The European Spallation Source will open a new discovery window in particle physics. HIBEAM/NNBAR is a proposed two-stage program of experiments that can exploit this remarkable
opportunity. The program starts with a general-purpose magnetically controlled beamline (HIBEAM) which, among other topics, enables the first competitive search for over thirty years of
free neutrons converting to antineutrons, giving an order of magnitude increase in sensitivity,
and the world’s most sensitive laboratory-based search for ultralight axion-like particles coupling
to nucleons. The second stage (NNBAR) is a dedicated search for free neutrons converting to
anti-neutrons using a greater neutron flux and a longer beamline than at HIBEAM. NNBAR provides a sensitivity increase of three orders of magnitude compared to the last search. This paper
summarises the design and potential of the HIBEAM/NNBAR program.