Strong coupling unification and extra dimensions

TL;DR

This paper explores how models with extra dimensions affect the unification of electroweak and strong forces, revealing bounds on unification and compactification scales based on experimental data.

Contribution

It provides a general analysis of coupling unification in models extending the MSSM with extra dimensions, deriving bounds on scales from experimental constraints.

Findings

In 4D theories, unification scale is limited to below 10^9 GeV.

In models with extra dimensions, the compactification scale is constrained to below 10^7 GeV.

Electroweak precision data influences the allowed parameter space for unification and extra dimensions.

Abstract

We analyze the implications of electroweak and strong coupling unification in a very general class of models extending the minimal supersymmetric standard model in $4+p$ dimensions $(p\geq 0)$. In general, electroweak precision data require the presence of large extra dimensions (low compactification scales, $M_c$) and/or low unification scale, $M_U$. In particular, the actual experimental value of the strong coupling at $M_Z$ imposes an upper bound on the compactification and unification scales. In four dimensional theories ($M_c\simeq M_U$) with canonical hypercharge assignment we find $M_U\lesssim 10^{9}$ GeV. In theories with extra dimensions ($M_c<M_U$) we find $M_c\lesssim 10^7$ GeV, for a supersymmetric spectrum at the TeV scale.