Limits on Anomalous Spin-Spin Couplings between Neutrons

Abstract

We report experimental limits on new spin-dependent macroscopic forces between neutrons. We measured the nuclear Zeeman frequencies of a ${}^3\mathrm{He}/{}^{129}\mathrm{Xe}$ maser while modulating the nuclear spin polarization of a nearby ${}^3\mathrm{He}$ 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 [$g_p{g}_p/(4\pi \hslash c)$] at the $3\times {10}^{-7}$ 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.