\centerline{\bf HYDROGEN - ANTIHYDROGEN COLLISIONS}
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\centerline{P. Froelich $^\ast$, S. Jonsell $^\ast$ and A. Saenz
$^\sharp$}
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$^\ast$Department of Quantum Chemistry, Uppsala University, Box 518,
75120 Uppsala, Sweden \\
$^\sharp$Max-Planck Institute for Quantum Optics, Hans-Kopfermann-Str.
1,
D-85748  Garching,  Germany
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Matter - antimatter interaction is studied on the example of
hydrogen - antihydrogen collisions. Cross sections for
the rearrangement reaction resulting in formation of  (excited)
protonium
and positronium
in the final channel, according to
${\rm H} +  \bar{\rm H} \rightarrow p\bar{p} + e^+e^-$,
are calculated for the first time in the fully quantum mechanical
approach. Implications on the experiments intending  to trap and cool
the antihydrogen are discussed.