Establish whether the vanilla QCD axion is sourced inside neutron stars

Establish whether the vanilla QCD axion with domain-wall number N=1 is sourced inside neutron stars, accounting for the vacuum structure and stellar backreaction specific to the N=1 model.

Background

The paper compares the vanilla light QCD axion with multi-branched Z_N axion models. For Z_N models with N≥3, the authors find, under a free-Fermi-gas description trusted up to nuclear density, that sourcing occurs inside neutron stars. They caution that the N=1 case has a different vacuum structure and correspondingly different backreaction on the star.

Whether the standard QCD axion is sourced depends on dense-matter physics and on the reliability of the equation-of-state and nuclear-interaction description at the relevant densities. This unresolved issue affects whether the axion-hair electrodynamics and associated bounds apply to the vanilla QCD axion.

References

If the vanilla QCD axion with $N=1$ is sourced inside NSs is an open question, see e.g. .

— Axion Hair and Pulsar Electrodynamics: modelling, discharge dynamics, and particle-in-cell simulations  (2609.08840 - Witte et al., 8 Sep 2026) in Section 2, subsection “The special case of the Z_N axion”