# Observation of a correlated free four-neutron system

A neutron can be bound either in an atomic nucleus or in a neutron star. The free neutron has a lifetime of just under 15 min and decays into a proton, electron and antineutrino. The system made of two neutrons, the dineutron, is known to be unbound by only about 100 keV. Whether multi-neutron systems can exist as weakly bound states or very short-lived unbound resonant states has been a long-standing question1. The next simplest system of three neutrons is less likely to exist owing to the odd number of nucleons and therefore weaker binding; yet, a recent calculation has suggested its existence5. Following these considerations, the four-neutron system, the tetraneutron, is an appropriate candidate to address this question. An overview of previous experiments and theoretical approaches is given in ref. 1

Numerous attempts have been made to find a hint for the existence of the tetraneutron as a bound or resonant state. Among these attempts, experiments were performed searching for possible bound tetraneutrons produced in uranium fission reactions (see, for example, ref. 6). Other attempts, sensitive to both bound and resonant states, used pion-induced double-charge-exchange (DCX) reactions, mainly the $$^4\rmH\rme(\rm\pi ^-,\,\rm\pi ^+)$$ reaction (see, for example, ref. 7), as well as transfer reactions such as $$^8\rmH\rme(\rmd,^6\rmL\rmi)$$ (ref. 8). None of the experiments yielded a positive signal.

Most of the past experiments were performed with stable nuclei. Towards the twenty-first century, with the development of radioactive-ion beam facilities, it became possible to use extremely neutron-rich nuclei in which one can expect an enhanced formation of a tetraneutron system. The first indication for a possible bound tetraneutron was reported in 20022 from a break-up reaction of 14Be into 10Be + 4n. The result stimulated several theoretical studies, all agreeing on the same conclusion: a bound tetraneutron state cannot be obtained theoretically without significantly changing our understanding of the nuclear forces9,10,11. However, the possibility of the four-neutron system existing as a resonant quasi-bound state with a very short lifetime on the order of a few 10−22 s, before decaying, has remained an open and challenging question. It was later found that the result reported in ref. 2 is also consistent with such a resonant state with the limit on its energy $$E_\rmr\lesssim 2\,\rmMeV$$ (ref. 3).

A decade later, in 2016, an indication of a tetraneutron resonance was reported4. A DCX reaction was used, but in contrast to 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(4He, 8Be) reaction channel was investigated. The advantage of using a radioactive beam is the freedom of selecting the reaction partner in a so-called recoil-less production (without momentum transfer) of the four-neutron system. The energy of the state was found to be Er = 0.8 ± 1.4 MeV, and an upper limit on its width was estimated as Γ ≤  2.6 MeV. However, owing to the large experimental uncertainty, the possibility of a bound state could not be excluded by this experiment.

In this work, we used the quasi-elastic knockout of an α-particle (4He nucleus) from a high-energy 8He projectile induced by a proton target to populate a possible tetraneutron state. The inverse-kinematics knockout reaction $$^8\rmHe(\rmp,\,\rmp^4\rmHe)$$ at large momentum transfer is well suited because the 8He nucleus has the pronounced cluster structure of an α-core (4He) and four valence neutrons with small 4n centre-of-mass motion, such that after the sudden removal of the α-particle, a rather localized four-neutron system with small relative energy between the neutrons is produced, which may have a large overlap with a tetraneutron state12,13. The chosen kinematics at large momentum transfer between the proton and the α-particle ensures that the four-neutron system will recoil only with the intrinsic momentum of the 4He core in the 8He rest frame, without any further momentum transfer, thus allowing the recoil-less production. Furthermore, final-state interactions between the four neutrons and the charged particles are also minimized owing to the large momentum transfer, separating charged reaction partners from the neutron spectators in momentum space (Fig. 1).

The experiment took place at the Radioactive Ion Beam Factory operated by the RIKEN Nishina Center and the Center for Nuclear Study, University of Tokyo, using the Superconducting Analyzer for Multi-particles from Radio Isotope Beams (SAMURAI)14. A primary beam of 18O was directed onto a beryllium production target producing a cocktail of radioactive nuclei from fragmentation. The secondary 8He beam was separated using the BigRIPS fragment separator and transported with an energy of 156 MeV per nucleon to a 5-cm-thick liquid-hydrogen target15 located at the SAMURAI spectrometer (Fig. 2).

