Precision measurement of the decay rate of the negative positronium ion Ps-

The negative positronium ion Ps(-) is a bound system consisting of two electrons and a positron. Its three constituents are pointlike leptonic particles of equal mass, which are subject only to the electroweak and gravitational force. Hence, Ps(-) is an ideal object in which to study the quantum mechanics of a three-body system. The ground state of Ps(-) is stable against dissociation but unstable against annihilation into photons. We report here on a precise measurement of the Ps(-) ground-state decay rate Gamma, which was carried out at the high-intensity NEutron induced POsitron source MUniCh (NEPOMUC) at the research reactor FRM II in Garching. A value of Gamma = 2.0875(50) ns(-1) was obtained, which is three times more precise than previous experiments and in agreement with most recent theoretical predictions. The achieved experimental precision is at the level of the leading corrections in the theoretical predictions.