Gravitational Effects Induced by Spin–Mass Interactions

It is well known that axions or axion-like particles can mediate spin-dependent interactions. If such interactions exist, they may violate the equivalence principle of general relativity, causing a polarized fermion with nonzero mass to experience different gravitational effects for different spin orientations. In this work, we derive the spin-dependent gravitational interaction generated by a spherically symmetric celestial body and apply the formalism to the Earth. We compare existing experimental searches for spin-dependent gravitational effects with the constraints implied by axion-mediated interactions. We further note that a rotating polarized neutron star may generate a time-dependent spin–mass interaction field acting on an unpolarized probe mass, which could be detected with high-frequency, high-Q torsion oscillators.