  The violation of Baryon Number, $\mathcal{B}$, is an essential ingredient for
the preferential creation of matter over antimatter needed to account for the
observed baryon asymmetry in the universe. However, such a process has yet to
be experimentally observed. The HIBEAM/NNBAR %experiment program is a proposed
two-stage experiment at the European Spallation Source (ESS) to search for
baryon number violation. The program will include high-sensitivity searches for
processes that violate baryon number by one or two units: free
neutron-antineutron oscillation ($n\rightarrow \bar{n}$) via mixing,
neutron-antineutron oscillation via regeneration from a sterile neutron state
($n\rightarrow [n',\bar{n}'] \rightarrow \bar{n}$), and neutron disappearance
($n\rightarrow n'$); the effective $\Delta \mathcal{B}=0$ process of neutron
regeneration ($n\rightarrow [n',\bar{n}'] \rightarrow n$) is also possible. The
program can be used to discover and characterise mixing in the neutron,
antineutron, and sterile neutron sectors. The experiment addresses topical open
questions such as the origins of baryogenesis, the nature of dark matter, and
is sensitive to scales of new physics substantially in excess of those
available at colliders. A goal of the program is to open a discovery window to
neutron conversion probabilities (sensitivities) by up to three orders of
magnitude compared with previous searches. The opportunity to make such a leap
in sensitivity tests should not be squandered. The experiment pulls together a
diverse international team of physicists from the particle (collider and low
energy) and nuclear physics communities, while also including specialists in
neutronics and magnetics.
