Does the QCD axion exist?
In plain words
The axion would be a very light, very weakly interacting particle that removes the strong CP puzzle. Detectors search for it turning into a photon inside strong magnets.
Precise statement
Determine whether a pseudoscalar a with $m_a \sim 5.7\ \mathrm{micro-eV}\ (1e12\ \mathrm{GeV}/f_a)$ and photon coupling g_agamma inside the QCD axion band (KSVZ to DFSZ and beyond) exists, for $f_a\ \text{between}\ \sim 1e8\ \text{and}\ \sim 1e17\ \mathrm{GeV}$. An answer is a detection with $m_a$ and g_agamma consistent with the QCD mass-decay constant relation, or exclusion of the band over the full range.
What would settle it
Haloscope, helioscope (IAXO) and nuclear-coupling (CASPEr-type) searches covering the QCD axion band across micro-eV to meV masses.
Status in the literature
Unverified note
Haloscopes reach DFSZ sensitivity only in narrow windows of a few micro-eV as of 2025.
Related problems
- Special case of Why is the QCD $\theta$ angle smaller than $1e-10$?
- More general than Is dark matter the QCD axion?