The negative ion of positronium: measurement of the decay rate and prospects for further experiments

The negative positronium (Ps(-)) ion consisting of two electrons and a positron (e(+)e(-)e(-)), 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. We present a new determination of the decay rate of Ps(-), using a beam-foil method and a stripping-based detection technique. The measured value of Gamma = 2.089(15) ns(-1) is a factor of six times more precise than the previous experimental result, and there is excellent agreement both with the latter and with the theoretical value. With the new high-intensity positron source NEPOMUC at the FRM-II research reactor in Munich being available, a further improvement in precision seems possible. Moreover, the high flux of low-energy positrons at this facility brings other properties of this exotic system within reach of experiments. The prospects for such investigations are discussed.