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arXiv:nucl-th/9705007 · Physical Review A

Low-energy antiproton interaction with helium

William R Gibbs

Published Nov 1, 1997• 8 citations
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Abstract

An ab initio potential for the interaction of the neutral helium atom with antiprotons and protons is calculated using the Born-Oppenheimer approximation. Using this potential, the annihilation cross section for antiprotons in the energy range 0.01 $\ensuremath{\mu}$eV to 1 eV is calculated.

DOI
10.1103/physreva.56.3553

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An ab initio potential for the interaction of the neutral helium atom with antiprotons and protons is calculated using the Born-Oppenheimer approximation. Using this potential, the annihilation cross section for antiprotons in the energy range 0.01 \ensuremathμ\ensuremath{\mu}\ensuremathμeV to 1 eV is calculated.
We calculate cross sections for helium–antihydrogen scattering for energies up to 0.01 atomic unit. Our calculation includes elastic scattering, direct antiproton–alpha particle annihilation and rearrangement into and ground-state positronium. Elastic scattering is calculated within the Born–Oppenheimer approximation using the potential calculated by Strasburger et al (2005 J. Phys. B: At. Mol. Opt. Phys. 38 3091). Matrix elements for rearrangement are calculated using the T -matrix in the distorted wave approximation, with the initial state represented by Hylleraas-type functions. The strong force, leading to direct annihilation, was included as a short-range boundary condition in terms of the strong-force scattering length.
A relationship between the low-energy antiproton-atom scattering parameters and Coulomb level shifts is derived. The corresponding scattering lengths are calculated for the p\ifmmode\bar\else\textasciimacron\fi{}-He+{\mathrm{He}}^{+}He+ system on the basis of energy level shifts evaluated earlier by means of the Born-Oppenheimer approximation.
We calculate cross sections for elastic scattering and annihilation in antihydrogen-helium collisions at low energies. The calculations are based on the Born-Oppenheimer approximation, and incorporate the effects of the strong interaction through a scattering length approach. We find that the strong nuclear force not only causes significant annihilation, but also cannot be neglected in the elastic channel. In the zero energy limit we obtain the scattering length $a=\ensuremath{-}7.69\ensuremath{-}3.80i\phantom{\rule{0.3em}{0ex}}\mathrm{a.u.}$ for ground state antihydrogen-helium collisions. Annihilation is found to dominate over elastic scattering up to a temperature about 3K3\phantom{\rule{0.3em}{0ex}}\mathrm{K}3K. Loosely bound metastable antihydrogen-helium states are also investigated, and it is found that a number of relatively long-lived states with up to three units of angular momentum exist.
The stability of antimatter in contact with matter has been investigated. The interaction between hydrogen and antihydrogen is considered as the prototype reaction. We have focused interest on the rates for proton-antiproton and/or electron-positron annihilation during hydrogen-antihydrogen collisions at low energies. In particular, we have concentrated on the calculation of the rates for the rearrangement reaction leading to formation of protonium and positronium, ending inevitably in particle-antiparticle annihilation. The cross section for the rearrangement collision has been calculated in a fully quantum-mechanical treatment. Additionally, we have calculated cross sections for direct annihilation during the collision process, which was found to be comparable to the rearrangement cross section. The elastic cross section and its low-energy limit, given by the scattering length, have been calculated, allowing, by comparison to the inelastic processes, a prediction for the efficiency of cooling antihydrogen via collisions with ultracold hydrogen atoms.
The capture of slow antiprotons (energies less than 1.0 au, i.e. roughly 27 eV) by the rare gas atoms helium, neon and argon is considered. Appropriate to this low velocity, the capture cross sections are calculated using the adiabatic `hidden-crossing' theory in which the collision complex is viewed as a transient diatomic molecule with the positively-charged atomic ion and antiproton as nuclei. In addition to the total capture cross section, estimates of the percentage population of long-lived `circular' states is given. Our calculations suggest that a few per cent of captured antiprotons occupy these states in helium but in the case of argon or neon the probability of primary capture into such long-lived states is negligible. These results are at variance with previous calculations of antiproton capture cross sections.
Accurate quantum-mechanical calculations on the system hydrogen-antihydrogen (HH‾\mathrm{H}\overline{\mathrm{H}}HH) are presented. The energy is calculated using the Ritz variational principle, and is obtained as a function of the proton-antiproton distance in the Born-Oppenheimer approximation. It is everywhere lower and therefore more accurate than that recently calculated by Junker and Bardsley. In particular, the maximum in the interatomic interaction reported by these authors is not seen in this calculation. The cross section for rearrangement annihilation is calculated and found to be uniformly larger than that reported by Junker and Bardsley.