Cortina Gil, María DoloresDuer, M.Zhukov, M.V.Fernández Domínguez, Beatriz2024-02-062024-02-062022-06-22Duer, M., Aumann, T., Gernhäuser, R. et al. Observation of a correlated free four-neutron system. Nature 606, 678–682 (2022). https://doi.org/10.1038/s41586-022-04827-60028-0836http://hdl.handle.net/10347/32386A long-standing question in nuclear physics is whether chargeless nuclear systems can exist. To our knowledge, only neutron stars represent near-pure neutron systems, where neutrons are squeezed together by the gravitational force to very high densities. The experimental search for isolated multi-neutron systems has been an ongoing quest for several decades1, with a particular focus on the four-neutron system called the tetraneutron, resulting in only a few indications of its existence so far2–4, leaving the tetraneutron an elusive nuclear system for six decades. Here we report on the observation of a resonance-like structure near threshold in the four-neutron system that is consistent with a quasi-bound tetraneutron state existing for a very short time. The measured energy and width of this state provide a key benchmark for our understanding of the nuclear force. The use of an experimental approach based on a knockout reaction at large momentum transfer with a radioactive high-energy 8 He beam was key.engThis article is licensed under a Creative Commons Attribution 4.0 International License.http://creativecommons.org/licenses/by-nc-nd/4.0/Neutron systemNuclear physicsRadioactive high-energyjournal article10.1038/s41586-022-04827-61476-4687open access