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1. Overview of the Cosmic Axion Spin Precession Experiment (CASPEr)

   Jackson Kimball, D. F.; Afach, S.; Aybas, D.; Blanchard, J. W.; Budker, D.; Centers, G.; Engler, M.; Figueroa, N. L.; Garcon, A.; Graham, P. W.; et al (January 2020, Springer proceedings in physics)

   Carosi, G.; Rybka, G. (Ed.)

   An overview of our experimental program to search for axion and axion-like-particle (ALP) dark matter using nuclear magnetic resonance (NMR) techniques is presented. An oscillating axion field can exert a time-varying torque on nuclear spins either directly or via generation of an oscillating nuclear electric dipole moment (EDM). Magnetic resonance techniques can be used to detect such an effect. The first-generation experiments explore many decades of ALP parameter space beyond the current astrophysical and laboratory bounds. It is anticipated that future versions of the experiments will be sensitive to the axions associated with quantum chromodynamics (QCD) having masses <10^(−9) eV/c^2. 

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2. Constraints on bosonic dark matter from ultralow-field nuclear magnetic resonance

   https://doi.org/10.1126/sciadv.aax4539  

   Garcon, Antoine; Blanchard, John W.; Centers, Gary P.; Figueroa, Nataniel L.; Graham, Peter W.; Jackson Kimball, Derek F.; Rajendran, Surjeet; Sushkov, Alexander O.; Stadnik, Yevgeny V.; Wickenbrock, Arne; et al (October 2019, Science Advances)

   The nature of dark matter, the invisible substance making up over 80% of the matter in the universe, is one of the most fundamental mysteries of modern physics. Ultralight bosons such as axions, axion-like particles, or dark photons could make up most of the dark matter. Couplings between such bosons and nuclear spins may enable their direct detection via nuclear magnetic resonance (NMR) spectroscopy: As nuclear spins move through the galactic dark-matter halo, they couple to dark matter and behave as if they were in an oscillating magnetic field, generating a dark-matter–driven NMR signal. As part of the cosmic axion spin precession experiment (CASPEr), an NMR-based dark-matter search, we use ultralow-field NMR to probe the axion-fermion “wind” coupling and dark-photon couplings to nuclear spins. No dark matter signal was detected above background, establishing new experimental bounds for dark matter bosons with masses ranging from 1.8 × 10 −16 to 7.8 × 10 −14 eV. 

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3. Adiabatic conversion of ALPs into dark photon dark matter

   https://doi.org/10.1007/JHEP03(2025)215  

   Broadberry, Edward; Das, Saurav; Hook, Anson; Marques-Tavares, Gustavo (March 2025, Journal of High Energy Physics)

   A<sc>bstract</sc> We introduce a mechanism by which a misaligned ALP can be dynamically converted into a dark photon in the presence of a background magnetic field. An abundance of non-relativistic ALPs will convert to dark photons with momentum of order the inhomogeneities in the background field; therefore a highly homogeneous field will produce non-relativistic dark photons without relying on any redshifting of their momenta. Taking hidden sector magnetic fields produced by a first order phase transition, the mechanism can reproduce the relic abundance of dark matter for a wide range of dark photon masses down to 10⁻¹³eV. 

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4. Calibration of the Solid‐State Nuclear Magnetic Resonance Search for Axion‐Like Dark Matter

   https://doi.org/10.1002/andp.202300252  

   Winter, Andrew J.; Marić, Tanja; Balabanova, Viktoriya A.; Ádám, János; Randall, Glenn; Wickenbrock, Arne; Jackson Kimball, Derek F.; Budker, Dmitry; Sushkov, Alexander O. (November 2023, Annalen der Physik)

   Abstract Calibration of nuclear‐magnetic‐resonance‐based searches for axion‐like dark matter can be performed by free induction decay (FID) measurements. This manu‐ script describes FID experiments on several solid materials, motivated by the Cosmic Axion Spin Precession Experiment (CASPEr) program. Experiments with²⁰⁷Pb nuclear spins in ferroelectrics, lead magnesium niobate‐lead titanate (PbMg_1/3Nb_2/3O₃) (PbTiO₃)_1/3(PMN‐PT) and lead zirconium titante PbZr_0.52Ti_0.48O₃(PZT) are directly relevant to the CASPEr‐electric search for the electric dipole moment interaction of axion‐like dark matter. Experiments with³¹P nuclear spins in gadolinium‐doped hydroxypyromorphite Pb_4.95Gd_0.05(PO₄)₃OH (HPM:Gd) are used for apparatus calibration. The measurements characterized the nuclear spin ensemble coherence time and the magnetic resonance detection sensitivity for these samples. Calibration is performed using small tip‐angle pulses. 

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5. Predictions for axion couplings from ALP cogenesis

   https://doi.org/10.1007/JHEP01(2021)172  

   Co, Raymond T.; Hall, Lawrence J.; Harigaya, Keisuke (January 2021, Journal of High Energy Physics)

   null (Ed.)

   A bstract Adding an axion-like particle (ALP) to the Standard Model, with a field velocity in the early universe, simultaneously explains the observed baryon and dark matter densities. This requires one or more couplings between the ALP and photons, nucleons, and/or electrons that are predicted as functions of the ALP mass. These predictions arise because the ratio of dark matter to baryon densities is independent of the ALP field velocity, allowing a correlation between the ALP mass, m a , and decay constant, f a . The predicted couplings are orders of magnitude larger than those for the QCD axion and for dark matter from the conventional ALP misalignment mechanism. As a result, this scheme, ALP cogenesis, is within reach of future experimental ALP searches from the lab and stellar objects, and for dark matter. 

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