Limits on Composition-Dependent Interactions Using a Laboratory Source: Is There a "Fifth Force" Coupled to Isospin?

Abstract

Previous "fifth-force" experiments searched for differential acceleration of test bodies placed near terrestrial sources. Because these sources have $N\ensuremath{\approx}Z$ the results had little sensitivity to forces coupled to $B\ensuremath{-}2L=N\ensuremath{-}Z$. We searched for such forces by placing a 1-ton Pb source close to a Be/Al torsion balance. Our null results rule out (at $2\ensuremath{\sigma}$) the possibility that all previous composition-dependence results could be due to a force coupling predominantly to $B\ensuremath{-}2L$ with a range $\ensuremath{\lambda}<1000$ m. We also set improved constraints on new interactions with $0.3\ensuremath{\le}\ensuremath{\lambda}\ensuremath{\le}10$ m.