Limits on Anomalous Spin-Spin Interactions Using Noble-Gas Nuclear Magnetic Resonance

Numerous theories beyond the standard model of particle physics propose the existence of ultralight hypothetical bosons, which are prominent dark-matter candidates. One of the most promising approaches to search for these bosons is through precision measurements. The basic principle is to measure energy shifts of standard-model particles induced by hypothetical bosons with ultrahigh energy resolution. Here we develop a sensitive magnetometer based on noble-gas nuclear magnetic resonance, achieving an energy resolution of ∼10^{-23} eV. Using this magnetometer, we measure the energy shifts of nuclear spins induced by anomalous Z^{'} boson-mediated spin-spin interactions. The null results set the most stringent constraints on anomalous neutron-neutron and neutron-proton spin couplings in the Z^{'}-boson mass ranges of 6.0-157.4 and <129.6 μeV, respectively. Notably, our constraints surpass previous ones by up to 17 orders of magnitude in aforementioned mass ranges. Our experiments further set the most stringent constraints on neutron-proton spin couplings mediated by massless γ^{'} paraphotons, surpassing previous constraints by at least 25 orders of magnitude. Our technique paves the way for intriguing future directions in the search for various anomalous spin-spin interactions, including those mediated by axions with projected sensitivity surpassing astrophysical limits.