Abstract
The field of experimental positronium physics has advanced significantly in the last few decades, with new areas of research driven by the development of techniques for trapping and manipulating positrons using Surko-type buffer gas traps. Large numbers of positrons (typically ≥106) accumulated in such a device may be ejected all at once, so as to generate an intense pulse. Standard bunching techniques can produce pulses with ns (mm) temporal (spatial) beam profiles. These pulses can be converted into a dilute Ps gas in vacuum with densities on the order of 107 cm−3 which can be probed by standard ns pulsed laser systems. This allows for the efficient production of excited Ps states, including long-lived Rydberg states, which in turn facilitates numerous experimental programs, such as precision optical and microwave spectroscopy of Ps, the application of Stark deceleration methods to guide, decelerate and focus Rydberg Ps beams, and studies of the interactions of such beams with other atomic and molecular species. These methods are also applicable to antihydrogen production and spectroscopic studies of energy levels and resonances in positronium ions and molecules. A summary of recent progress in this area will be given, with the objective of providing an overview of the field as it currently exists, and a brief discussion of some future directions.Graphical abstract
Highlights
The modern concept of antimatter was first revealed in the form of anti-electrons, predicted to exist by the relativistic quantum theory of Dirac [2,3,4,5]
Despite employing an intense pulsed positron beam, an efficient source of Ps atoms, and a high-power broadband pulsed laser, the signal observed in the Lawrence Livermore national laboratory (LLNL) experiments was extremely weak, primarily because of the detection method used; Ps annihilation was monitored using a system designed for TOF measurements that consisted of a collimated plastic scintillation detector whose field of view was offset from the target by about 1 cm so as to reduce background events from the target [364]
The hydrogen atom has long been the testing ground for quantum physics owing to its simplicity; as a one-electron atom H is extremely well described by the Dirac equation, and its properties have been calculated with sufficient accuracy that even the structure of the proton has to be considered [487]
Summary
The modern (cf. [1]) concept of antimatter was first revealed in the form of anti-electrons (positrons), predicted to exist by the relativistic quantum theory of Dirac [2,3,4,5]. Some examples include new measurements of the Ps ground state hyperfine interval that may solve the long-standing discrepancy between theory and experiment [81], the resolution of the (perhaps related) positronium “lifetime puzzle” [82,83], advances in positron scattering from atoms and molecules [84,85,86], antihydrogen research [87,88,89,90], positron beam [91,92,93] and trap [66,69] development, materials science [65,94], and surface physics [95] (including positron diffraction [96,97] and positron induced Auger emission [98,99]) These areas, and others, will not be discussed here
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