Population of Tetra-Neutron System Using RI Beams

Abstract

Introduction Multineutron systems have attracted long-standing attention in nuclear physics. The attractive force acting on two neutrons is slightly insufficient to create a bound state and causes a strong correlation just above the threshold. As the number of neutrons increases, the number of neutron pairs in the system also increases. If an attractive force acts on them, bound states or strongly correlated phenomena, such as resonances, may appear. Such correlated multineutron systems can be considered to affect the stabilities of neutron-rich nuclei and properties of neutron matters such as neutron stars through clustering and/or condensation phenomena. For several decades, experimental attempts have been made with a particular focus on the tetra-neutron (4n) system. In the previous century it was not clarified whether there is a bound or a resonant state [1–3]. Early this century, a possible signature of 4n cluster (or resonance) was reported in coincidence measurements of neutron(s) and fragments produced by the fragmentation of a neutronrich rare isotope (RI) beam, 14Be [4]. At the Rikagaku Kenkyūjo (RIKEN) RI Beam Factory, two experiments using neutron-rich 8He beams were performed to observe mass-spectra of the tetra-neutron system with almost recoil-less conditions and their results were published in 2016 [5] and 2022 [6]. Here, the essences of their basic idea, analysis, and results are described, as well as future perspectives.