Limits on composition-dependent interactions using a laboratory source: is there a "fifth force" coupled to isospin?

Previous "fifth-force" experiments searched for differential acceleration of test bodies placed near terrestrial sources. Because these sources have N≈Z the results had little sensitivity to forces coupled to B-2L = N-Z. The authors searched for such forces by placing a 1 ton Pb source close to a Be/Al torsion balance. No evidence for an isospin-coupled force was found.