The incoming beam was measured upstream of the target on an event-by-event basis using scintillators for charge identification as well as momentum measurement, and two drift chambers for tracking (Extended Data Fig. 1).

The outgoing charged fragments (α-particle and proton) emerging from the quasi-elastic scattering were detected using a combination of detectors downstream of the target. Three planes of silicon-strip detectors, where each plane consists of two orthogonal layers enabling position measurements in both horizontal and vertical directions, served for tracking, energy-deposition measurement and reconstruction of the reaction vertex inside the target (Extended Data Figs. 2 and 3).

Behind the silicon planes, both charged fragments were bent through the magnetic field of the SAMURAI spectrometer, which was operated at a nominal magnetic field of 1.25 T in the centre of the magnet. The experiment was designed to detect an α-particle and a proton that emerge from quasi-elastic scattering close to 180° in the centre-of-mass frame (Fig. 1). Under these kinematical conditions, their resulting outgoing momenta are very different from each other in the laboratory frame, as shown in Figs. 1 and 2. The knocked-out α-particle is slowed down from its initial momentum, that is, with the incoming beam velocity, to a momentum of about 330 MeV/c per nucleon after the reaction (where c is the speed of light). In contrast, the proton, which was at rest in the initial state, becomes the fastest particle in the reaction, gaining a typical momentum of about 860 MeV/c. At the focal plane, a drift chamber is used for tracking of the fragments after the magnet, and two scintillator walls located side by side, which cover a wide momentum range, are used for energy-deposition and time-of-flight measurements. The α-particle and proton are identified from a combination of their measured energy deposition, each in a different scintillator wall, and their position in the drift chamber (Extended Data Fig. 4). Their momenta are determined precisely from their reconstructed trajectories through the SAMURAI spectrometer.

As no additional momentum is transferred to the neutrons in the reaction, they continue moving with nearly beam velocity and can be detected, in principle, by the neutron detectors placed at a forward angle behind the SAMURAI spectrometer. The detection efficiency for neutrons is much lower than that for charged particles and decreases quickly as a function of the number of detected neutrons. The small p–4He elastic cross-section at backwards centre-of-mass angles of less than 1 microbarn (ref. 16) resulted in the relatively low statistics of 422 events obtained for the $$^8\rmH\rme(\rmp,\,\rmp^4\rmH\rme)$$ reaction. These factors made it impossible to detect more than two neutrons in coincidence with the charged particles. Therefore, the neutron detection is not a part of the current study, aside from a consistency check (provided in Supplementary Information) of the near recoil-less production of the free neutrons.

The combined selection of event-by-event identification of incoming 8He-beam particles in coincidence with the knocked-out α-particle and the scattered proton defines the $$^8\rmH\rme(\rmp,\,\rmp^4\rmH\rme)$$ channel. From a precise measurement of the momenta of the charged particles, the energy spectrum of the 4n system is reconstructed assuming energy and momentum conservation through the missing mass:

$$E_4\rmn=\sqrt{E_\rmmiss^2-\bfP_\rmmiss^2}-4m_\rmn,$$

(1)

where Emiss (Pmiss) is the energy (momentum) component of the missing-momentum four-vector, and mn is the neutron mass. Using this notation, a bound 4n system will appear at E4n < 0 whereas a resonant state will appear at E4n > 0. The missing momentum in equation (1) is defined by $$\barP_\rmmiss=\barP_^8\rmHe+\barP_\rmp(\rmtgt)-\barP_^4\rmHe-\barP_\rmp$$, where the four-momenta $$\barP$$ on the right-hand side of the equation are those of the incoming beam, target proton, knocked-out α-particle and scattered proton, respectively.

