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 $^{ 85}$Rb and $^{ 87}$Rb 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 $3 \times 10^{ -18}~{ \rm eV}$, improving laboratory constraints on long-range monopole-dipole interactions by over three orders of magnitude.
Categories: physics.atom-ph nucl-ex quant-ph