Gauge Coupling Unification in Realistic Free-Fermionic String Models

TL;DR

This paper investigates gauge coupling unification in realistic free-fermionic string models, analyzing various effects and thresholds, and finds that additional states can reconcile string unification with experimental data.

Contribution

It systematically evaluates heavy string threshold corrections and other effects, demonstrating that additional color triplets and electroweak doublets can achieve unification consistent with experiments.

Findings

Heavy string thresholds are too small to resolve discrepancies.

Non-standard hypercharge normalizations do not fix unification issues.

Extra color triplets and doublets can enable unification at the string scale.

Abstract

We discuss the unification of gauge couplings within the framework of a wide class of realistic free-fermionic string models which have appeared in the literature, including the flipped SU(5), SO(6)xSO(4), and various SU(3)xSU(2)xU(1) models. If the matter spectrum below the string scale is that of the Minimal Supersymmetric Standard Model (MSSM), then string unification is in disagreement with experiment. We therefore examine several effects that may modify the minimal string predictions. First, we develop a systematic procedure for evaluating the one-loop heavy string threshold corrections in free-fermionic string models, and we explicitly evaluate these corrections for each of the realistic models. We find that these string threshold corrections are small, and we provide general arguments explaining why such threshold corrections are suppressed in string theory. Thus heavy thresholds cannot resolve the disagreement with experiment. We also study the effect of non-standard hypercharge normalizations, light SUSY thresholds, and intermediate-scale gauge structure, and similarly conclude that these effects cannot resolve the disagreement with low-energy data. Finally, we examine the effects of additional color triplets and electroweak doublets beyond the MSSM. Although not required in ordinary grand unification scenarios, such states generically appear within the context of certain realistic free-fermionic string models. We show that if these states exist at the appropriate thresholds, then the gauge couplings will indeed unify at the string scale. Thus, within these string models, string unification can be in agreement with low-energy data.