Further evidence for low-energy protonium production in vacuum

We describe an experiment performed in the ATHENA apparatus in which there is evidence
that the antiproton-proton bound state, protonium, has been produced at very low energies in vacuum
following the interaction of cold antiprotons with a trapped cloud of molecular hydrogen ions. The latter
were confined in a centrifugally separated belt outside a positron plasma used for antihydrogen formation.
Studies have been performed at low positron plasma temperatures in which the protonium annihilation
signal has been identified along with that from antihydrogen, and we discuss how their contributions can
be disentangled. With the positron plasma heated to around 10000K the ions become distributed in the
positrons, and the majority of the annihilation signal can be explained in terms of protonium formation, as
antihydrogen creation is heavily suppressed. In this case we compare the observed protonium formation rate
with expectations from theory and find reasonable accord, when experimental systematics are taken into
account. The effect on the annihilation signals of the passage of an electron current through a pre-loaded
positron plasma has been studied in detail, and the results are presented here for the first time.