Search for an intermediate-range composition-dependent force coupling to N-Z

We report a search for composition dependence in the forces acting on two test masses made primarily of lead and copper, due to an attracting mass made primarily of lead. The test masses are mounted on a torsion balance of high symmetry. The attracting mass is a 320-kg lead ring in an aluminum shell, positioned so that the torsion balance lies on the ring's axis at a distance from its center approximately equal to √3/2 times the ring's mean radius. When the ring is moved periodically between symmetric positions on opposite sides of the balance, the resulting change in gravitational field experienced by the balance is spatially uniform to a very high degree: all derivatives of the change in field at the center of the balance vanish through third order. We find the apparent gravitational force per unit mass on the two test masses due to the attracting ring mass to be equal to within 1.1±1.2 ppm. Assuming a parametrization of a composition-dependent force in the notation of Fischbach, we find ξ=(5.7±6.3)×10^-5 for θ₅=90° (N-Z coupling), ξ=(-1.2±1.3)×10^-3 for θ₅=0° (B coupling).