Positronium (Ps), a bound state of a positron (e+) and an electron (e+), can further bind to another electron to form the positronium negative ion, Ps-(e-e+e-). Since its constituents are solely point-like particles with the samemass, this system provides an excellent testing ground for the three-body problem in quantum mechanics. A laser spectroscopy study of Ps- has been performed, observing a strong shape resonance of 1Po symmetry near the Ps (n=2) formation threshold, in good agreement with the result of three-body calculations.
Nature Communications 7, 11060 (2016) doi:10.1038/ncomms11060.
