Five dimensional $SU(5) \times U(1)_{\rm PQ}$ grand unification

TL;DR

This paper introduces a five-dimensional grand unified model that naturally incorporates the QCD axion, predicts proton decay within experimental reach, and unifies the unification and axion scales.

Contribution

It presents a novel 5D GUT framework that explains the axion as a gauge field and links unification with axion physics, ensuring axion quality through gauge symmetry.

Findings

Proton decay predicted within upcoming experimental sensitivity.

Unified scales of grand unification and axion decay constant.

Model naturally explains axion as a gauge field and addresses the strong CP problem.

Abstract

We propose a grand unified model in five dimensions that addresses the strong CP problem. In this framework, the boundary conditions that break the unified gauge group into the Standard Model gauge group simultaneously explain the mass splitting of the new matter content required for gauge coupling unification and the emergence of the QCD axion as the $U(1)_{\rm PQ}$ gauge field. The quality of the axion solution is inherently ensured by the gauge symmetry of the model. We further investigate the implications of this setup, including its predictions for the proton decay rate, the detectability of axion dark matter, and its cosmological impact. Notably, the model predicts a proton decay signal within the sensitivity of upcoming experiments, a result driven by the trade-off between the unification scale and the reheating temperature. In our scenario, the scales of grand unification and the axion decay constant are naturally unified.