“…Both, the fact that the Peccei-Quinn mechanism provides an elegant solution to the strong CP -problem, and that it at the same time might provide a solution to the missing matter problem of the universe, accounts for the unwavering interest in the axion, even though it has not been detected yet and its existence hence remains hypothetical. The original ("visible") Peccei-Quinn-Weinberg-Wilczek (PQWW) axion with a decay constant at the electroweak scale and a mass in the keV/MeV region seems to be ruled out experimentally [4,[23][24][25], although there are still attempts to make the experimental data compatible with the original model [26,27]. However, these models require a lot of additional assumptions including that these axions restrictively couple to the first generation fermions (up and down quark, electron) and are accidentally pion-phobic.…”