An Axion from the G2 Constraint Sector: Decay Constant, Mass, Photon Coupling and the Experiment That Would Test It

The Peccei-Quinn solution to the strong CP problem predicts an axion whose properties follow from a single unknown scale, the decay constant. We propose that within the G2 algebraic framework of prior work this scale is set by the constraint sector, the sector governing CPT and conservation and the natural home for a CP-conserving mechanism: f_a = M_GUT × w_constraint = 4.28 × 10¹⁴ GeV, with M_GUT = 2 × 10¹⁶ GeV and w_constraint = 0.0214 the constraint-sector weight from the symmetry-breaking analysis. The standard QCD axion mass relation then gives m_a ≈ 13–14 neV. This version adds two further results. First, the axion-photon coupling: the framework's complete generation content, 16 = 2(3) + 2(3̄) + 4(1), with the universal Peccei-Quinn charge natural to a global sector, gives the anomaly ratio E/N = 8/3 and hence g_aγγ ≈ 2.1 × 10⁻¹⁸ GeV⁻¹, placing the prediction on the DFSZ line. Second, its experimental location: 13–14 neV lies in a currently unprobed window, above the 8.3 neV ceiling reached by ABRACADABRA-10 cm and below the 20 neV floor of DMRadio-m3, and far below the microwave-cavity range of ADMX. The proposed DMRadio-GUT experiment, sensitive across 0.4–120 neV at DFSZ sensitivity, covers it squarely. The prediction is therefore a complete and plottable point in parameter space that no experiment has yet excluded and one planned experiment would test. We state honestly which parts are distinctive to this framework and which are generic to grand-unified axion models.