The unfinished picture of low-energy antineutron interactions: open issues and hints for future research possibilities

Low-energy antineutron interactions provide a unique laboratory for investigating the strong interaction in the non-perturbative regime. Owing to the absence of Coulomb interactions and the well-defined isospin ( I = 1 ) of the n ̄ p system, antineutrons serve as particularly clean probes for investigating antinucleon annihilation dynamics. Most of the available measurements were performed more than two decades ago at CERN Low Energy Antiproton Ring (LEAR), yet leaving several fundamental questions unresolved. This review revisits the principal results obtained with low-energy antineutron beams and highlights the key open issues that emerged from these studies. These include discrepancies in antineutron–nucleus annihilation cross sections, the anomalous low-energy behavior of the n ̄ p elastic channel, the poorly understood isospin dependence of antinucleon annihilation, and inconsistencies among charge-exchange measurements. Open questions related to strangeness production, meson spectroscopy investigations, baryonium searches, and neutron–antineutron oscillation studies are also addressed. Finally, this review outlines how future low-energy antineutron beams, potentially accessible through upgrades of CERN Antiproton Decelerator (AD) complex, could deliver the precision measurements required to resolve these long-standing puzzles and significantly advance our understanding of low-energy antinucleon physics.