Can Four Neutrons Tango?

Abstract

A candidate resonant tetraneutron state is found in the missing-mass spectrum obtained in the doublecharge-exchange reaction 4 Heð 8 He; 8 BeÞ at 186 MeV=u. The energy of the state is 0.83 AE 0.65ðstatÞ AE 1.25ðsystÞ MeV above the threshold of four-neutron decay with a significance level of 4.9σ. Utilizing the large positive Q value of the ð 8 He; 8 BeÞ reaction, an almost recoilless condition of the four-neutron system was achieved so as to obtain a weakly interacting four-neutron system efficiently. DOI: 10.1103/PhysRevLett.116.052501 Multineutron systems have attracted considerable attention in nuclear physics [1]. Few-body systems comprising chargeless nucleons such as the tetraneutron system have long fascinated nuclear physicists and have motivated investigations of pure neutron-neutron interactions possible for these systems. These systems are located beyond the neutron drip line and can serve as a unique test case for understanding nuclear structure at the limit of nuclear stability [2]. They are also closely related to the structure of neutron-rich hypernuclei [3]. Information on multineutron forces obtained in studies of multineutron systems is a critical input into theories of neutron stars [4,5]. Possible testing of universality in many fermion systems is another interesting application of these studies. The weakly coupled many-neutron systems can provide an opportunity to investigate the universality in systems governed by a strong interaction; the BCS-BEC crossover found in cold atoms is a good example [6,7].Several attempts have been made to find a bound tetraneutron system by using a uranium fission reaction [1,8], a pion double-charge-exchange (DCX) reaction 4 Heðπ − ; π þ Þ [9-11], and several transfer reactions [12,13]. The existence of the bound tetraneutron system was also discussed in theoretical studies [14][15][16]. Previously, no evidence was reported for the existence of either a bound tetraneutron system or resonant tetraneutron state. However, Marqués et al. [17] reported the possible existence of a bound tetraneutron observed in a breakup reaction of the 14 Be → 10 Be þ 4n channel. Following this experimental result, several theoretical studies were performed to confirm this result, but all results from these studies were negative [18][19][20].The possibility of the tetraneutron system forming a resonant state remains an open and fascinating question. Ab initio calculations incorporating realistic nuclear interactions [20] suggest that a broad resonance of a tetraneutron state may exist at 2 MeV above the threshold based on the extrapolation of the calculated energy with an external potential well. An investigation of a resonant state by solving the Faddeev-Yakubovsky equations with a complex energy plane shows that a resonant state can be generated only in the presence of a strong four-body force, which is incompatible with the current understanding of nuclear interactions [21]. A calculation assuming a compound system with coexisting 3 n þ n and 2 n þ 2 n coupled cluster configu...