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 He-3-K comagnetometer located about 15 cm away. We place improved constraints on the products of scalar and pseudoscalar coupling constants, g(p)(n)g(s)(N) < 4.2 x 10(-30) and g(p)(e)g(s)(N) < 1.7 x 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.