Running Gauge Couplings and Thresholds in the Type II Superstring

TL;DR

This paper studies how gauge couplings evolve and how threshold effects influence unification in four-dimensional Type II string models with non-simply-laced gauge groups, highlighting a specific SU(3)×U(1)×U(1) example.

Contribution

It provides a detailed analysis of running couplings and one-loop threshold corrections in general Type II string models with multiple gauge levels, including a concrete SU(3)×U(1)×U(1) case.

Findings

Threshold corrections cause a 6% increase in the unification scale.

Unification can occur with non-simply-laced gauge groups in Type II string models.

The study extends understanding of gauge coupling evolution in string theory.

Abstract

A distinctive feature of string unification is the possibility of unification by a non-simply-laced group. This occurs most naturally in four dimensional type~II string models where the gauge symmetry is realized by Kac-Moody algebras at different levels. We investigate the running coupling constants and the one-loop thresholds for such general models. As a specific case, we examine a $\rm SU(3)\times U(1)\times U(1)$ model and find that the threshold corrections lead to a small $6\%$ increase in the unification scale.