Axion induced spin effective couplings

TL;DR

This paper re-examines axion-induced spin couplings in electromagnetic fields, highlighting a new electron spin effective coupling related to magnetic fields and analyzing implications for ultralight axion dark matter detection.

Contribution

It identifies a novel axion-electromagnetic interaction that differs from traditional OEDM effects and clarifies the role of shift symmetry in ultralight axion models.

Findings

Axion-photon interaction induces a spin effective coupling related to magnetic fields.

Ultralight axion models without shift symmetry face strong constraints.

Results extended to axions with finite velocity.

Abstract

Detecting axionic dark matter induced electron or nucleon oscillating electric dipole moment (OEDM) has become a new way for dark matter searches. We re-examine such axion-spin couplings in external electromagnetic fields. We point out that axion-photon interaction induces an electron spin effective coupling, which is different from an OEDM. In particular, the axion-spin effective coupling is directly related to magnetic field rather than electric field. For axion-electron or axion-nucleon couplings, an OEDM of fermion is introduced, whose effect in ultralight axion cases depends on whether axion shift symmetry is manifest. Specifically, ultralight axionic dark matter interactions that do not obey the shift symmetry will be strongly constrained. We also extend the results to the case where axion has a finite velocity.