Dark Matter as Ultralight Axion-Like particle in $E_6 \times U(1)_X$ GUT with QCD Axion

TL;DR

This paper proposes a GUT model incorporating an ultralight axi-Higgs as a dark matter candidate, which naturally arises from anomaly cancellation mechanisms and addresses both the strong CP problem and small-scale structure issues.

Contribution

It constructs a gauged $E_6 imes U(1)$ model with both an invisible QCD axion and an ultralight axi-Higgs, linking string-inspired anomaly cancellation to dark matter phenomenology.

Findings

The axi-Higgs mass range is $10^{-22}$ to $10^{-20}$ eV, suitable for dark matter.

The model naturally includes an invisible QCD axion solving the strong CP problem.

The axi-Higgs can address small-scale structure problems in cosmology.

Abstract

Axion-like fields are naturally generated by a mechanism of anomaly cancellation of one or more anomalous gauge abelian symmetries at the Planck scale, emerging as duals of a two-form from the massless bosonic sector of string theory. This suggests an analogy of the Green-Schwarz mechanism of anomaly cancellation, at field theory level, which results in one or more Stueckelberg pseudoscalars. In the case of a single Stueckelberg pseudoscalar $b$, vacuum misalignments at phase transitions in the early Universe at the GUT scale provide a small mass - due to instanton suppression of the periodic potential - for a component of $b$, denoted as $\chi$ and termed the "axi-Higgs", which is a physical axion-like particle. The coupling of the axi-Higgs to the gauge sector via Wess-Zumino terms is suppressed by the Planck mass, which guarantees its decoupling, while its angle of misalignment is related to $M_{GUT}$. We build a gauged $E_6\times U(1)$ model with anomalous $U(1)$. It contains both an automatic invisible QCD axion and an ultra-light axi-Higgs. The invisible axion present in the model solves the strong CP problem and has mass in the conventional range while the axi-Higgs, which can act as dark matter, is sufficiently light ($10^{-22} \textrm{ eV} < m_{\chi} < 10^{-20} \textrm{ eV}$) to solve short-distance problems which confront other cold dark matter candidates.