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 ${}^{85}\mathrm{Rb}$ and ${}^{87}\mathrm{Rb}$ 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 $3\times {10}^{-18}\mathrm{eV}$, improving laboratory constraints on long-range monopole-dipole interactions by over 3 orders of magnitude.

• Received 5 July 2017

DOI:https://doi.org/10.1103/PhysRevD.96.075004