Heavy meson chiral perturbation theory constraints on a charm-coupled GeV-scale QCD axion

2026/07/19 by Bo-Qiang Lu
#hep-ph #gr-qc #hep-ex #hep-th

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Abstract

The QCD axion with charm-only Peccei-Quinn coupling evades the isospin-violation problem of light-quark models in χPT. Since m_c>Λ_{\rm QCD}, we employ heavy meson chiral perturbation theory (HMChPT) to constrain this GeV-scale axion, including its CP-even radial partner σ and the axion a. In HMChPT, σ enters at leading order with O(1) coupling, while a couples only at O(1/m_c) and manifests via axion-pion mixing with θ_aπ≃ f_π/f_a∼0.04. We analyze D-meson masses, D-D^* splitting, charmonium, D-D scattering, B_s mixing, rare D decays, and the channels K⁺→π⁺a, B→ K a. No fatal HMChPT constraint emerges, unlike the χPT isospin issue. The strongest bound is from σ-mediated B_s- B_s mixing; axion-pion mixing opens testable rare decays, though rates are slightly below current sensitivity.

Citations

• Naturally quality-safe GeV axion with charm coupling
• Hadronic molecules

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