Magnetic resonance searches for Earth-sourced exotic nuclear spin–dependent interactions

Searches for physics beyond the standard model motivate investigations into ultralight exotic bosons, which may mediate long-range spin-spin interactions. Here, we develop an unreported method based on nuclear magnetic resonance (NMR) performed at the surface of Earth to search for exotic interactions sourced by astrophysical bodies. The key aspects of the work are modeling the internal nuclear spin polarization of Earth and developing an NMR detector to sense the exotic fields produced by such polarization. We create a distribution map of underground polarized nucleons by combining nuclear physics and geophysics data, identifying approximately 5.3 × 10 37 polarized geoprotons. The searches are enabled by a dual-xenon-isotope NMR sensor combined with phase cycling techniques, which suppresses systematic errors by more than 200-fold compared with a measurement without using these reversals. As a result, the experiment can detect exotic fields corresponding to equivalent static magnetic fields at the femtotesla level. Our work sets the most stringent limits on axion-mediated couplings g P n g P p , as well as on Z ′ boson–mediated couplings g A n g A p and g A n g V p . In particular, the limits on g A n g A p improve upon previous bounds by more than 15 orders of magnitude for the force range beyond 4.0 kilometers. Our technique opens an avenue for a wide range of terrestrial and space-based quantum sensors in studying macroscopic-scale fundamental physics.