Physicists may have finally spotted elusive clusters of four neutrons

The clearest indication of a rumored four of a kind has been discovered by physicists.

Tetraneutrons, or clusters of four neutrons, have been the subject of six decades of investigation. But there hasn't been much solid proof of their existence. Scientists claim to have now seen what look to be tetraneutron neutron clusters. The outcome supports the idea that the fab four is more than just a product of physicists' fantasies. However, other researchers question if the alleged tetraneutrons exist as described.

The alleged tetraneutrons appear to be in quasi-bound, or resonant, states, as opposed to an atomic nucleus, where protons and neutrons are strongly linked together. According to the researchers' paper published in the June 23 issue of Nature, this indicates that the clusters only exist for brief moments, in this example, less than a billionth of a trillionth of a second.

Tetraneutrons are a source of fascination for physicists as, if proven true, the clusters would enable researchers to isolate and study enigmatic neutron-neutron interactions and the inner workings of atomic nuclei. Scientists don't fully grasp the forces operating inside groupings of merely neutrons since all atomic nuclei contain one or more protons.

The four-neutron assembly would be definitively discovered for the first time. “Up to now, there was no real observation of … such a system that is composed only from neutrons,” explains Meytal Duer, a nuclear physicist at the Technical University of Darmstadt in Germany.

Duer and colleagues first used a beam of helium-8, a radioactive form of helium rich in neutrons produced at RIKEN in Wako, Japan, to produce the neutron quartets. The scientists then directed that beam onto a proton-containing target. An alpha particle, also known as a group of two protons and two neutrons, was created when a proton and helium-8 nucleus collided. There were two protons and six neutrons in each of the original helium-8 nuclei, leaving four neutrons on their own.

The researchers calculated the energy of the four neutrons by measuring the momenta of the alpha particle and the proton that was ricocheting. The test showed a resonance's telltale spike on a map of the neutrons' energy over several encounters.

Particle smashup

The momenta of the ricocheting proton and the fleeing alpha particle, which is a clump of two neutrons and two protons, were recorded when a helium-8 nucleus collided with a target proton. These results showed evidence that the four neutrons released after the collision produced the long-sought tetraneutron cluster.

Tetraneutrons may have existed in the past, but Marlène Assié of the Laboratoire de Physique des 2 Infinis Irène Joliot-Curie in Orsay, France, believes that “there were indications, but it was never very clear”. In 2016 (SN: 2/8/16), Assié and colleagues found signs of just a few tetraneutrons. In the latest investigation, the researchers claim to have seen about 30 clusters. She claims that the bump in the new storyline is considerably more obvious. “I have no doubts on this measurement.”

However, doubts about the possibility of a tetraneutron resonance have been raised by theoretical simulations of what occurs when four neutrons meet. According to theoretical nuclear physicist Natalia Timofeyuk of the University of Surrey in Guildford, England, certain forms of atomic nuclei that are not known to exist should exist if the interactions between neutrons were strong enough to produce a tetraneutron resonance.

She interprets this inconsistency as evidence that the researchers did not actually notice a resonance but rather some other, as-yet-unidentified effect. She cites the possibility that the neutrons' "memory" of their spatial arrangement inside the helium-8 nucleus may be the cause of the hump.

The new findings are more in line with other kinds of theoretical computations.

“Indeed, theoretical results are very controversial, as they either predict a tetraneutron resonance in good agreement with the results presented in this paper, or they don’t predict any resonance at all,” says Los Alamos National Laboratory theoretical nuclear scientist Stefano Gandolfi. To fully comprehend the experiment's findings, more computations will be required.

Additional research might be beneficial. The researchers didn't directly examine the four neutrons because it is more challenging to detect charged particles than neutrons, which have no electric charge. Future research by Duer and colleagues aims to locate the neutrons and more precisely define the characteristics of the tetraneutrons.

Future research may definitively establish whether tetraneutrons are genuine or not.

M. Duer et al. Observation of a correlated free four-neutron system. Nature. Vol. 606, June 23, 2022, p. 678. doi: 10.1038/s41586-022-04827-6.