Search for Axionlike Dark Matter Using Liquid-State Nuclear Magnetic Resonance

We search for dark matter in the form of axionlike particles (ALPs) in the mass range $5.576741\mathrm{n}\mathrm{e}\mathrm{V}\mathrm{/}{\mathrm{c}}^2$ - $5.577733\mathrm{n}\mathrm{e}\mathrm{V}\mathrm{/}{\mathrm{c}}^2$ 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 $g_{\mathrm{a}\mathrm{p}}\approx 3\times {10}^{-2}{\mathrm{G}\mathrm{e}\mathrm{V}}^{-1}$. 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 $4.1\text{peV}\mathrm{/}{\mathrm{c}}^2$ to $17\text{neV}\mathrm{/}{\mathrm{c}}^2$ with hyperpolarized samples to boost the sensitivity beyond the astronomical limits.