Production of Positronium atoms, ions, and molecules

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Positronium is an atomic system made from an electron-positron pair, which has many similarities to hydrogen in terms of its atomic properties. Being composed of a particle-antiparticle pair makes the Ps system more complex than hydrogen, however, because the possibility of both self and virtual annihilation processes plays a large role in the fundamental energy level structure and in the practicalities of performing experiments. Nevertheless, recent advances in positron trapping technology have made it possible to produce high-density Ps gases in which Ps-Ps scattering is readily observed, and which can be probed with lasers. Experiments have succeeded in producing Ps2 molecules and exciting them with laser light, observing Ps-Ps scattering, and the subsequent spin polarization of a Ps gas. The shift of Ps energy levels caused by interactions with the internal surfaces of mesoscopic porous films has also been observed.With an increased beam density it will be possible to generate Ps2 molecules with higher efficiency, and therefore to study their properties in much greater detail. A more robust source of Ps2 molecules also allows for the production of both positive and negative Ps ions, which can be created using lasers to break up a Ps2 molecule. These ions and molecules are stable atomic systems (although they do self-annihilate) and can be studied optically. Because they are composed of three or four bodies of equal mass approximations like the Born-Oppenheimer approach (where electrons and nuclei are treated independently) cannot be used. Thus, Ps ions and molecules present an interesting challenge to theorists. They also possess unique properties that have so far not been widely studied experimentally. The availability of a source of cold Ps atoms is also a key step in several experimental endeavors, including high resolution spectroscopy and (anti)matter wave interferometry, which can be used to test bound state QED theory and search for new physics. Ps atoms are pure QED systems, as QED is a theory of light and leptons, and so they are especially sensitive to non-QED effects, such as unknown particles or forces. If they can be studied in sufficient detail these simplest of systems may reveal some of the best kept secrets in the universe.