Bulletin of the American Physical Society
42nd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 56, Number 5
Monday–Friday, June 13–17, 2011; Atlanta, Georgia
Session C1: Positron-matter Interactions and Antihydrogen |
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Chair: Robert McConnell, Harvard University Room: A601 |
Tuesday, June 14, 2011 2:00PM - 2:30PM |
C1.00001: Antihydrogen Trapped Invited Speaker: In 2010 the ALPHA collaboration succeeded in trapping antihydrogen atoms for the first time.\footnote{``Trapped antihydrogen,'' G.B. Andresen \textit{et al}., \textit{Nature} 468, 673 (2010)} Stored antihydrogen promises to be a unique tool for making high precision measurements of the structure of this first anti-atom. Achieving this milestone presented several substantial experimental challenges and this talk will describe how they were overcome. The unique design features of the ALPHA apparatus will be explained.\footnote{``A Magnetic Trap for Antihydrogen Confinement,'' W. Bertsche \textit{et al}., \textit{Nucl. Instr. Meth. Phys. Res. A} 566, 746 (2006)} These allow a high intensity positron source and an antiproton imaging detector similar to the one used in the ATHENA\footnote{``Production and detection of cold antihydrogen atoms,'' M.Amoretti \textit{et al}., \textit{Nature} 419, 456 (2002).} experiment to be combined with an innovative magnet design of the anti-atom trap. This seeks to minimise the perturbations to trapped charged particles which may cause particle loss and heating.\footnote{``Antihydrogen formation dynamics in a multipolar neutral anti-atom trap.'' G.B. Andresen \textit{et al., Phys. Lett. B} 685, 141 (2010)} The diagnostic techniques used to measure the diameter, number, density, and temperatures of both plasmas will be presented as will the methods developed to actively compress and cool of both plasma species to sizes and temperatures\footnote{``Evaporative Cooling of Antiprotons to Cryogenic Temperatures,'' G.B. Andresen \textit{et al}. \textit{Phys. Rev. Lett} 105, 013003 (2010)}$^,$\footnote{``Compression of Antiproton Clouds for Antihydrogen Trapping,'' G. B. Andresen \textit{et al}. \textit{Phys. Rev. Lett} 100, 203401 (2008)}$^,$\footnote{``Autoresonant Excitation of Antiproton Plasmas,'' G.B. Andresen \textit{et al}., \textit{Phys. Rev. Lett.} 106, 025002 (2011)} where trapping attempts with a reasonable chance of success can be tried. The results of the successful trapping experiments will be outlined as well as some subsequent experiments to improve the trapping rate and storage time. [Preview Abstract] |
Tuesday, June 14, 2011 2:30PM - 3:00PM |
C1.00002: Synthesis of cold antihyrogen in a cusp trap Invited Speaker: We report here the first successful synthesis of cold antihydrogen atoms employing a cusp trap[1], which is not to trap but to extract an intensified antihydrogen beam in a field-free region for high precision microwave spectroscopy. This success opens a new path to make a stringent test of the CPT symmetry via observation of ground-state hyperfine transitions of antihydrogen atoms. The cusp trap consists of superconducting anti-Helmholtz coils and a stack of multiple ring electrodes, which provides non-uniform but axially symmetric magnetic and electric fields. Because of this axial symmetry, a large number of antiprotons and positrons are stably stored in the trap. At the same time, antihydrogen atoms in low-field-seeking states synthesized in the cusp trap can be selectively and effectively extracted along the trap axis. \\[4pt] [1] Y. Enomoto et al., Phys.Rev.Lett.105, 243401 (2010) [Preview Abstract] |
Tuesday, June 14, 2011 3:00PM - 3:30PM |
