Gauge unification, non-local breaking, open strings

TL;DR

This paper explores non-local GUT symmetry breaking in open string models, demonstrating how freely acting orbifolds can break GUT symmetry without localized effects, with the breaking scale determined by compactification moduli.

Contribution

It introduces a new open string orbifold model where GUT breaking occurs non-locally via freely acting orbifolds, avoiding localized GUT-breaking terms and fixing the unification scale through compactification.

Findings

GUT breaking is possible only in the purely parallel orbifold case.

The unification scale is set by the size of the compactification dimension.

The model achieves gauge symmetry breaking without localized GUT-breaking contributions.

Abstract

The issue of non-local GUT symmetry breaking is addressed in the context of open string model building. We study ZNxZM' orbifolds with all the GUT-breaking orbifold elements acting freely, as rotations accompanied by translations in the internal space.We consider open strings quantized on these backgrounds, distinguishing whether the translational action is parallel or perpendicular to the D-branes. GUT breaking is impossible in the purely perpendicular case, non-local GUT breaking is instead allowed in the purely parallel case. In the latter, the scale of breaking is set by the compactification moduli, and there are no fixed points with reduced gauge symmetry, where dangerous explicit GUT-breaking terms could be located. We investigate the mixed parallel+perpendicular case in a Z2xZ2' example, having also a simplified field theory realization.It is a new S1/Z2xZ2' orbifold-GUT model, with bulk gauge symmetry SU(5)xSU(5) broken locally to the Standard Model gauge group. In spite of the locality of the GUT symmetry breaking, there is no localized contribution to the running of the coupling constants, and the unification scale is completely set by the length of S1.