These two are not the only candidates for axion. I add here my comment to the blog of Lubos. The first evidence for pion/axion-like particles created in heavy-nucleus collisions near Coulomb threshold in MeV range came for more than 20 years ago. I developed a TGD based model for their productions characteristics for 15 years ago.
TGD explanation was based on what I called leptohadron physics with quarks replaced by color octet excitations of electrons allowed by TGD view about color. The production mechanism was creation of leptopions in strong non-orthogonal E and B with action given by E.B. Coupling to photons is dictated by partically conserved axial current hypothesis and identical with the coupling of pion/axion. The mass of the lowest leptopion state is very nearly to 2 times electron masses. Also heavier excitations on Regge trajectory are predicted: actually entire spectroscopy of leptomesons and leptobaryons. Heavier resonances were also observed.
Exotic quarks with MeV mass scale and corresponding to Mersenne prime M127=2k-1, k=127, characterizing electron, play a key role in TGD based model of nuclei as string like structure with threads connecting nucleons having quark and antiquark at their ends. The model explains tetraneutron and predicts a new exotic states of nuclei.
Quite generally, TGD predicts entire hierarchy of p-adically scaled up variants of standard model physics with mass scales coming as powers of half octaves. p≈ 2k. k prime or power of prime, are favored. Hence pionlike states should exist at various length scales and serve as a signature of various scaled variants of QCD like physics.
- Mass scale 8 MeV (compare with 7 MeV) would correspond to k= 112, and thus power of prime.
- Mass scale of 16 MeV (compare with 19 MeV) to p=about 119=7×17.
Below a list of references about early evidence for pionlike states which people for some reason probably related to shortcomings of ancient theories;-) want to call axions.
1. A.T. Goshaw et al(1979), Phys. Rev. Lett. 43,
2.J.Schweppe et al(1983), Phys. Rev. Lett. 51, 2261.
3. M. Clemente et al (1984), Phys. Rev. Lett. 137B, 41.
4. P.V. Chliapnikov et al(1984), Phys. Lett. B 141, 276.
5. L. Kraus and M. Zeller (1986), Phys. Rev. D 34, 3385.
6. A. Chodos (1987), Comments Nucl. Part. Phys., Vol 17, No 4, pp. 211, 223.
7.W. Koenig et al(1987), Zeitschrift fur Physik A, 3288, 1297.
8. C. I. Westbrook ,D. W Kidley, R. S. Gidley, R. S Conti and A. Rich (1987), Phys. Rev. Lett. 58 , 1328.