NASDUCK SERF: New constraints on axion-like dark matter from a SERF comagnetometer

Abstract: Ultralight axion-like particles are well-motivated relics that might compose the cosmological dark matter and source anomalous time-dependent magnetic fields. We report on new terrestrial bounds on the coupling of axion-like particles to neutrons and protons using the nuclei of noble-gas and alkali-metal atoms in a comagnetometer detector operating in the Spin-Exchange Relaxation-Free~(SERF) regime. Conducting a month-long search, we cover the mass range of $1.4\times 10^{-12}$ eV/$c^2$ to $2\times 10^{-10}$~eV/$c^2$ and provide world-leading limits which supersede robust astrophysical bounds and improve upon previous terrestrial constraints by up to two orders of magnitudes for many masses within this range. These are the first reliable terrestrial bounds reported on the coupling of protons with axion-like dark matter, covering a new and unexplored terrain in its parameter space.