A neutron can bind to an atomic nucleus or a neutron star. The free neutron has a lifespan of just under 15 minutes and decays into a proton, an electron, and an antineutrient. The two-neutron system, dineutron, is known to be disconnected by only about 100 keV. Whether multineutron systems can exist as weakly bound states or very short-lived unbound resonant states has been a long-term question1. The next simpler three-neutron system is less likely to exist due to the odd number of nucleons and therefore a weaker junction; however, a recent calculation has suggested its existence5. Following these considerations, the four-neutron system, the tetraneutron, is a suitable candidate to address this question. At ref. 1.
Numerous attempts have been made to find a clue to the existence of the tetraneutron as a bound or resonant state. Among these attempts, experiments were performed for possible bound tetraneutrons produced in uranium fission reactions (see, e.g., ref. 6). Other attempts, sensitive to both bound and resonant states, used pion-induced double-charge exchange (DCX) reactions, mainly \ ({} ^ {4} {\ rm {H}} {\ rm { e}} ({{\ rm {\ pi}}} ^ {-}, \, {{\ rm {\ pi}}} ^ {+}) \) (see, for example, ref. 7), as well as transferring reactions such as \ ({} ^ {8} {\ rm {H}} {\ rm {e}} ({\ rm {d}}, {} ^ {6} {\ rm {L} } {\ rm {i}}) \) (ref. 8). None of the experiments gave a positive signal.
Most past experiments were performed with stable cores. Towards the 21st century, with the development of radioactive ion beam installations, it has been possible to use extremely neutron-rich nuclei in which improved formation of a tetraneutron system can be expected. The first indication of a possible bound tetraneutron was reported in 20022 from a 14Be rupture reaction in 10Be + 4n. The result stimulated several theoretical studies, all agreeing on the same conclusion: a state of bound tetraneutrons cannot be obtained theoretically without significantly changing our understanding of nuclear forces9,10,11. However, the possibility that the four-neutron system exists as a near-resonant state with a very short lifespan of about 10-22 s, before decaying, has remained an open and challenging issue. It was later verified that the result reported in ref. 2 is also consistent with this state of resonance with the limit of its energy \ ({E} _ {{\ rm {r}}} \ lesssim 2 \, {\ rm {MeV}} \) (ref. 3) .
A decade later, in 2016, a tetraneutron resonance was reported4. A DCX reaction was used, but unlike previous attempts, this time the reaction was induced by a high-energy 8He radioactive beam. 8He is the most neutron-rich bound isotope and the 8He reaction channel (4He, 8Be) was investigated. The advantage of using a radioactive beam is the freedom to select the reaction partner in a so-called production without retraction (without motion transfer) of the four neutron system. It was found that the energy of the state was Er = 0.8 ± 1.4 MeV, and an upper limit of its width was estimated as ⁇ ≤ 2.6 MeV. However, due to the great experimental uncertainty, this experiment could not rule out the possibility of a linked state.
In this work, we used the quasi-elastic knockout of an α particle (4He nucleus) of a high-energy 8He projectile induced by a proton target to populate a possible tetraneutron state. The reverse kinematics elimination reaction \ ({} ^ {8} {\ rm {He}} ({\ rm {p}}, \, {{\ rm {p}}} ^ {4} {\ rm {He}}}) \) at a large moment transfer is very suitable because the 8He nucleus has the pronounced cluster structure of an α nucleus (4He) and four valence neutrons with a small center of mass motion 4th, so that after the sudden removal of the α particle occurs, a fairly localized four-neutron system with a small relative energy between neutrons, which can have a large overlap with a tetraneutron state12,13. The chosen kinematics at a large moment transfer between the proton and the α particle ensures that the four neutron system will only recede with the intrinsic moment of the 4He nucleus in the 8He rest frame, without any additional impulse transfer, allowing thus the setback. less production. In addition, the final state interactions between the four neutrons and the charged particles are also minimized due to the large momentum transfer, separating the charged reaction partners from the neutron spectators in the space of the moment (Figure 1).
FIG. 1: Schematic illustration of the quasi-elastic reaction investigated in this work.
Above: quasi-elastic scattering of the 4He nucleus of an 8He projectile from a proton target in the laboratory frame. The length of the arrows represents the momentum per nucleon (the velocity) of the incoming and outgoing particles. Zthe beam is the axis of the beam. Below: the elastic dispersion p – 4He equivalent in its frame of the center of mass, where we consider the reactions at backward angles close to 180 °, ⁇cm ≳ 160 °. In this framework, the moment of the proton equals that of 4He, \ ({{\ bf {P}}} _ {{\ rm {p}}} = – {{\ bf {P}}} _ {{} ^ {4} {\ rm {He}}} \), that is, the proton is four times faster than 4He.
