Volumes of the Series published from 1961 to 1973 are not officially numbered. The parenthetical numbers shown are designed to aid librarians and bibliographers to check the completeness of their holdings. Titles published in this series prior to 1987 appear under either the W. A. Benjamin or the Benjamin/Cummings imprint; titles published since 1986 appear under the Westview Press imprint.

We compare the relative precession frequencies of Hg and Cs magnetometers as a function of the position of two 475 kg 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 20 cm, the products of the scalar and pseudoscalar couplings ͑g s g p ͞hc͒ are found to be less than 2.3 3 10 229 and 3.6 3 10 229 for couplings to the electron and neutron spins, respectively. The e ͑n͒ limits are approximately 3 (4) orders of magnitude better than earlier experiments.[S0031-9007(96)01097-6] PACS numbers: 14.80. Mz, 32.80.Bx, 34.20.Cf Axions are well known as an elegant solution to the strong CP problem as well as an interesting darkmatter candidate [1]. One of the most remarkable and least explored predictions associated with the axion is that it would yield a parity and time-reversal violating, monopole-dipole coupling between spin and matter of thewhere V is the two-fermion interaction potential, sh͞2 is the fermion spin, r is the displacement vector between the mass and the spin, g p and g s are the coupling constants at the vertices of the polarized and unpolarized particles, respectively, and m p is the mass of the polarized particle. Experimental and astrophysical observations imply that the mass of the axion must lie between 1 meV and 1 meV, corresponding to a range, l, between 20 cm and 0.2 mm [3]. This range is commonly referred to as the "axion window." For ranges larger than about a meter, good experimental limits on the product g s g p are obtained from experiments that search for a coupling between either Be 1 or 199 Hg and the Earth [4,5]. A torsion-pendulum experiment explicitly establishes limits on the product g s g p in the axion window, but is only sensitive to terms involving electron spin [6]. As we shall see in the theoretical analysis, nuclear spin provides a more sensitive and more model-independent probe for possible axion couplings.To search for this axion coupling, we compare the precession frequencies of atomic 199 Hg and Cs when a large mass is positioned near the cells, relative to an applied magnetic field B. The monopole-dipole potential described in Eq. (1), when integrated over the mass distribution, produces a frequency splitting K between adjacent magnetic sublevels of the atomic ground state. The total atomic precession frequency (including the magnetic and monopole-dipole coupling) can then be written for each atom as f gB 6 K ϵ gB eff , where g Cs 350 kHz͞G and g Hg 759 Hz͞G are the atomic groundstate gyromagnetic ratios and the ͑1͒ or ͑2͒ sign depends on the relative position of the mass with respect to B. The difference between the effective magnetic fields, B eff , measured by the Cs and Hg oscillators, is

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The question as to whether an even or odd spin of the mediating field results in an attractive or repulsive force between like sources is examined within the framework of classical theory. It is shown that the assertion holds in a Lorentz-invariant theory constructed in analogy with electrodynamics provided that the current does not depend on any intrinsic direction of the source. Counterexamples are presented to show that the connection does not exist when the current does depend on the intrinsic direction.