Assess second-order effects for the ultralight-photon benchmark

Calculate the second-order axial-photon effects for the benchmark photon mass \(m_\gamma=10^{-26}\,\mathrm{eV}\), including their consequences for laboratory haloscopes and polarimeters, and determine whether these effects dominate the corresponding first-order signals.

Background

For a photon mass far below the main benchmark, effects enhanced by inverse powers of mγm_\gamma can become much larger. The paper notes that second-order contributions may then dominate laboratory observables, but it does not calculate them or determine the resulting experimental reach.

References

We have not worked out second-order effects in detail, but since they scale with an additional factor of $\sqrt{\rho_\DM} / (f_\gamma m_\gamma)$, they may become dominant for laboratory haloscopes and polarimeters.

— Ultralight Axial Dark Matter  (2609.10671 - Hook et al., 9 Sep 2026) in Appendix A.3, paragraph “Benchmark Without Vortex Formation”