Constraints on long-range spin-gravity and monopole-dipole couplings of the proton

Results of a search for a long-range monopole-dipole coupling between the mass of the Earth and rubidium (Rb) nuclear spins are reported. The experiment simultaneously measures the spin precession frequencies of overlapping ensembles of Rb85 and Rb87 atoms contained within an evacuated, antirelaxation-coated vapor cell. The nuclear structure of the Rb isotopes makes the experiment particularly sensitive to spin-dependent interactions of the proton. The spin-dependent component of the gravitational energy of the proton in the Earth’s field is found to be smaller than 3×10−18 eV, improving laboratory constraints on long-range monopole-dipole interactions by over 3 orders of magnitude.