Limits on Spin-Mass Couplings within the Axion Window

We compare the relative precession frequencies of Hg and Cs magnetometers as a function of the position of two 475kg lead masses with respect to an applied magnetic field. Our observations limit the size of a possible monopole-dipole interaction expected to be produced by a pseudoscalar particle such as the axion. For a range of 20cm, the products of the scalar and pseudoscalar couplings (gsgp/ℎc) are found to be less than 2.3x10-29 and 3.6x10-29 for couplings to the electron and neutron spins, respectively. The e(n) limits are approximately 3 (4) orders of magnitude better than earlier experiments. copyright 1996 The American Physical Society