Minimal Standard Heterotic String Models

TL;DR

This paper develops a string-theoretic mechanism using asymmetric boundary conditions to reduce Higgs multiplets and moduli in heterotic-string models, aiming for minimal Standard Model-like spectra.

Contribution

It introduces a novel string-level method to eliminate extra Higgs representations via asymmetric boundary conditions, improving model minimality.

Findings

Successfully reduces Higgs pairs to one in explicit models

Significantly decreases the supersymmetric moduli space

Provides a general framework for string model minimality

Abstract

Three generation heterotic-string vacua in the free fermionic formulation gave rise to models with solely the MSSM states in the observable Standard Model charged sector. The relation of these models to Z_2 x Z_2 orbifold compactifications dictates that they produce three pairs of untwisted Higgs multiplets. The reduction to one pair relies on the analysis of supersymmetric flat directions, that give superheavy mass to the dispensable Higgs states. We explore the removal of the extra Higgs representations by using the free fermion boundary conditions and hence directly at the string level, rather than in the effective low energy field theory. We present a general mechanism that achieves this reduction by using asymmetric boundary conditions between the left- and right-moving internal fermions. We incorporate this mechanism in explicit string models containing three twisted generations and a single untwisted Higgs doublet pair. We further demonstrate that an additional effect of the asymmetric boundary conditions is to substantially reduce the supersymmetric moduli space.