We search for dark matter in the form of axionlike particles (ALPs) in the mass range 5. 576741 \, neV/c² - 5. 577733\, neV/c² by probing their possible coupling to fermion spins through the ALP field gradient. This is achieved by performing proton nuclear magnetic resonance spectroscopy on a sample of methanol as a technical demonstration of the Cosmic Axion Spin Precession Experiment Gradient (CASPEr-Gradient) Low-Field apparatus. Searching for spin-coupled ALP dark matter in this mass range with associated Compton frequencies in a 240 Hz window centered at 1. 348570 MHz resulted in a sensitivity to the ALP-proton coupling constant of g₀ 3 10^-2\, GeV^-1. This narrow-bandwidth search serves as a proof-of-principle and a commissioning measurement, validating our methodology and demonstrating the experiment's capabilities. CASPEr-Gradient Low-Field will probe the mass range from 4. 1\, /c² to 17\, /c² with hyperpolarized samples to boost the sensitivity beyond the astronomical limits.
Walter et al. (Tue,) studied this question.
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