Antimatter Atomic Physics

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Science  05 Nov 2010:
Vol. 330, Issue 6005, pp. 762-763
DOI: 10.1126/science.1197822

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Positronium (Ps), the bound state of an electron and its antiparticle, the positron, is the lightest neutral atomic species. The atomic nucleus is replaced by a positron that has only 1/1836 the mass of a proton. One way to characterize Ps atoms is to study how they scatter off other atoms and molecules, and it would be reasonable to expect Ps scattering to be some kind of coherent combination of electron and positron scattering. On page 789 of this issue, Brawley et al. (1) show experimentally that for impact energies up to 250 eV (2), Ps scatters as if it were just a free electron moving at the same speed. This result implies that the positron's interaction with the target is somehow “cloaked.” Whether each component of the total scattering, for example, the ionization contribution, is cloaked, or whether cancellation effects are at work, remains an outstanding and substantial theoretical challenge.