The Negative Ion Of Positronium: Decay Rate Measurements And Prospects For Future Experiments

Consisting of two electrons and a positron (e(+)e(-)e(-)), the negative ion of positronium (Ps(-)) represents the simplest three-body system with a bound state. Its constituents are stable, point-like particles, and it is essentially free from perturbations by strong interaction effects. Together with the rather unique mass ratio, these properties make the positronium ion an interesting object for studying the quantum-mechanical three-body problem. Accordingly, there is a considerable number of theoretical publications on this exotic ion, but experimental data are very scarce. In this article, after giving a short overview of the theoretical results on Ps-, we review the measurements of its decay rate, and we discuss the prospects for further experiments now becoming possible at the new high-intensity positron source NEPOMUC at the FRM-II research reactor in Garching (Germany).