It must be noted that this plot and the 1/fa plot are, at best, challenging to interpret from any theoretical point of view. They should not be taken any more seriously than simply a comparison of experimental sensitivities. The area outside of the QCD line here requires significant (and rarely natural) model-building. In essence one needs a "QCD" axion whose mass is different from than the mass arising just from the anomalous gluon coupling of the axion (which defines 1/f_a). There are very few models that achieve this naturally, such as the Z_N axion model of Hook (https://arxiv.org/abs/1802.10093), developed phenomenologically by Di Luzio et al. (https://arxiv.org/pdf/2102.00012.pdf)
- Beam EDM: limit, reference
- CASPEr-electric: limit, reference
- CASPEr-electric projections: phase I, phase II, phase III, reference
- HfH+: limit, reference
- JEDI: limit, reference
- nEDM: limit, reference
- Storage Ring EDM (Projection): limit, reference
- Polarisation haloscope (Projection): projection (scanning), projection (post-discovery), reference