Investigation of Quasi--Realistic Heterotic String Models with Reduced Higgs Spectrum

TL;DR

This paper explores heterotic-string models with reduced Higgs spectra, analyzing their flat directions and symmetry properties, extending previous work to models with unbroken or reduced SO(10) symmetry.

Contribution

It extends previous analysis of free fermionic heterotic-string models to include cases with unbroken or reduced SO(10) symmetry, identifying flat directions and Higgs spectrum properties.

Findings

Models with unbroken or reduced SO(10) symmetry admit flat directions.

Reduced Higgs spectrum achieved through boundary conditions.

Standard-like models without flat directions remain unchanged.

Abstract

Quasi--realistic heterotic-string models in the free fermionic formulation typically contain an anomalous U(1), which gives rise to a Fayet-Iliopolous term that breaks supersymmetry at the one--loop level in string perturbation theory. Supersymmetry is restored by imposing F- and D-flatness on the vacuum. In Phys. Rev. D 78 (2008) 046009, we presented a three generation free fermionic standard-like model which did not admit stringent F- and D-flat directions, and argued that the all the moduli in the model are fixed. The particular property of the model was the reduction of the untwisted Higgs spectrum by a combination of symmetric and asymmetric boundary conditions with respect to the internal fermions associated with the compactified dimensions. In this paper we extend the analysis of free fermionic models with reduced Higgs spectrum to the cases in which the SO(10) symmetry is left unbroken, or is reduced to the flipped SU(5) subgroup. We show that all the models that we study in this paper do admit stringent flat directions. The only examples of models that do not admit stringent flat directions remain the strandard-like models of reference Phys. Rev. D 78 (2008) 046009.