  With the construction of the European Spallation Source, a remarkable
opportunity has emerged to conduct high sensitivity searches for neutron
oscillations, including a first search for thirty years for free neutrons
converting to antineutrons. Furthermore, searches can be made for transitions
of neutrons and antineutrons to sterile neutron states. Upgrades to the ESS
infrastructure allow an improved HIBEAM design that would provide an increase
in sensitivity by an order of magnitude compared to previous work.
  The HIBEAM program focuses on processes that violate baryon number by one or
two units. The observation of a process satisfying one of the Sakharov
conditions addresses the open question of the origin of the matter-antimatter
asymmetry in the Universe. Sterile neutron states would belong to a `dark'
sector of particles which may explain dark matter. As electrically neutral,
meta-stable objects that can be copiously produced and studied, neutrons
represent an attractive portal to a `dark' sector. The HIBEAM instrument can
also be utilised for other purposes such as direct searches for ultralight
axion dark matter.
  This paper describes the capability, design, infrastructure, and potential of
the HIBEAM program. This includes a dedicated beamline, neutron optical system,
magnetic shielding and control, and detectors for neutrons and antineutrons.
