Unparticles as suggested recently by Georgi have surprising phenomenological implications, distinctive from any other new physics that we know of. But they must interact very feebly with ordinary matter to have avoided detection thus far. We determine how feebly they can interact with the electron, using the precisely measured quantities in QED: the electron $g-2$ and the bounds on invisible and exotic positronium decays. The most stringent bound comes from invisible orthopositronium decays: the effective energy scale entering the vector unparticle-electron interaction must exceed $4\times {10}^5\mathrm{TeV}$ for a scaling dimension $\frac{3}{2}$ of the vector unparticle. The lower bounds on scales for other unparticles range from a few tens to a few hundreds TeV. This makes the detection of unparticles challenging in low energy electron systems.

• Received 13 May 2007

DOI:https://doi.org/10.1103/PhysRevD.76.056006