Spin without Magnetism: Neutron Scattering in search of a Fifth Force*

Current experimental limits allow for unobserved, fundamental forces of nature many times stronger than gravity at sub-millimeter length scales. Such forces arise in unified theories, and could be mediated by candidate particles for dark matter and dark energy. These forces can couple to fundamental properties such as mass and spin. Constraints on spin-dependent interactions are less stringent given the difficulty of producing polarized test masses in the absence of magnetic fields which can lead to backgrounds, and constraints on momentum-dependent interactions are even weaker. We describe an experiment designed to search for exotic spin and momentum-dependent interactions between neutrons and electrons. The experiment uses polarized neutron spin rotation, which has emerged as a powerful technique to probe fundamental interactions down to the micron range. The polarized scattering target consists of a rare-earth iron garnet that exhibits orbital compensation of the magnetism associated with the electron spins. The compensation is expected to be greatest near temperatures of 250 K. We describe the target fabrication and cooling apparatus in detail, and discuss the projected sensitivity of the experiment.