Threshold Corrections and Gauge Symmetry in Twisted Superstring Models

TL;DR

This paper calculates threshold corrections in superstring models with free fermions, showing how they affect gauge unification scales, symmetry enhancements, and mixing angles, with implications for model building.

Contribution

It introduces calculations of threshold corrections for specific superstring models and explores how boundary conditions influence gauge symmetry and mixing angles.

Findings

Threshold corrections slightly increase the unification scale.

Different boundary conditions can describe the same particle spectrum.

Complex twisted fermions can enhance gauge symmetry groups.

Abstract

Threshold corrections to the running of gauge couplings are calculated for superstring models with free complex world sheet fermions. For two N=1 $SU(2)\times U(1)^5$ models, the threshold corrections lead to a small increase in the unification scale. Examples are given to illustrate how a given particle spectrum can be described by models with different boundary conditions on the internal fermions. We also discuss how complex twisted fermions can enhance the symmetry group of an N=4 $SU(3)\times U(1)\times U(1)$ model to the gauge group $SU(3)\times SU(2)\times U(1)$. It is then shown how a mixing angle analogous to the Weinberg angle depends on the boundary conditions of the internal fermions.