Making Ends Meet: String Unification and Low-Energy Data

TL;DR

The effects that may resolve the disagreement between string-scale unification and low-energy data include string threshold corrections, nonstandard hypercharge normalizations, light supersymmetric thresholds, intermediate gauge structure, and extra matter beyond the minimal supers asymmetric standard model.

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.

Figures (1)

• Figure 1: Scatter plot of $\lbrace\sin^2\theta_W(M_Z), \alpha_{\rm strong}(M_Z)\rbrace$ for various values of $\lbrace m_0,m_{1/2},\newline m_h,m_{\tilde{h}},M_{\rm string}\rbrace$. Region (a) assumes the MSSM spectrum as discussed in the text, while (b) also includes the effects of the string-predicted extra matter.