The experiment took place at the radioactive ion beam plant operated by the RIKEN Nishina Center and the Center for Nuclear Studies at the University of Tokyo, using the superconducting analyzer for multiple particles of radio isotope beams ( SAMURAI) 14. An 18O primary beam was targeted at a beryllium production target producing a cocktail of radioactive nuclei from fragmentation. The secondary 8He beam was separated by the BigRIPS fragment separator and transported with an energy of 156 MeV per nucleon to a 5 cm thick liquid hydrogen target15 located on the SAMURAI spectrometer (Fig. 2).
FIG. 2: Moment of experimental assembly and loaded fragments.
Left: Schematic view of the experimental setup. The 15H MeV secondary beam at 156 MeV per nucleus is transported from the BigRIPS (Big RIKEN projectile fragment separator) to the SAMURAI configuration, where it reaches a liquid hydrogen (LH2) target. In a quasi-elastic reaction \ (({\ rm {p}}, \, {{\ rm {p}}} ^ {4} {\ rm {He}}) \), the 4He nucleus is removed from the 8He projectile. Flashing detectors and drift chambers are used for beam identification and tracking. The trajectories of the protruding fragments are tracked by three silicon (Si) planes and then bent through the SAMURAI spectrometer to the focal plane detectors. Two arrays of neutron detectors were placed at a forward angle behind the SAMURAI. An additional scintillating wall with a smaller bending angle was placed to detect the unreacted 8He beam. Right: measured moments of the knocked 4He and the scattered proton after the quasi-elastic scattering (symbols). The distribution of the amount of motion of the incoming 8He beam is shown by comparison. Solid curves are the results of the simulation. The cyan (magenta) dotted line represents the upper (lower) limit of the expected 4He (proton) momentum of the simulation assuming a quasi-elastic scatter, and the orange line indicates the center beam momentum.
Source data
The incoming beam was measured upstream of the target event by event using scintillators for load identification, as well as momentum measurement, and two drift chambers for monitoring (Figure 1 of extended data) .
The protruding charged fragments (α particle and proton) that emerge from the quasi-elastic scattering were detected by a combination of downstream targets. Three planes of silicon tape detectors, where each plane consists of two orthogonal layers that allow the position to be measured in both horizontal and vertical directions, were used to monitor, measure the energy deposition and reconstruct the reaction vertex within the target (Extended data Figs. 2 and 3).
Behind the silicon planes, both charged fragments bent through the magnetic field of the SAMURAI spectrometer, which operated with a nominal magnetic field of 1.25 T in the center of the magnet. The experiment was designed to detect an α particle and a proton emerging from the near-elastic scattering close to 180 ° within the center of mass (Fig. 1). Under these kinematic conditions, their resulting exit times are very different from each other within the framework of the laboratory, as shown in Figs. 1 and 2. The removed particle α decelerates from its initial moment, that is to say, with the speed of the incoming beam, until a moment of about 330 MeV /c per nucleon after the reaction (where c is the speed of light). Instead, the proton, which was at rest in the initial state, becomes the fastest particle in the reaction, gaining a typical impulse of about 860 MeV /c. In the focal plane, a drift chamber is used to track the fragments after the magnet, and two flickering walls next to each other, which cover a wide range of momentum, are used to measure energy deposition and flight time. The α particle and the proton are identified from a combination of their measured energy deposition, each in a different scintillation wall, and their position in the drift chamber (Extended Data Fig. 4). Its moments are determined precisely from its trajectories reconstructed using the SAMURAI spectrometer.
Because no additional momentum is transferred to the neutrons in the reaction, they continue to move at almost beam speed and can be detected, in principle, by neutron detectors located at a forward angle behind the SAMURAI spectrometer. The detection efficiency of neutrons is much lower than that of charged particles and decreases rapidly depending on the number of neutrons detected. The small elastic section p – 4He at the angles of the center of mass backwards of less than 1 micrograner (ref. 16) resulted in the relatively low statistics of 422 events obtained for \ ({} ^ {8} {\ rm {H}} {\ rm {e}} ({\ rm {p}}, \, {{\ rm {p}}}} {4} {\ rm {H}} {\ rm {e}} ) \) reaction. These factors made it impossible to detect more than …