Rare e-/e+/e- State.

The best study of the rare "atom" consisting of two electrons and one positron is being reported.

Positronium (abbreviated Ps) is a very "clean" two-body object: it consists of an electron and a positron which after about 150 nanoseconds annihilate each other. For studying the theory of quantum electrodynamics (QED), Ps is in some ways better even than the hydrogen atom: with pointlike constituents and with no complicating nuclear forces (the size of the proton and its own internal structure interject uncertainties into QED estimates of hydrogen behavior), Ps is a simpler, albeit fragile, quantum system.

An even more fragile "atom" is the tripartite object consisting of two electrons and one positron. Ps^-, as it is known, is less suitable for QED studies than Ps, but has the great virtue of being the simplest three-body system in physics. Again, it is simpler than H^-, H₂⁺, and helium because of its pointlike constituents and the absence of nuclear forces.

Ps^- is, like Ps, a bound state with discrete quantum energy states, although only the ground state is calculated to be stable against dissociation into Ps and a free electron. Very little is known about Ps^- beyond its lifetime.

Now, a new experiment carried out at the Max Planck Institute for Nuclear Physics in Heidelberg has measured the lifetime of Ps^- with a sixfold increase in precision (the new value is half a nanosecond). Ps^- is formed by shooting a positron beam into a thin carbon foil, and its size is actually a bit bigger than a hydrogen atom.

Fleischer et al., Physical Review Letters, upcoming article
Contact: Frank Fleischer, f.fleischer@mpi-hd.mpg.de
(+49) 6221-516-516