Viability of an extended Z_N axion model with cross-sector nucleon–pion couplings under existing constraints

Investigate whether the extended Z_N-symmetric axion model that introduces additional cross-sector nucleon–pion couplings, defined by the interaction term ΔL ⊃ ∑_i (1/2) c~ (⟨χ_+^(i+1)⟩ + ⟨χ_+^(i−1)⟩) N̄^(i)N^(i), can evade existing experimental and phenomenological constraints while preserving consistency with nuclear physics and the axion solution to the strong CP problem.

Background

To mitigate density-induced modifications of the axion potential (e.g., inside Earth) that can generate strong bounds on ultralight axions, the authors consider extending the Z_N axion framework by coupling nucleons in one Standard Model copy to pion-sector operators of neighboring copies. This modifies the density corrections to the axion mass and could remove Earth, Sun, and white dwarf density-effect bounds while leaving clock-based limits intact.

However, such cross-sector couplings may face strong constraints (e.g., from missing-energy searches) and must not spoil low-energy QCD predictions or the strong CP solution. The authors note that a full assessment of whether this extended construction remains viable within all relevant constraints has not yet been carried out.