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Journal of Physics Conference Series

The production of positronium and its resonant scattering

Simon Brawley, A. I. Williams, M. Shipman, Gaetana Laricchia

Published Nov 5, 2012• 0 citations
Publisher

Abstract

The total cross section of positronium scattering from a variety of atoms and molecules has been found to be unexpectedly close to that of a bare electron moving at the same velocity, despite Ps being neutral and having twice the mass [Science 330, 789 (2010)]. This behaviour appears to extend to velocities where effects such as the Ramsauer-Townsend minima and shape resonances occur for electrons. The latter has now been observed in collisions with CO 2 . Results are presented along with recent production efficiency measurements of Ps at high energies.

DOI
10.1088/1742-6596/388/7/072031

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The total cross section of positronium scattering from a variety of atoms and molecules has been found to be unexpectedly close to that of a bare electron moving at the same velocity, despite Ps being neutral and having twice the mass [Science 330, 789 (2010)]. This behaviour appears to extend to velocities where effects such as the Ramsauer-Townsend minima and shape resonances occur for electrons. The latter has now been observed in collisions with CO 2 . Results are presented along with recent production efficiency measurements of Ps at high energies.
A pseudopotential for positronium-atom interaction, based on electron-atom and positron-atom phase shifts, is constructed, and the phase shifts for Ps-Kr and Ps-Ar scattering are calculated. This approach allows us to extend the Ps-atom cross sections, obtained previously in the impulse approximation [I. I. Fabrikant and G. F. Gribakin, Phys. Rev. Lett. 112, 243201 (2014)], to energies below the Ps ionization threshold. Although experimental data are not available in this low-energy region, our results describe well the tendency of the measured cross sections to drop with decreasing velocity at v<1v<1v<1 a.u. Our results show that the effect of the Ps-atom van der Waals interaction is weak compared to the polarization interaction in electron-atom and positron-atom scattering. As a result, the Ps scattering length for both Ar and Kr is positive, and the Ramsauer-Townsend minimum is not observed for Ps scattering from these targets. This makes Ps scattering quite different from electron scattering in the low-energy region, in contrast to the intermediate energy range from the Ps ionization threshold up to $v\ensuremath{\sim}2$ a.u., where the two are similar.
We have continued our investigation into the elastic scattering of positronium atoms by hydrogen atoms, extending the work to include the spin-one (triplet) state of the two electrons. Once again we have extracted the SSS-wave phase shifts from normalizable trial functions by examining the spatial behavior of the center-of-mass wave functions. An effective-range expansion linear in the positronium energy fits the results very well and leads to a scattering length ${a}^{(\ensuremath{-})}\ensuremath{\simeq}2.36{a}_{0}$, compared with the static result ${a}^{(\ensuremath{-})}=2.476{a}_{0}$ of Hara and Fraser. The SSS-wave total and ortho-para conversion cross sections are calculated by making use of both 1S^{1}S1S and 3S^{3}S3S results. In the 3S^{3}S3S case, we see no indication of any resonances below the excitation threshold at 5.1 eV.
A three-state and a six-state close-coupling approximation have been employed to investigate the e − -Ps scattering below the first excitation threshold of the Ps atom. Present s - and p -wave phase shifts and total cross-sections are compared with the corresponding predictions of Ward et al. ( J. Phys. B , 20 (1987) 127). All the Feshbach resonances below the n = 2 threshold, including 3 S e and 3 P e resonances as predicted by Ho ( Phys. Lett. A , 120 (1984) 348 and Phys. Rev. A , 60 (1999) 2834), have been obtained.
Scattering of ortho positronium (Ps) by cesium and rubidium atoms has been investigated employing a three-Ps-state coupled-channel model with Ps(1s,2s,2p) states using a time-reversal-symmetric regularized electronexchange model potential. We find a narrow S-wave singlet resonance at 5.057 eV of width 0.003 eV in the Ps-Rb system and at 5.067 eV of width 0.003 eV in the Ps-Cs system. Singlet P-wave resonances in both systems are found at 5.3 eV of width 0.4 eV. Singlet D-wave structures are found at 5.4 eV in both systems. The pronounced P- and D-wave resonances in these systems lead to easily detectable local minima in the low-energy elastic cross sections. We also report results for elastic and Ps-excitation cross sections for Ps scattering by Rb and Cs. PACS Number(s): 34.10.+x, 36.10.DrRecent measurements of ortho positronium (Ps) scattering cross section by H2, N2, He, Ne, Ar, C4H10, and C5H12 [1–7] have initiated new theoretical investigations [8–14] in this subject. We suggested [15–17] a regularized, symmetric, nonlocal electron-exchange model
Lasers are used to choose the binding energies of highly excited positronium atoms (Ps*) created via charge exchange collisions. The lasers directly excite cesium (Cs) atoms to highly excited states (Cs*). These large Cs*atoms have a large cross section for resonant charge exchange collisions with trapped positrons. Highly excited Ps* is formed with a binding energy that is determined by the initial laser excitation. These Ps* should have a large cross section for resonant charge exchange collisions with trapped antiprotons—suggesting a possible new way to produce cold antihydrogen.