Unification without Unification

TL;DR

The paper addresses whether gauge coupling unification must come from embedding the SM into a traditional grand unified group, noting that precision data suggest unification near $M_{GUT} \sim 10^{16}$ GeV but that conventional GUTs face proton decay and doublet/triplet splitting problems. It proposes a collapsed GUT framework with a weakly coupled sector $G_W$ and a strongly coupled sector $G_S$ that are Higgsed to the diagonal $SU(3)\otimes SU(2)\otimes U(1)$, yielding effective couplings with $\frac{1}{g_{eff}^{2}} = \frac{1}{g_W^{2}} + \frac{1}{g_S^{2}}$ and, for $g_S \gg g_W$, $g_{eff} \approx g_W$, thereby reproducing apparent unification without a conventional GUT. The approach connects to dimensional deconstruction/moose models and higher-dimensional Wilson-line breaking, while acknowledging losses such as uncertain hypercharge normalization and the lack of universal Yukawa relations. It also discusses TeV-scale phenomenology, including possible new diagonal gauge bosons and threshold effects, providing a novel route to unification that is consistent with current constraints and potentially testable at colliders.

Abstract

The logarithmic running of the gauge couplings alpha_1, alpha_2 and alpha_3, indicates that they may unify at some scale M_GUT ~ 10^16. This is often taken to imply that the standard model gauge group is embedded into some larger simple group in which quarks and leptons are placed in the same multiplet. These models have generic features, such as proton decay, and generic problems, namely the splitting of the Higgs doublet and triplet. Inspired by the recent discusion of dimensional deconstruction, we propose an interesting alternative: we postulate a strongly coupled SU(3)xSU(2)xU(1), which is not the remnant of a GUT, and is Higgsed with a weakly coupled SU(3)xSU(2)xU(1), which is the remnant of a GUT, or with a GUT group directly, into the diagonal subgroup. In this ``collapsed GUT'' mechanism, unification of coupling constants in the low energy theory is expected, but proton decay and the doublet/triplet splitting problem are entirely absent.