Bulletin of the American Physical Society
48th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 62, Number 8
Monday–Friday, June 5–9, 2017; Sacramento, California
Session N5: Antimatter Collisions at Low EnergiesInvited
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Sponsoring Units: DAMOP GEC Chair: Thomas M. Miller, Boston College Room: 310 |
Thursday, June 8, 2017 10:30AM - 11:00AM |
N5.00001: Physics with Trapped Antihydrogen Invited Speaker: Michael Charlton For more than a decade antihydrogen atoms have been formed by mixing antiprotons and positrons held in arrangements of charged particle (Penning) traps [1,2]. More recently, magnetic minimum neutral atom traps have been superimposed upon the anti-atom production region, promoting the trapping of a small quantity of the antihydrogen yield [3-5]. We will review these advances, and describe some of the first physics experiments performed on anrtihydrogen including the observation of the two-photon 1S-2S transition [6], invesigation of the charge neutrailty of the anti-atom [7,8] and studies of the ground state hyperfine splitting [9]. We will discuss the physics motivations for undertaking these experiments and describe some near-future initiatives. 1. M. Amoretti \textit{et al}. (ATHENA Collaboration), Nature \textbf{419} (2002) 456 2. G. Gabrielse \textit{et al.} (ATRAP Collaboration), Phys. Rev. Lett. \textbf{89} (2002) 213401 3. G.B. Andresen \textit{et al}. (ALPHA Collaboration), Nature \textbf{468} (2010) 673 4. G.B. Andresen \textit{et al}. (ALPHA Collaboration), Nature Phys. \textbf{7} (2011) 558 5. G. Gabrielse \textit{et al.} (ATRAP Collaboration), Phys. Rev. Lett. \textbf{108} (2012) 113002 6. M. Ahmadi \textit{et al}. (ALPHA Collaboration), Nature \textbf{541} (2017) 506 7. C. Amole \textit{et al}. (ALPHA Collaboration), Nature Commun. \textbf{5} (2014) 3955 8. M. Ahmadi \textit{et al}. (ALPHA Collaboration), Nature \textbf{529} (2016) 373 9. C. Amole \textit{et al}. (ALPHA Collaboration) Nature \textbf{483} (2012) 439 [Preview Abstract] |
Thursday, June 8, 2017 11:00AM - 11:30AM |
N5.00002: Positronium collisions with atoms, protons, and antiprotons Invited Speaker: Ilya Fabrikant Recently observed similarities between positronium (Ps) scattering and electron scattering from several atoms and molecules [1] in the intermediate energy range were explained [2,3] by the dominance of the electron exchange interaction with the target atom or molecule. An explicit proof of this equivalence was given using the framework of the impulse approximation [2], valid above the Ps ionization threshold. For lower collision energies a pseudodopotential method [3] was developed. It was successfully applied to the calculation of Ps scattering from heavy rare gas atoms, and gave results in good agreement with those of the beam experiments [1]. The same method was applied to Ps collisions with molecular hydrogen [4]. In general we observe the similarity between electron and Ps scattering at energies above the Ps ionization threshold. However, below the threshold the two sets of cross sections are different because of the different nature of the long-range interaction between the projectile and the target, the polarization interaction in the case of electron collisions and the van der Waals interaction in the case of Ps collisions. In particular the Ramsauer-Townsend minimum is not seen in theoretical cross sections for the heavy rare gas atoms. The second part of this talk will summarize recent results on the threshold behavior of Ps collisions with protons and antiprotons [5]. Partial cross sections for elastic and quasielastic scattering exhibit oscillations as functions of $\ln E$ where $E$ is the Ps energy. The quantum-mechanical threshold behavior of hydrogen and antihydrogen formation show features which make them different from results of classical trajectory Monte Carlo simulations. $^1$ S. J. Brawley, S. Armitage, J. Beale, D. E. Leslie, A. I. Williams, and G. Laricchia, Science {\bf 330}, 789 (2010). $^2$ I. I. Fabrikant and G. F. Gribakin, Phys. Rev. Lett. {\bf 112}, 243201 (2014). $^3$ I. I. Fabrikant and G. F. Gribakin, Phys. Rev. A {\bf 90}, 052717 (2014). $^4$ R. S. Wilde and I. I. Fabrikant, Phys. Rev. A {\bf 92}, 032708 (2015). $^5$ I. I. Fabrikant, A. W. Bray, A. S. Kadyrov, and I. Bray, Phys. Rev. A {\bf 94}, 012701 (2016). [Preview Abstract] |
Thursday, June 8, 2017 11:30AM - 12:00PM |
N5.00003: Resonances in Positron Annihilation on Molecules -- Which Bells Ring? Invited Speaker: Clifford M. Surko Positron collisions with molecules can result in the excitation of high-Q vibrational Feshbach resonances -- temporary positron-molecule bound states that exhibit greatly enhanced annihilation rates.\footnote{G. F. Gribakin, J. A. Young, and C. M. Surko, {\it Rev. Mod. Phys.} {\bf 82}, 2557 (2010).} A simple theory agrees well with data for annihilation spectra as a function of incident positron energy for selected molecules, such as methyl halides, in which infrared-active vibrations dipole-couple the incident positron to the bound state.\footnote{G. F. Gribakin and C. M. R. Lee, {\it Phys. Rev. Lett.} {\bf 97}, 193201 (2006).} However additional effects appear to be prominent in most molecules, including the excitation of combination and overtone modes.\footnote{A. C. L. Jones, J. R. Danielson, M. R. Natisin, C. M. Surko, and G. F. Gribakin, {\it Phys. Rev. Lett.} {\bf 108}, (2012).} Until now, limited energy resolution has inhibited the study of these effects. A recently developed, high-energy-resolution, cryogenic trap-based beam is used to investigate two other ways to ``ring the molecule's bells'': positron coupling to infrared-inactive modes and the excitation of combination modes. The operation of the new beam system will be briefly described,\footnote{M. R. Natisin, J. R. Danielson, and C. M. Surko, {\it Appl. Phys. Lett.} {\bf 108}, 024102 (2016).} followed by a discussion of high-resolution data for molecules that provide evidence of resonant annihilation due to infrared {\it inactive} modes. Data exploring the possible excitation of combination modes will also be discussed and related to broad and featureless regions of the annihilation spectra observed in many molecules.$^{3,5}$ [Preview Abstract] |
Thursday, June 8, 2017 12:00PM - 12:30PM |
N5.00004: Low Energy Positron Scattering, Transport, and Applications Invited Speaker: Stephen Buckman Relatively intense, high energy-resolution beams of low-energy positrons are now available through the use of buffer-gas (Surko) traps. These have led to measurements of interaction cross sections for a broad range of atoms and molecules, including molecules of biological interest. The increased energy resolution, and experimental techniques developed for scattering in strong magnetic fields has also enabled highly accurate measurements of discrete excitation processes such as electronic and vibrational excitation, positronium formation and ionization in a range of atomic and molecular species. This talk will review some of these measurements and discuss their application in new and sophisticated models of positron transport which aim, for example, to provide a better understanding of the atomic and molecular processes which occur when positrons are emitted in the body during a Positron Emission Tomography scan. This work is part of a broad collaboration between the ANU (James Sullivan, Joshua Machacek), Flinders University (Michael Brunger), James Cook University (Ronald White and co-workers) CSIC Madrid (Gustavo Garcia) and the Institute of Physics, Belgrade (Zoran Petrovic and colleagues). [Preview Abstract] |
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