The $$^6\rmH\rme(\rmp,\rmp^4\rmH\rme)$$ knockout reaction was measured with almost exactly the same experimental conditions as for 8He, except for some small differences in the energy of the incoming beam and the beam profile (Supplementary Table 2), and served as a benchmark for verifying the analysis and calibration procedures. In the case of 6He, the 2n system is produced by the sudden removal of the 4He core. The two-neutron relative-energy spectrum is expected to be well described by theory taking into account both the well established ground-state wavefunction and the final-state scattering wave of the two neutrons, predicting a low-energy peak around 100 keV. Similarly to the 8He case, we define the missing mass ($$\barP_^8\rmHe\to \barP_^6\rmHe$$ and $$4m_\rmn\to 2m_\rmn$$).The measured missing-mass spectrum for 6He is shown in the right panel of Fig. 3 together with the theoretical calculation17 convoluted with the experimental acceptance and resolution (blue curve). The energy range shown represents the one covered by the experimental acceptance. The calculation is compared with the data by implementing it into an event generator for the quasi-elastic reaction, which uses the measured p–4He differential elastic cross-section16 as an input, as well as the measured internal momentum distribution of the α-particle in 6He (ref. 18). The generated events are transported through the experimental set-up in Geant4 simulations to account for the experimental acceptance and detector resolutions. The excellent agreement of the simulated theoretical distribution with the measured spectrum confirms the analysis and the calibration for determining the missing mass. The missing-mass resolution obtained in the measurement is approximately 1 MeV sigma, and is almost constant over the measured energy range. The systematic uncertainty for the determination of the absolute energy was estimated from this measurement to be 0.4 MeV and that of the energy width to be 0.27 MeV (Methods). Also shown in the right panel of Fig. 3 (green curve) is a possible small background contribution coming from two-step process where 4He is produced in a first step (see Methods and following discussion for 8He). This background was estimated from the measured cross-section to contribute 1% of the total number of measured events.

The measured missing-mass spectrum of the four-neutron system from the $$^8\rmHe(\rmp,\,{\rmp}^4\rmH\rme)$$ reaction is shown in the left panel of Fig. 3. Two components are observed: a well pronounced peak in the low-energy region with an energy around 2 MeV and a broad distribution at higher energies attributed to a non-resonant continuum response13, a direct four-body decay.

The shape of the non-resonant continuum spectrum of the four neutrons has been studied theoretically for the case of the four-neutron structure formed after the sudden removal of the α-core from 8He (ref. 13). The creation of the system is investigated by introducing into the Schrödinger equation a source term that accounts for the reaction mechanism producing the four-body system, and that depends explicitly on the internal structure of the parent nucleus 8He. The 8He ground-state wavefunction (without final-sate interaction) was treated using the five-body ($$^4\rmHe+4\rmn$$) cluster orbital shell model approximation (COSMA)12. The exact shape of the non-resonant continuum is sensitive to the hyperradius of the source, ρsour an internal radius of the 4n system, described in the hyperspherical harmonics basis. A hyperradius of 5.6 fm is considered by the theory as the most realistic, as it reproduces the correct experimental radius of 8He in the COSMA model. This results in a broad distribution centred around 30 MeV, in good agreement with the observed experimental spectrum.

We model the spectrum as follows:

$$f(E_4\rmn)=af_\rmres(E_4\rmn)+bf_\rmcon(E_4\rmn)+cf_\rmbkg(E_4\rmn),$$

(2)

where a, b, and c are constants, fres is a Breit–Wigner function representing the possible resonance structure, and fcon is the non-resonant continuum part presented above with the hyperradius as a parameter. The last term in equation (2), fbkg, represents possible background events coming from competing processes. Several processes were investigated and quantified (Methods), where the only non-negligible contribution found is from a two-step process involving 6He (4He) production: proton-induced break-up of 8He into 6He (4He) followed by a p–4He quasi-elastic scattering. The resulting energy distribution is broadened and shifted to lower energies compared with the pure 6He case (right panel of Fig. 3) owing to the two-step process, which has been taken into account in the simulation of fbkg. This background was estimated from measured cross-sections to contribute 2.6% to the total number of measured events (Methods), which has been used to determine the normalization constant c.

The experimental spectrum was then fitted with the energy-dependent function given in equation (2), where the fit function was convoluted with the experimental response, taking into account acceptance and detector resolutions. The experimental acceptance is not constant over the measured energy range. It is maximal in the region $$20\,\rmMeV < E_4\rmn < 40\,\rmMeV$$ (Extended Data Fig. 5).

The result of the χ2 minimization is presented by the solid blue curve in the left panel of Fig. 3, together with the individual contributions. The statistical significance of the peak structure is well beyond the 5σ level (Methods).

