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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 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 8He beam was key.

Background: Odd-odd nuclei, around doubly closed shells, have been extensively used to study proton-neutron interactions. However, the evolution of these interactions as a function of the binding energy, ultimately when nuclei become unbound, is poorly known. The 26 F nucleus, composed of a deeply bound π 0d 5/2 proton and an unbound ν0d 3/2 neutron on top of an 24 O core, is particularly adapted for this purpose. The coupling of this proton and neutron results in a J π = 1 1 + − 4 1 + multiplet, whose energies must be determined to study the influence of the proximity of the continuum on the corresponding proton-neutron interaction. The J π = 1 1 + , 2 1 + , 4 1 + bound states have been determined, and only a clear identification of the J π = 3 1 + is missing. Purpose: We wish to complete the study of the J π = 1 1 + − 4 1 + multiplet in 26 F, by studying the energy and width of the J π = 3 1 + unbound state. The method was first validated by the study of unbound states in 25 F, for which resonances were already observed in a previous experiment. Method: Radioactive beams of 26 Ne and 27 Ne, produced at about 440A MeV by the fragment separator at the GSI facility were used to populate unbound states in 25 F and 26 F via one-proton knockout reactions on a CH 2 target, located at the object focal point of the R 3 B/LAND setup. The detection of emitted γ rays and neutrons, added to the reconstruction of the momentum vector of the A − 1 nuclei, allowed the determination of the energy of three unbound states in 25 F and two in 26 F. Results: Based on its width and decay properties, the first unbound state in 25 F, at the relative energy of 49(9) keV, is proposed to be a J π = 1/2 − arising from a p 1/2 proton-hole state. In 26 F, the first resonance at 323(33) keV is proposed to be the J π = 3 1 + member of the J π = 1 1 + − 4 1 + multiplet. Energies of observed states in 25,26 F have been compared to calculations using the independent-particle shell model, a phenomenological shell model, and the ab initio valence-space in-medium similarity renormalization group method. Conclusions: The deduced effective proton-neutron interaction is weakened by about 30-40% in comparison to the models, pointing to the need for implementing the role of the continuum in theoretical descriptions or to a wrong determination of the atomic mass of 26 F.

Measurement of root-mean-square radii of proton distributions of 17–22N from charge-changing cross section shows the emergence of thick neutron skin towards the neutron-drip line. Signature of N = 14 shell gap has been found in nitrogen isotopes along with the emergence of neutron halo in 22N. The measured radii are in good agreement with the shell model calculations.