String Unification and Threshold Corrections

TL;DR

This paper investigates how heavy particle thresholds and Wilson line effects influence gauge coupling unification in string theories, highlighting the model dependence and implications for string unification without grand unified groups.

Contribution

It provides detailed calculations of heavy particle thresholds in realistic string models with Standard Model gauge groups, extending previous analyses limited to unbroken E6 models.

Findings

Threshold effects show strong model dependence.

Wilson line backgrounds significantly affect unification.

Results challenge the universality of string unification scenarios.

Abstract

The interpretation of the apparent unification of gauge couplings within supersymmetric theories depends on uncertainties induced through heavy particle thresholds. While in standard grand unified theories these effects can be estimated easily, the corresponding calculations are quite complicated in string unified theories and do exist only in models with unbroken $E_6$. We present results for heavy particle thresholds in more realistic models with gauge group $SU(3)\times SU(2)\times U(1)$. Effects of Wilson line background fields as well as the universal part of the (rather mild) threshold corrections indicate a strong model dependence. We discuss the consequences of our results for the idea of string unification without a grand unified gauge group.