Abstract
The binding energies of antihydrogen atoms formed when antiprotons are mixed with positron plasmas having densities ranging from 1013–1015 m−3, and at temperatures of 5–30 K, have been investigated using simulations. Major changes in the distribution of binding energies are observed, with more strongly bound states evident at the higher densities, and at lower temperatures. For deeper binding, the distribution of binding energies follows a power-law which is found to be strongly dependent upon plasma properties and the strength of the applied magnetic field. The underpinning role of collisions in determining the binding energies is explored.
Highlights
Introduction tational studies ofHformation via the three-body reaction, The last two decades have witnessed remarkable progress in the development of experimental capabilities for the formation and study of neutral antimatter as antihydrogen, H, the antiproton–positron ( ̄p–e+) bound state
[18, 19]: recently with a study [20] involving simulations of the formation of Hat kinetic energies low enough to be held in the 0.54 K deep magnetic minimum trap used by the ALPHA experiment [21]
A key parameter is the Hbinding energy, denoted as EB, which plays a crucial role in determining whether the anti-atoms created in the e+ plasma survive to be detected and/or trapped
Summary
Comprehensive accounts of the methodology used in our simulations have been given elsewhere [18,19,20], such that we present a brief survey here with emphasis on the details most pertinent for the discussion of the Hbinding energies. Using guidance from [3√4], a crude estimate shows that Hwith EB lower than about 9 E K, with E in Vcm−1, corresponding to ∼28 K, will be ionised by the electric field Though this value for the electric field is somewhat of bound Hwith arbitrar√y, tests EB < 9 E K reveal that while the number changes with the magnitude of the applied field, the distribution of more deeply bound anti-atoms is not affected. Collisional effects play an important role in increasing EB as the Hmoves through the e+ plasma This is so as Te is lowered, where the anti-atom is initially formed in very high Rydberg-like states. In order to remove this problem, we (i) introduced a softening of the Coulomb potential and (ii) froze the internal state of any antihydrogen with binding energy EB > 300 K. Our results for binding energies above 300 K cannot be trusted, but this should, for all temperatures simulated, be safely above the so-called ‘bottleneck’ at a few kBT e3
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