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Very light particles with CP-violating couplings to ordinary matter, such as axions or axionlike 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 comagnetometer located about 15 cm away. We place improved constraints on the products of scalar and pseudoscalar coupling constants, g^ngₒ^N<4. 210^-30 and g^egₒ^N<1. 710^-30 (95% C. L. ) for axionlike particle masses less than 10^-6 eV, which represents an order of magnitude improvement over the best previous neutron laboratory limit.
Lee et al. (Tue,) studied this question.