Search for Spin-Dependent Gravitational Interactions at Earth Range

Among the four fundamental forces, only gravity does not couple to particle spins according to the general theory of relativity. We test this principle by searching for an anomalous scalar coupling between the neutron spin and the Earth's gravity on the ground. We develop an atomic gas comagnetometer to measure the ratio of nuclear spin-precession frequencies between 129Xe and 131Xe, and search for a change of this ratio to the precision of 10-9 as the sensor is flipped in Earth's gravitational field. The null results of this search set an upper limit on the coupling energy between the neutron spin and the gravity on the ground at 5.3 x 10-22 eV (95% confidence level), resulting in a 17-fold improvement over the previous limit. The results can also be used to constrain several other anomalous interactions. In particular, the limit on the coupling strength of axion-mediated monopole-dipole interactions at the range of Earth's radius is improved by a factor of 17.

Automated summary

Among the four fundamental forces, only gravity does not couple to particle spins. This principle was tested by searching for an anomalous scalar coupling between the neutron spin and the Earth's gravity on the ground. The results set an upper limit on the coupling energy between the neutron spin and gravity at 5.3 x 10-22 eV (95% confidence level), improving the previous limit by a factor of 17. These results also have implications for constraining other anomalous interactions, such as axion-mediated monopole-dipole interactions.