2026/07/14 by Bo-Qiang Lu · 1 citation
#hep-ph #gr-qc #hep-ex #hep-th
The GeV-scale QCD axion -- where Peccei-Quinn (PQ) symmetry is broken by the QCD condensate at fa\simO(1)~GeV -- faces a structural isospin problem: the PQ spurion coupling to light quarks (u,d,s) breaks SU(2) isospin, generating an unacceptable ∼ 15% π0--π^± mass splitting. We show that coupling the PQ scalar to the charm quark instead eliminates this violation entirely, and lowering mϕ∼ 3--4~MeV makes the charm Yukawa κc∝ mϕ perturbative (κc<1). The resulting fa\simGeV≪ M\rm Pl solves the axion quality problem: even the lowest-dimension d=6 Planck-suppressed operator gives m\rm PQ/ma∼ 10-14, without any additional symmetry. The model predicts a distinctive \it negative ΔN\rm eff∼ -0.1 for mϕ∼ 3~MeV, testable by CMB-S4. The B→ Kσ penguin predicts BR∼ 2× 10-5, consistent with the Belle~II evidence for B+→ K+νν at (2.3±0.7)× 10-5~\citeBelleII:2024knv. All ten classes of experimental, astrophysical, and cosmological constraints are satisfied in the viable window mϕ∼ 3--4~MeV, with the Bs mixing constraint pending a dedicated lattice calculation.