A 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 far^2,3,4, 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 ⁸He beam was key.

核物理学中一个长期存在的问题是无电荷核系统是否可以存在。据我们所知，只有中子星代表近乎纯的中子系统，其中中子被引力挤压在一起，密度非常高。对孤立的多中子系统的实验性探索已经持续了几十年¹，特别关注被称为四中子的四中子系统，迄今为止只有少数迹象表明它存在^2,3,4，使四中子成为一个难以捉摸的核系统长达六年之久。在这里，我们报告了在四中子系统中对阈值附近的类共振结构的观察，该结构与存在很短时间的准束缚四中子状态一致。这种状态的测量能量和宽度为我们理解核力提供了一个关键基准。关键是使用基于大动量传递的敲除反应和放射性高能⁸ He 束的实验方法。