Simulation of the formation of antihydrogen

Detailed simulations of antihydrogen formation have been performed under the conditions of the ATHENA experiment, using several densities of the positron plasma in the range ne = 5 × 1013 m−3 to 1015 m−3. The simulations include only collisional effects, typically resulting in the formation of weakly bound antihydrogen via the three-body process, e+ + e+ + p → H + e+. (Radiative processes, which are much slower than collisional effects, are neglected.) The properties of these weakly bound anti-atoms are affected not only by further collisions in the plasma but also by the inherent electric fields. The role of field ionization in influencing the distribution of binding energies of the antihydrogen is clarified and the mechanism for this process in the strong B-field nested Penning trap used in the experiment is elucidated. The fate of antihydrogen is explained and the properties of the population detected after having reached the wall of the Penning trap electrodes, as well as those field ionized, are recorded. We find that the yield of detected antihydrogen varies with positron density roughly as n1.7 e , rather than the ne expected from the underlying formation process. As ne is increased, antihydrogen formation is sufficiently rapid that epithermal effects begin to play an important role. In general, the simulated timescales for antihydrogen formation are much shorter than those found from the experiment.