The probability of populating a four-neutron system in a resonant state after the sudden removal of the α-core in 8He is determined by the overlap of the 4n wavefunction in the final state and the relative motion of the four neutrons in the 8He initial state. This overlap integral defines the ratio of cross-sections for the population of the resonance and the non-resonant continuum. Unconvoluting with the acceptance of the set-up, following the energy dependence of equation (2), we extract a probability of Pr = 18.7 ± 2.3%. For comparison, the relative motion of the four neutrons studied in the COSMA model12,13 yields a probability of about 30%. This value is obtained by considering the hyperradius of 5.6 fm, whereas the resulting value from the fit to the experimental data is 5.0 ± 0.1 fm, which would yield a smaller probability to populate the resonant state.

Assuming a resonant state, its energy and width as determined from the fit are

$$\beginarraycE_\rmr=2.37\pm 0.38(\rmstat.)\pm 0.44(\rmsys.)\,\rmMeV,\\ \varGamma =1.75\pm 0.22(\rmstat.)\pm 0.30(\rmsys.)\,\rmMeV.\endarray$$

For comparison, Fig. 4 shows our result (full red symbol) together with the previous experimental result obtained from the DCX measurement4 (open red symbol). The energy of the resonance is in agreement within the uncertainty, despite the fact that different reactions were used to probe the 4n system, and is also in agreement with the upper limit given in ref. 3.

From the theory side, there is no consensus among the different theories and their predictions are partly contradictory, although, there is a general agreement that a bound 4n does not exist. In 2003, Pieper10 studied this possibility using Green’s function Monte Carlo calculations. His conclusion was that the existence of a bound 4n state has to be excluded, unless nuclear forces are drastically modified. However, his calculation suggested that a possible resonance might exist near 2 MeV, but in such a case it must be very broad.

Using a similar approach, the quantum Monte Carlo (QMC) framework based on two-body and three-body chiral interactions was used to calculate the energy of a 4n resonance5. The result supports the existence of a resonant state with an energy of 2.1(2) MeV, while no prediction has been made for its width (blue band). An extended no-core shell model (NCSM) approach using a harmonic-oscillator representation predicts different resonant states (full stars) including their corresponding widths19 (see also ref. 20 cited in ref. 20). Calculations have been performed also in the framework of the no-core Gamow shell model (NCGSM)21. These resulted in Er ≈ 7 MeV and Γ ≈ 3.7 MeV (cross), where the conclusion was in fact that the energy of a 4n resonance might be compatible with the experimental value of ref. 4, albeit with a significantly larger width. As pointed out in a later study20, these calculations were incomplete, as they were performed only in truncated model spaces or with unphysically overbinding interactions. The authors of this work20 concluded that both the energy and the width of such a resonance are comparable with the experimental data (open star). At the same time, other calculations claim that to generate such a resonance, nuclear forces have to be significantly changed22,23,24,25,26,27, which would not be consistent with the present understanding. We note that some theories26,27 predict a non-resonant low-energy enhancement of the density of states in the four-neutron spectrum. Whether such a prediction is consistent with our observed resonance-like feature cannot be currently ascertained, as the energy spectrum of the four-neutron system is not given. The drastically different predictions resulting from different theoretical approaches highlight the importance of the current firm experimental observation.

In conclusion, we have presented the experimental observation of a resonance-like structure consistent with a tetraneutron state near threshold after 60 years of experimental attempts to clarify the existence of this state. The use of a high-energy knockout reaction in inverse kinematics allowed a precise measurement. The use of a radioactive 8He beam as the parent system and a direct, large momentum-transfer reaction opened up the opportunity to create the 4n system in a well defined one-step process and in a recoil-less undisturbed way. The optimized detection system enabled a precise determination of the final state and a high-resolution measurement. A well developed peak structure has been observed at an energy of 2.37 ± 0.38(stat.) ± 0.44(sys.) MeV with a striking statistical level. This is in agreement with the result of ref. 4 and the upper limit given in ref. 3. Both the energy and the extracted width of Γ = 1.75 ± 0.22(stat.) ± 0.30(sys.) MeV are consistent with a tetraneutron state that is unbound with a corresponding lifetime of (3.8 ± 0.8) × 10−22 s. Next-generation experiments using different reaction mechanisms and possibly detecting the four neutrons in coincidence will reveal more insights into the properties of the four-neutron system, including correlations among the neutrons. More elaborated ab initio nuclear theories accounting fully for the effect of the continuum are necessary to understand the observed low-energy peak and its origin.