C1.00003: Atomic Physics with Positronium Invited Speaker: Positronium, the metastable hydrogen-like bound state between an electron and its antiparticle, the positron, is a leptonic atomic system whose properties may be studied using laser spectroscopy in much the same way as for any other atomic system. However, such measurements are complicated by the difficulties associated with producing these short-lived atoms in sufficient quantities. The introduction of positron trapping techniques [1] has made it possible to produce intense bursts of slow positrons with spatiotemporal densities approaching $\sim $ 10$^{20}$ e$^{+}$cm$^{-2}$s$^{-1}$ [2]. By implanting these positrons into various materials we may produce short bursts of positronium atoms that are well suited to pulsed laser spectroscopy, and that we have used to perform a variety of laser-Ps experiments [3] as well as measurements of Ps-Ps interactions [4]. In this presentation I shall outline the techniques we have used to do so, and describe how this work fits into our long-term goal of producing a Bose-Einstein condensate of positronium [5]. A condensate of this sort would provide a nearly ideal weakly interacting system of fundamental interest that could be used for precision spectroscopy, and may one day form the basis of a positronium annihilation gamma ray laser [6]. \\[4pt] [1] C. M. Surko and R. G. Greaves, Phys. Plasmas \textbf{11}, 2333 (2004).\\[0pt] [2] D. B. Cassidy, S. H. M. Deng, R. G. Greaves and A. P. Mills Jr., Rev. Sci. Instrum. \textbf{77}, 073106 (2006).\\[0pt] [3] D.B. Cassidy, \textit{et al}., Phys. Rev. A \textbf{81}, 012715 (2010); D. B. Cassidy \textit{et al.}, Phys. Rev. Lett. \textbf{106}, 023401 (2011).\\[0pt] [4] D. B. Cassidy and A. P. Mills Jr, Nature \textbf{449}, 195 (2007); D. B. Cassidy and A. P. Mills, Jr, Phys. Rev. Lett, \textbf{100} 013401 (2008); D. B. Cassidy, V. E. Meligne, and A. P. Mills, Jr., Phys. Rev. Lett. \textbf{104}, 173401 (2010).\\[0pt] [5] P.M. Platzman and A.P. Mills, Jr., Phys. Rev. B \textbf{49}, 454 (1994).\\[0pt] [6] E.P. Liang and C. D. Dermer, Opt. Commun. \textbf{65}, 419 (1988). [Preview Abstract] |
Tuesday, June 14, 2011 3:30PM - 4:00PM |
C1.00004: Positron binding to molecules Invited Speaker: While there is theoretical evidence that positrons can bind to atoms,\footnote{Mitroy, {\it et. al.}, {J. Phys. B} \textbf{35}, R81 (2002).} calculations for molecules are much less precise.\footnote{Strasburger, {J. Chem. Phys.} \textbf{114}, 615 (2001).} Unfortunately, there have been no measurements of positron-atom binding, due primarily to the difficulty in forming positron-atom bound states in two-body collisions. In contrast, positrons attach to molecules via Feshbach resonances (VFR) in which a vibrational mode absorbs the excess energy. Using a high-resolution positron beam, this VFR process has been studied to measure binding energies for more than 40 molecules. New measurements will be described in two areas: positron binding to relatively simple molecules, for which theoretical calculations appear to be possible;\footnote{Danielson, {\it et. al.}, {Phys.~Rev.~Lett.}, \textbf{104}, 233201 (2010).} and positron binding to molecules with large permanent dipole moments, which can be compared to analogous, weakly bound electron-molecule (negative-ion) states. Binding energies range from 75 meV for CS$_2$ (no dipole moment) to 180 meV for acetonitrile (CH$_3$CN). Other species studied include aldehydes and ketones, which have permanent dipole moments in the range 2.5 - 3.0 debye. The measured binding energies are surprisingly large (by a factor of 10 to 100) compared to those for the analogous negative ions,\footnote{Hammer, {\it et. al.}, J. Chem. Phys. 119, 3650 (2003).} and these differences will be discussed. New theoretical calculations for positron-molecule binding are in progress, and a recent result for acetonitrile will be discussed.\footnote{Tachikawa, {\it et. al.}, Phys. Chem. Chem. Phys. {\bf 13}, 2701 (2011).} This ability to compare theory and experiment represents a significant step in attempts to understand positron binding to matter. [Preview Abstract] |
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