Laser Spectroscopy of Dense Positronium

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In this project Scientists at the University of California, Riverside (UCR) produce short bursts of millions of positronium atoms and cool them far below room temperature. The purpose is to prepare positronium ensembles suitable for making precision laser spectroscopy measurements and producing a Bose-Einstein condensed state. Positronium (chemical symbol Ps) is composed of an electron and a positron, the electron's antiparticle, bound together by the attraction of their opposite charges. Positronium is chemically like hydrogen but different because a positronium atom weighs only one-thousandth as much as a hydrogen atom and because the electron and positron in a positronium atom annihilate each other within about 100 nanoseconds. In previous work the UCR team has used dense bursts of positronium atoms at room temperature and above to measure Ps-Ps spin exchanging collision rates and to observe optical transitions between the ground and first excited states of Ps2 molecules. In the new work the team of UCR scientists and their students will use a new ps chirped pulse laser instrument developed under NSF MRI Award #1040590 to study the coherent excitation and deexcitation of Ps atoms in the adiabatic fast regime most familiar from NMR experiments. They will implement laser cooling of Ps in free space, using this technique to produce the rapid cooling needed for these short-lived atoms. In future experiments the methods developed in this project are expected to be used to produce and study the first Bose-Einstein condensate (BEC) formed from a dense gas of Ps atoms. The experiments of this project are of wide and compelling interest. Precision spectroscopy of purely electron-positron matter will give important information that can be compared with the essentially exact theory of quantum electrodynamics, which in turn will enable a unique search for new physics. The matter-antimatter BEC should exhibit fascinating quantum properties associated with collections of identical particles at comparatively high temperatures due to the light mass of the Ps atoms. The availability of cold Ps atoms emitted from a Ps BEC would be essential to possible measurements of the gravitational free-fall of Ps that would give us direct information about the nature of antimatter gravity for the first time. The possibility of a BEC producing stimulated electron-positron annihilation photons might someday be the basis for a gamma ray laser with important implications for defense and energy production. The project has a strong educational component since it will be partly carried out by graduate students who will receive high level training in a wide variety of leading edge techniques, in data analysis, and in the writing of scientific papers for high impact journals. The ongoing activities in the laboratory will form the basis for antimatter-based programs directed toward pre-college students and will be the centerpiece for laboratory tours and discussions for high school visitors and summer intern high school teachers.