HIBEAM-NNBAR is a proposed two-stage program of experiments at the European Spallation Source (ESS) designed to search for neutrons converting into antineutrons and/or sterile neutrons. Such an observation would indicate baryon number violation, a fundamental condition for baryogenesis, the physical process that is hypothesized to have taken place during the early universe to produce baryonic asymmetry, i.e. the imbalance of matter (baryons) and antimatter (antibaryons) in the observed universe. In addition to shedding light on the baryon asymmetry of the universe, neutron conversions would provide the first falsification outside of the neutrino sector of the Standard Model (SM) of particle physics.

Taking advantage of the unique potential of the ESS, the high precision searches for neutron conversions to be performed by the HIBEAM-NNBAR collaboration will culminate in an ultimate sensitivity increase of three orders of magnitude beyond the previously attained limit obtained at the Institut Laue Langevin (ILL) in the early 1990s.