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 and , and search for a change of this ratio to the precision of 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 (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.
- Received 23 January 2023
- Revised 6 March 2023
- Accepted 15 March 2023
© 2023 American Physical Society
Atomic, Molecular & OpticalGravitation, Cosmology & AstrophysicsParticles & Fields