New Constraints on Axion-Mediated Spin Interactions Using Magnetic Amplification.

Axions are highly motivated hypothetical particles beyond the standard model that can be dark matter candidates and address the strong CP problem. Here we search for axion-mediated interactions generated between two separated ^{129}Xe gas ensembles, monitoring and polarizing the ^{129}Xe nuclear spins through spin-exchange interactions with Rb vapor. Our method exploits the magnetic amplification through effective fields from Rb-Xe collisions, increasing the sensitivity for axion-mediated interactions by up to 145-fold relative to conventional methods. Moreover, we employ template filtering to extract exotic interactions with a maximum signal-to-noise ratio. By combining two techniques, axion-mediated interactions are constrained to be less than 10^{-5} of normal magnetic interactions on a length scale of 60 mm. We establish new constraints on the neutron-neutron pseudoscalar couplings for a mass range that expands into the well-motivated "axion window" (10  μeV-1  meV), improving previous constraints by up to 50-fold within it and 118-fold outside it. We further discuss promising applications in searches for other axion-nucleon interactions, including axion dark matter and black hole axion bursts with sensitivity well beyond astrophysical limits by several orders of magnitude.