We search for dark matter in the form of axionlike particles (ALPs) in the mass range $5.576741 \,\mathrm{ neV/c^2}$ - $5.577733\,\mathrm{ neV/c^2}$ 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 $g_{ \mathrm{ ap}} \approx 3 \times 10^{ -2}\,\mathrm{ GeV}^{ -1}$. 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 $4.1\,\mathrm{ \peV/c^2}$ to $17\,\mathrm{ \neV/c^2}$ with hyperpolarized samples to boost the sensitivity beyond the astronomical limits.
Categories: hep-ex physics.atom-ph
