On the fermionic couplings of axionic dark matter
Résumé
In the non-relativistic limit, two types of dark matter axion interactions with fermions are thought to dominate: one is induced by the spatial gradient of the axion field and called the axion wind, and the other by the time-derivative of the axion field, generating axioelectric effects. By generalizing Schiff theorem, it is demonstrated that this latter interaction is actually strongly screened. For a neutral fermion, it can be entirely rotated away and is unobservable. For charged fermions, the only effect that can peek through the screening is an axion-induced electric dipole moment (EDM). These EDMs are not related to the axion coupling to gluons, represent a prediction of the Dirac theory analogous to the g=2 magnetic moments, and are not further screened by the original Schiff theorem, at least when axions are not too light. The two main phenomenological consequences are first that the axion-induced neutron EDM could be several orders of magnitude larger than expected from the axion gluonic coupling, and second, that the electron EDM would also become available, and would actually be highly sensitive to relic axions.