QCD axion dark matter in the dark dimension

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 ≲ M₅ ~ 10⁹ – 10¹⁰ GeV, where M₅ is the five-dimensional Planck mass. When combined with observational lower bounds, this implies that f_a falls within a narrow range f_a ~ 10⁹ – 10¹⁰ GeV, corresponding to the axion mass m_a ~ 10⁻³ – 10⁻² eV. At this scale, the QCD axion constitutes a minor fraction of the total cold dark matter (DM) density ~ 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 ~ 10⁻⁵ eV and a decay constant of f_A ~ 10¹¹ 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.