Unparticles as suggested by Georgi are identities that are not constrained by dispersion relations but are governed by their scaling dimension $d$. Their coupling to particles can result in macroscopic interactions between matter that are generally an inverse nonintegral power of distance. This is totally different from known macroscopic forces. We use the precisely measured long-ranged spin-spin interaction of electrons to constrain unparticle couplings to the electron. For $1<d<1.5$ the axial vector unparticle coupling is excluded, and for $1<d<1.3$ the pseudoscalar and vector couplings are also ruled out. These bounds and the ones for other ranges of $d$ exceed or are complementary to those obtained previously from exotic positronium decays.

• Received 16 June 2007

DOI:https://doi.org/10.1103/PhysRevLett.99.191804