Limits on Anomalous Spin-Spin Couplings between Neutrons

We report experimental limits on new spin-dependent macroscopic forces between neutrons. We measured the nuclear Zeeman frequencies of a ³He/¹²⁹Xe maser while modulating the nuclear spin polarization of a nearby ³He ensemble in a separate glass cell. We place limits on the coupling strength of neutron spin-spin interactions mediated by light pseudoscalar particles like the axion [gₚgₚ/(4πc)] at the 3×10^-7 level for interaction ranges longer than about 40 cm. This limit is about 10^-5 the size of the magnetic dipole-dipole interaction between neutrons.