Atomic collisions using slow antiprotons
The beam of very low energy antiprotons which can be created
in the ASACUSA beam line offers a unique opportunity to
investigate the processes which lead to ionization and
excitation in collisions between charged particles and
atoms/molecules in general. This is because antiprotons do not
capture electrons, as positive particles do, and because they
have still ample energy to excite/ionize even in collisions
where their velocity is much slower than those of the target
electrons.
This means that we can create benchmark data with which we can
test atomic collision theory, especially for quasi-adiabatic
collisions. The basic model of the theory is simple: We regard
a few point-like, charged particles which interact via the
Coulomb force. The complication comes from the facts that we
have to treat a dynamically evolving system, and that the
atomic electrons are subject to strong correlation. The
resulting many-body problem is not yet solved to a
satisfactory accuracy: Often, total ionization cross sections
cannot be calculated to better than 30%.
We have measured total cross sections for single and multiple
ionization for a number of targets, especially helium and
molecular hydrogen, and much progress has been achieved. An
example is shown in the second figure. Here we show the total
single ionization cross section measured by our group at the
LEAR (black points) and the AD (blue points) accelerators,
compared with a few of the plethora of theoretical
calculations which have been published during the last decade.
Clearly, our data are able to distinguish between some of the
calculations, but it would be nice with a more selective
measurement. 
We are now trying to move forward in that direction, in that
we would like to be able to measure highly differential
ionization cross sections. For this, we propose to build a
-Y´Reaction Microscope¡ into the structure of the coming
Extra Low ENergy Accelerator ELENA, at CERN’s AD complex: In
the third figure, such a reaction microscope is shown in
principle, and its suggested position in the ELENA structure.
We should then be able to measure differential cross sections
like those shown in the fourth figure, which are calculated by
McGowern et al PRA 79 042707 (2009) for single ionization of
helium by 3 keV antiproton impact. Please note the two very
different results which, however, when integrated give
approximately the same total cross section. (Mesh: First Born,
red: Advanced model).