While all atomic nuclei except hydrogen are composed of protons and neutrons, physicists have been searching for a particle consisting of two, three, or four neutrons for over half a century. Experiments by a team of physicists of the Technical University of Munich (TUM) at the accelerator laboratory...now indicate that a particle comprising four bound neutrons may well exist. Should such particle exist, parts of the theory of the strong interaction would need to be rethought. In 2016, a group in Japan attempted to produce tetra-neutrons from helium-4 by bombarding it with a beam of radioactive helium-8 particles. This reaction...