Interactions between $\overline P$ and H at Intermediate Antiproton Energies

Abstract

Cross sections for direct excitation and ionisation in slow $\overline p + H$ collisions (at few keV lab),

$$\overline p + H(1s) \to \overline p + {H^*}(n\ell m)$$

(1)

$$\overline p + H(1s) \to \overline p + p + {e^ - },$$

(2)

are currently of particular interest in view of the recent proposal (1) to produce antihydrogen, $\overline H$, under laboratory conditions. It has been pointed out (2) that cross sections for formation of $\overline H$ via the capture of positron by antiproton $\overline p$ in collisions between $\overline p$ and positronium, Ps, are likely to be larger by several orders of magnitude than the cross sections for radiative capture to $\overline H$ in the $\overline p + Ps$ collisions. The Born cross sections for formation of $\overline H$ via positron capture in $\overline p + Ps$ collisions computed by Darewych (3) and semiclassical and classical calculations (4) of the same process point at.rather large (geometrical size) cross sections for the formation of $\overline H$ at low collision energies, thus giving additional support to the antihydrogen project. In these newly proposed experiments, the major source of background may be reactions between the trapped keV beam of antiprotons and the remnant hydrogen gas H₂ in the vacuum chamber. It is likely that this reaction can be estimated by considering a simpler process of interactions between antiproton and atomic hydrogen. For energies E > 100 keV lab (E is impact energy in the laboratory system where H is assumed to be at rest), cross sections for incident $\overline p$ are close to those for incident p. However this may not be the case for lower energies, hence the current interest in reactions (1) and (2) at E < 100 keV lab.