QCD Axion Dark Matter in the Dark Dimension

TL;DR

This paper explores how the QCD axion, predicted by the dark dimension scenario and constrained by the Weak Gravity Conjecture, can account for all dark matter through a resonant conversion with an axion-like particle.

Contribution

It demonstrates that the QCD axion can constitute the entire dark matter abundance via a two-axion mixing mechanism in the dark dimension framework.

Findings

QCD axion mass range is $10^{-3}$ to $10^{-2}$ eV.

QCD axion accounts for the total dark matter with specific ALP parameters.

Resonant conversion mechanism enables full dark matter explanation.

Abstract

The recently proposed dark dimension scenario reveals that axions can be localized on the Standard Model brane, thereby predicting the quantum chromodynamics (QCD) axion decay constant from the Weak Gravity Conjecture: $f_a\lesssim M_5 \sim 10^{9}-10^{10}\, \rm GeV$, where $M_5$ is the five-dimensional Planck mass. When combined with observational lower bounds, this implies that $f_a$ falls within a narrow range $f_a\sim 10^{9}-10^{10}\, \rm GeV$, corresponding to the axion mass $m_a\sim 10^{-3}-10^{-2}\, \rm eV$. At this scale, the QCD axion constitutes a minor fraction of the total cold dark matter (DM) density $\sim 10^{-3}-10^{-2}$. In this work, we investigate the issue of QCD axion DM within the context of the dark dimension and demonstrate that the QCD axion in this scenario can account for the entire DM abundance through a simple two-axion mixing mechanism. Specifically, we consider the resonant conversion of an axion-like particle (ALP) into the QCD axion. We find that, in a scenario where the ALP possesses a mass of approximately $m_A \sim 10^{-5} \, \rm eV$ and a decay constant of $f_A \sim 10^{11} \, \rm GeV$, the QCD axion in the dark dimension can account for the overall DM. The ALP required within this specific range may originate from the grand unification of gauge forces in the dark dimension.