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.
Highlights
It has been known from various observations that most of the substance in the Universe is invisible, in the form of dark matter and dark energy [1,2]
We argue that axionphoton interaction introduces an electron spin effective coupling, which is not the same as oscillating electric dipole moment (OEDM)
These interactions may manifest themselves in spin precession experiments or OEDM searches
Summary
It has been known from various observations that most of the substance in the Universe is invisible, in the form of dark matter and dark energy [1,2]. Numerous methods have been proposed to probe axions or axionic dark matter, including the microwave cavity experiments like ADMX [18,19], “light shining through wall” experiments such as OSQAR [20], and solar axion observations like CAST [21] These experiments have not found signal of axions, they have set constraints on axion interactions. Hill [26,27,28,29] proposed that axion-photon interaction introduces an OEDM of the electron, and the electric dipole radiation induced by axionic dark matter is discussed. We use the name “axion” but refer to general ALPs. We argue that axionphoton interaction introduces an electron spin effective coupling, which is not the same as OEDM. Greek indices take values in f0; 1; 2; 3g while Latin indices take values in f1; 2; 3g
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