On the fermionic couplings of axionic dark matter

TL;DR

This paper analyzes the interactions between axionic dark matter and fermions, revealing that certain effects are strongly screened and identifying the axion-induced electric dipole moments as key observable phenomena.

Contribution

It generalizes the Schiff theorem to show screening of axion interactions and predicts observable axion-induced electric dipole moments in fermions.

Findings

Axion-induced neutron EDM could be much larger than previously expected.

Electron EDM might be highly sensitive to relic axions.

Screening effects suppress certain axion-fermion interactions.

Abstract

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 operator 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, are not further screened by the original Schiff theorem, and are ultimately responsible for inducing the usual axioelectric ionization. The two main phenomenological consequences are then that first 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 could also become available, and could actually be highly sensitive to relic axions.