Improved Limits on Spin-Mass Interactions

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 He3-K comagnetometer located about 15 cm away. We place improved constraints on the products of scalar and pseudoscalar coupling constants, gpngsN<4.2×10-30 and gpegsN<1.7×10-30 (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.