gauge and matter structure beyond the MSSM; 6) strings without supersymmetry; and 7) strings at strong coupling.