String Theory and the Path to Unification: A Review of Recent Developments
Keith R. Dienes (Institute for Advanced Study, Princeton)

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…
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