  Searches for baryon number violation, including searches for proton decay and
neutron-antineutron transformation, are expected to play an important role in
the evolution of our understanding of beyond Standard Model physics. The
neutron-antineutron transformation is a key prediction of certain popular
theories of baryogenesis, and the experiments such as the Deep Underground
Neutrino Experiment and the European Spallation Source plan to search for this
process with bound- and free-neutron systems. Accurate simulation of this
process in Monte Carlo will be important for the proper reconstruction and
separation of these rare events from background. This article presents
developments towards accurate simulation of the annihilation process for use in
a cold, free neutron beam for neutron-antineutron searches from
antineutron-carbon annihilation, as carbon-12 is the target of choice for the
European Spallation Source's NNBar Collaboration. Initial efforts are also made
in this paper to perform analogous studies for intra-nuclear transformation
searches in argon-40 nuclei, although this latter work will be properly
addressed in our next publication.
