Some aspects of positronium physics
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ome Aspects of Positronium Physics* S. N. Gninenkoa, N. V. Krasnikova, V. A. Matveeva, and A. Rubbiab a
Institute for Nuclear Research, Russian Academy of Sciences, Moscow, 117312 Russia b ETH Zürich, Zürich, Switzerland e-mail: [email protected], [email protected], [email protected], [email protected] Abstract—Some aspects of both theoretical and experimental study of the positronium system to probe physics beyond the Standard Model are reviewed. In particular, new experiments to search for the invisible decay of orthopositronium (o-Ps) with the sensitivity in the branching ratio Br(o-Ps invisible) 10–8–10–7 are discussed. The experimental technique involves a specially designed high-efficiency pulsed slow positron beam, which is also applicable for other experiments with o-Ps in vacuum. Details of the beam design, as well as the first measurements results are presented. Possible applications of the slow-pulsed positron beam for materials research are discussed. PACS number: 13.66.Hk DOI: 10.1134/S1063779606030038
1. INTRODUCTION Quantum electrodynamics (QED) is the textbook example of the success of quantum field theory. Many physical quantities (anomalous electron and muon magnetic moments; hyperfine splitting of hydrogen, muonium, and positronium; Lamb shift, etc.) have been calculated very precisely. The measurements have been characterized by excellent agreement with the theoretical predictions and in general have provided very clean conditions for hunting for small deviations from the standard theory. Positronium (Ps), the positron–electron bound state, is the lightest known atom; it is bounded and self-annihilates through the same electromagnetic interaction. At the current level of experimental and theoretical precision, this is the only interaction present in this system. This feature has made positronium an ideal system for testing the accuracy of QED calculations for bound states, in particular, for the triplet (13S1) state of Ps, orthopositronium (o-Ps). Due to the odd–parity under C-transformation, o-Ps decays predominantly into three photons. As compared with the singlet (11S0) state (parapositronium), the “slowness” of the o-Ps decay rate due to the phase space and additional α suppression factors, gives an enhancement factor 103, making it more sensitive to an admixture of new interactions which are not accommodated in the Standard Model. Positronium was discovered experimentally in 1951 by Deutsch [1], who observed its decay in different gases. Since then, a lot of focus has been set on the determination of its basic properties, including decay lifetime, decay modes, spectroscopy, etc. In particular,
the measurement of the o-Ps lifetime attracted much attention. Since 1989, the precision on the o-Ps lifetime reached a value well under 1000 ppm. Much excitement was aroused when the measurements performed by the Michigan group did not agree with theory. This problem, called the o-Ps lifetime puzzle, ignited much experimental and theoretical activity devoted to its clarification. This a
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