Axions

Axions are ultra-light particles with masses from 10⁻⁶ eV to 10⁻² eV, originally solving the strong CP problem and now a top dark matter candidate. The Axion Dark Matter Experiment (ADMX) at the University of Washington uses 8 T magnetic fields and microwave cavities to exclude couplings above 10⁻¹⁵ GeV⁻¹ for the 2.7–4.3 μeV range, while CERN's IAXO helioscope will achieve 5600 m² sensitivity. Unlike WIMPs (#1) requiring high masses, axions offer a natural production via misalignment, fitting large-scale structure seamlessly. Sterile neutrinos (#4) rely on uncertain X-ray signals, but ADMX's null results over a decade haven't slowed progress; advanced cavities promise to scan the full axion window within five years.