Bulletin of the American Physical Society
39th Annual Meeting of the APS Division of Atomic, Molecular, and Optical Physics
Volume 53, Number 7
Tuesday–Saturday, May 27–31, 2008; State College, Pennsylvania
Session I2: Physics with Low-Energy Antimatter |
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Chair: Clifford Surko, University of California, San Diego Room: Kern Building 112 |
Thursday, May 29, 2008 8:00AM - 8:36AM |
I2.00001: Observation of Molecular Positronium, A Many-Positron Many-Electron System. Invited Speaker: The introduction of positron trapping techniques over the last twenty years or so has made possible a number of new experimental areas, and has revitalized the field of low energy positron physics. Positron plasmas have been used to create beams of unprecedented quality for precision atomic physics experiments, and have also been invaluable in the production of low energy antihydrogen. Another area in which these methods have proved to be useful is in studies of systems containing more than one positron. By capturing tens of millions of particles in an accumulator and then releasing them in a short burst it is possible to create instantaneous positron currents in excess of 10 mA. Implanting such bursts into an appropriate target can lead to the formation of positronium atoms that are able to interact with one another. An obvious outcome of such interactions is the formation of molecular positronium, which we have observed on both the internal surfaces of porous silica and on a clean metal surface. In this presentation I shall outline the techniques we have used to study interactions between positronium atoms, and in particular the first observation of molecular positronium. The experiments we have performed constitute the first step in a larger program to study multi-positronium interactions, specifically the formation of a Bose-Einstein condensate (BEC). With only minor modifications to our present system it should be possible to increase the density of interacting positronium atoms so that they may form a BEC with a critical temperature above 10 K. A condensate of this sort would provide a nearly ideal weakly interacting system of fundamental interest that could be used as the basis of a positronium ``atom laser''. This in turn would allow us to construct a Mach-Zender type interferometer and directly measure the matter-antimatter gravitational interaction. Since the CPT theorem implies that matter and antimatter should have been created in equal amounts following the big bang, that fact that the observable universe appears to consist almost entirely of matter remains an outstanding problem, one of literally astronomical proportions; any discovery of an unexpected asymmetry between matter and antimatter could help to resolve this mystery. [Preview Abstract] |
Thursday, May 29, 2008 8:36AM - 9:12AM |
I2.00002: Vibrational Feshbach Resonance Mechanism for Positron Annihilation on Molecules Invited Speaker: Thanks to a concerted experimental [1,2] and theoretical [3,4] effort, the hypothesis of the role of molecular vibrational Feshbach resonances in providing enhanced positron annihilation rates in molecules [5], has largely been confirmed and accepted. Recent experiments have provided a wealth of information on the two crucial aspects of the resonant annihilation mechanism, namely the positron-molecule binding, and its coupling to the vibrational degrees of freedom. In small molecules with simple vibrational spectra of infrared-active modes (e.g., methyl halides), resonant annihilation is described remarkably well by a theory [4] which contains only one free parameter, the positron binding energy. Application of this theory to other molecules highlights the role of overtones and combinations (methanol), as well as infrared-inactive vibrational excitations (acetylene, ethylene, etc.). In this talk I will review current theoretical understanding of positron-molecule resonant annihilation and discuss some outstanding questions for future research. \begin{enumerate}\setlength{\itemsep}{-3pt}\setlength{\itemindent}{-12pt} \item S. J. Gilbert, L. D. Barnes, J. P. Sullivan, and C. M. Surko, Phys. Rev. Lett. {\bf 88}, 043201 (2002); L. D. Barnes, S. J. Gilbert, and C. M. Surko, Phys. Rev. A {\bf 67}, 032706 (2003). \item L. D. Barnes, J. A. Young and C. M. Surko, Phys. Rev. A {\bf 74}, 012706 (2006). \item G. F. Gribakin and P. M. W. Gill, Nucl. Instrum. and Methods B {\bf 221}, 30 (2004). \item G. F. Gribakin and C. M. R. Lee, Phys. Rev. Lett. {\bf 97}, 193201 (2006). \item G. F. Gribakin, Phys. Rev. A {\bf 61}, 022720 (2000). \end{enumerate} [Preview Abstract] |
Thursday, May 29, 2008 9:12AM - 9:48AM |
I2.00003: Formation and Steps Toward Trapping of Antihydrogen - Results from ALPHA Invited Speaker: The ALPHA experiment is designed to trap antihydrogen atoms in a magnetic minimum trap configuration. Once trapped, the antihydrogen may be compared with hydrogen for precision tests of the CPT theorem. The antihydrogen is produced by merging plasmas of antiprotons and positrons in a cryogenic Penning trap co-located with the minimum-B trap. In this presentation I will describe the design and operation of the ALPHA apparatus and the most recent results from the ALPHA experiment including the first attempts at trapping antihydrogen. [Preview Abstract] |
Thursday, May 29, 2008 9:48AM - 10:24AM |
I2.00004: Antihydrogen Production within a Penning-Ioffe Trap (ATRAP) Invited Speaker: Slow antihydrogen atoms are produced in a Penning trap that is located within a quadrupole Ioffe trap. 5-MeV antiprotons provided by the CERN Antiproton Decelerator are slowed in a Be degrader and captured in the Penning trap where they are further cooled by collisions with cold trapped photoelectrons produced using a 20-mJ excimer laser pulse. Positrons from a Na-22 source are cooled with gas molecules and are trapped in a separate Penning trap and then transferred through a small aperture into the 1-T field of the main Penning trap where they are also cooled by electrons. Typically, 60 million positrons and 0.5 million antiprotons are collected within 15 minutes. Antihydrogen is formed as the positrons and antiprotons are mixed in a slowly-ramped nested well, and is detected by Stark-field ionization. The Ioffe trap, intended to ultimately confine extremely cold, ground-state H atoms, results in divergent magnetic fields, and we demonstrate that antihydrogen can be formed by combining its constituents in these fields. In fact, the number of detected antihydrogen atoms increases when the 400-mK Ioffe trap is turned on. This work is done by the ATRAP collaboration: G. Gabrielse (spokesperson), P. Larochelle, D. Le Sage, B. Levitt, W.S. Kolthammer, R. McConnell, P. Richerme, J. Wrubel, A. Speck, M.C. George, D. Grzonka, W. Oelert, T. Sefzick, Z. Zhang, A. Carew, D. Comeau, E.A. Hessels, C.H. Storry, M. Weel and J. Walz. [Preview Abstract] |
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