Abstract
It has been recently claimed that the axion coupling to fermions is responsible for an oscillating electric dipole moment (EDM) in the background of axion dark matter. In this work, we re-examine the derivation of this effect. Contrary to previous studies, we point out the physical relevance of an axion boundary term, which is crucial in restoring the axion shift symmetry and drastically affects the EDM phenomenology. To describe the latter, we introduce the notion of a time-averaged effective axion EDM, which encodes the boundary term and whose magnitude depends on the oscillation regime. For slow oscillations, the boundary term washes out the standard oscillating EDM, resulting in an exact cancellation in the static limit. Conversely, during fast oscillations, the boundary term amplifies the effective EDM relatively to the standard EDM contribution. This observable is especially interesting in the case of the electron EDM. For an \( \mathcal{O} \)(1) axion-electron coupling, the overall size of the effective EDM in the regime of intermediate or fast oscillations is comparable to the present static EDM limit.
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Acknowledgments
We thank Giovanni Carugno for bringing this problem to our attention, as well as Ramona Gröber, Sebastian Hoof, Gino Isidori, Marco Peloso, Pierre Sikivie and Christopher Smith for useful discussions. This work is funded by the European Union — NextGenerationEU and by the University of Padua under the 2021 STARS Grants@Unipd programme (Acronym and title of the project: CPV-Axion — Discovering the CP-violating axion). The work of LDL is also supported by the European Union — Next Generation EU and by the Italian Ministry of University and Research (MUR) via the PRIN 2022 project n. 2022K4B58X — AxionOrigins.
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Di Luzio, L., Gisbert, H. & Sørensen, P. On the oscillating electric dipole moment induced by axion-fermion couplings. J. High Energ. Phys. 2024, 76 (2024). https://doi.org/10.1007/JHEP04(2024)076
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DOI: https://doi.org/10.1007/JHEP04(2024)076