Very light particles with CP-violating couplings to ordinary matter, such as axions or axion-like particles, can mediate long-range forces between polarized and unpolarized fermions. We describe a new experimental search for such forces between unpolarized nucleons in two 250 kg Pb weights and polarized neutrons and electrons in a 3He-K co-magnetometer located about 15 cm away. We place improved constrains on the products of scalar and pseudoscalar coupling constants, gpngsN<4.2×10−30 and gpegsN<1.7×10−30 (95% CL) for axion-like particle masses less than 10−6 eV, which represents an order of magnitude improvement over the best previous neutron laboratory limit.
@article{arxiv.1801.02757,
title = {Improved Limits on Spin-Mass Interactions},
author = {Junyi Lee and Attaallah Almasi and Michael Romalis},
journal= {arXiv preprint arXiv:1801.02757},
year = {2018}
}