Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 56 (2023)
RIKEN
Accel. Prog. Rep. 56 (2023)

Observation of a correlated free four-neutron system†
M. Duer,∗1 T. Aumann,∗1,∗2,∗3 R. Gernhäuser,∗4 V. Panin,∗2,∗5 S. Paschalis,∗1,∗6 D. M. Rossi,∗1 N. L. Achouri,∗7D. Ahn,∗5 H. Baba,∗5 C. A. Bertulani,∗8 M. Böhmer,∗4 K. Boretzky,∗2 C. Caesar,∗1,∗2,∗5 N. Chiga,∗5 A. Corsi,∗9D. Cortina-Gil,∗10 C. A. Douma,∗11 F. Dufter,∗4 Z. Elekes,∗12 J. Feng,∗13 B. Fernández-Domı́nguez,∗10U. Forsberg,∗6 N. Fukuda,∗5 I. Gasparic,∗1,∗5,∗14 Z. Ge,∗5 J. M. Gheller,∗9 J. Gibelin,∗7 A. Gillibert,∗9K. I. Hahn,∗15,∗16 Z. Halász,∗12 M. N. Harakeh,∗11 A. Hirayama,∗17 M. Holl,∗1 N. Inabe,∗5 T. Isobe,∗5J. Kahlbow,∗1 N. Kalantar-Nayestanaki,∗11 D. Kim,∗16 S. Kim,∗1,∗16 T. Kobayashi,∗18 Y. Kondo,∗17 D. Körper,∗2P. Koseoglou,∗1 Y. Kubota,∗5 I. Kuti,∗12 P. J. Li,∗19 C. Lehr,∗1 S. Lindberg,∗20 Y. Liu,∗13 F. M. Marqués,∗7S. Masuoka,∗21 S. Masuoka,∗21 M. Matsumoto,∗17 J. Mayer,∗22 K. Miki,∗1,∗18 B. Monteagudo,∗7 T. Nakamura,∗17T. Nilsson,∗20 A. Obertelli,∗1,∗9 N. A. Orr,∗7 H. Otsu,∗5 S. Y. Park,∗15,∗16 M. Parlog,∗7 P. M. Potlog,∗23
S. Reichert,∗4 A. Revel,∗7,∗9,∗24 A. T. Saito,∗17 M. Sasano,∗5 H. Scheit,∗1 F. Schindler,∗1 S. Shimoura,∗21H. Simon,∗2 L. Stuhl,∗16,∗21 H. Suzuki,∗5 D. Symochko,∗1 H. Takeda,∗5 J. Tanaka,∗1,∗5 Y. Togano,∗17 T. Tomai,∗17H. T. Törnqvist,∗1,∗2 J. Tscheuschner,∗1 T. Uesaka,∗5 V. Wagner,∗1 H. Yamada,∗17 B. Yang,∗13 L. Yang,∗21
Z. H. Yang,∗5 M. Yasuda,∗17 K. Yoneda,∗5 L. Zanetti,∗1 J. Zenihiro,∗5,∗25 and M. V. Zhukov∗20
The high impact potential of multi-neutron systems
has led to many experimental searches for such isolated
systems starting in the early 60 s, with in particular, thefour-neutron system. Till date, only a few indications of
its existence have been found.1)
This study used the quasi-elastic knockout reaction
8He(p, pα) at maximum momentum transfer, removing
the α-core from 8 He as fast as possible, to ensure a recoilless production of the 4n as spectators.
The experiment was conducted at SAMURAI, wherea 156 MeV/nucleon 8 He beam was transported to aliquid-hydrogen target. From the combined selection of
charged particles in the reaction, the energy spectrumof the 4n system was reconstructed via the missing-mass
method, as shown in Fig. 1. Two components are observed: a pronounced peak at low energy which experimentally has a resonance-like structure, and a wide distribution at higher energies attributed to a non-resonantcontinuum response.2) As the non-resonant part cannot†

Condensed from the article in Nature 606, 678 (2022)
Institut für Kernphysik, Technische Universität Darmstadt∗2 GSI Helmholtzzentrum für Schwerionenforschung GmbH
∗3 Helmholtz Forschungsakademie Hessen für FAIR
∗4 Physik Department, Technische Universität München∗5 RIKEN Nishina Center∗6 Department of Physics, University of York∗7 Laboratoire de Physique Corpusculaire de Caen
∗8 Department of Physics, Texas A&M University∗9 IRFU, CEA, Université Paris-Saclay∗10 Molecular and Nuclear Physics, USC∗11 Faculty of Science and Engineering, University of Groningen∗12 Hungarian Academy of Sciences, Eötvös Loránd Research Network∗13 Department of Physics, Peking University∗14 Division of Experimental Physics, Rudjer Bošković Institute∗15 Department of Physics, Ewha Womans University∗16 Korea Basic Science Institute∗17 Department of Physics, Tokyo Institute of Technology∗18 Department of Physics, Tohoku University∗19 Department of Physics, University of Hong Kong
∗20 Department of Physics, Chalmers University of Technology∗21 Center for Nuclear Study, University of Tokyo∗22 Institut für Kernphysik, Universität zu Köln∗23 Department of Physics, Institute of Space Sciences∗24 GANIL, CEA Caen
∗25 Department of Physics, Kyoto University∗1Fig. 1. Missing-mass spectrum of the 4n system.

explain the sharp low-energy peak, the energy and widthof the resonance-like structure was determined assuming a Breit-Wigner shape as Er = 2.37 ± 0.58 MeV andΓ = 1.75 ± 0.40 MeV, respectively, with a striking signiﬁcance level.
From theoretical perspective there is no consensus
among the diﬀerent studies. While some predict a lowenergy resonance,3) others exclude this possibility4) andeven predict that a low-energy structure can appear asa consequence of neutron’s ﬁnal-state interaction, andthe reaction mechanism.5) Whether our observation is
attributed to a tetra-neutron correlation or other correlations between the neutrons in the ﬁnal state, needs
to be clariﬁed by ab initio theories accounting fully forthe continuum. Next-generation experiments, foreseen
at SAMURAI, using diﬀerent reaction mechanisms anddetecting the four neutrons in coincidence will shed lighton the properties of the four-neutron system and the origin of the low-energy peak.
References
1) F. M. Marqués et al., Eur. Phys. J. A 57, 105 (2021).
2) L. V. Grigorenko et al., Eur. Phys. J. A 19, 187 (2004).
3) S. Gandolﬁ et al., Phys. Rev. Lett. 118, 232501 (2017).
4) E. Hiyama et al., Phys. Rev. C 93, 044004 (2016).
5) R. Lazauskas et al., arXiv:2207.07575.

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