Making Ends Meet: String Unification and Low-Energy Data

TL;DR

This paper investigates various effects that could reconcile the discrepancy between string unification scale predictions and low-energy data, finding that extra matter in realistic models can achieve this alignment.

Contribution

It evaluates multiple potential modifications to string unification predictions within realistic free-fermionic models, identifying extra matter as the key factor.

Findings

Most effects do not significantly change minimal string predictions.

Extra matter can reconcile string unification with low-energy data.

Realistic models often naturally include the necessary extra matter.

Abstract

A long-standing problem in string phenomenology has been the fact that the string unification scale disagrees with the GUT scale obtained by extrapolating low-energy data within the framework of the minimal supersymmetric standard model (MSSM). In this paper we examine several effects that may modify the minimal string predictions and thereby bring string-scale unification into agreement with low-energy data. These include heavy string threshold corrections, non-standard hypercharge normalizations, light SUSY thresholds, intermediate gauge structure, and thresholds arising from extra matter beyond the MSSM. We explicitly evaluate these contributions within a variety of realistic free-fermionic string models, including the flipped SU(5), SO(6) x SO(4), and various SU(3) x SU(2) x U(1) models, and find that most of these sources do not substantially alter the minimal string predictions. Indeed, we find that the only way to reconcile string unification with low-energy data is through certain types of extra matter. Remarkably, however, many of the realistic string models contain precisely this required matter in their low-energy spectra.