Gauge Unification in Highly Anisotropic String Compactifications

TL;DR

This paper reevaluates gauge unification in heterotic string compactifications, highlighting that highly anisotropic orbifold models with large compactification radii can achieve precise gauge coupling unification, akin to MSSM predictions.

Contribution

It demonstrates that highly anisotropic string compactifications with large radii can realize precise gauge unification through non-local or localized GUT breaking mechanisms.

Findings

Highly anisotropic orbifold models facilitate gauge unification.

Non-local GUT breaking leads to MSSM-like unification.

Localized GUT breaking at singular points is also promising.

Abstract

It is well-known that heterotic string compactifications have, in spite of their conceptual simplicity and aesthetic appeal, a serious problem with precision gauge coupling unification in the perturbative regime of string theory. Using both a duality-based and a field-theoretic definition of the boundary of the perturbative regime, we reevaluate the situation in a quantitative manner. We conclude that the simplest and most promising situations are those where some of the compactification radii are exceptionally large, corresponding to highly anisotropic orbifold models. Thus, one is led to consider constructions which are known to the effective field-theorist as higher-dimensional or orbifold grand unified theories (orbifold GUTs). In particular, if the discrete symmetry used to break the GUT group acts freely, a non-local breaking in the larger compact dimensions can be realized, leading to a precise gauge coupling unification as expected on the basis of the MSSM particle spectrum. Furthermore, a somewhat more model dependent but nevertheless very promising scenario arises if the GUT breaking is restricted to certain singular points within the manifold spanned by the larger compactification radii.