Physicists may have finally spotted elusive clusters of four neutrons

The strongest 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 22 issue of Nature, this indicates that the clumps 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 forces and the inner workings of atomic nuclei. Scientists don't fully understand the forces operating within groupings of merely neutrons because all atomic nuclei contain one or more protons.

The four-neutron assembly would be definitively discovered for the first time. Nuclear physicist Meytal Duer of the Technical University of Darmstadt in Germany claims that "up to now, there was no true observation of... such a system that is constituted only of neutrons."

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 team then directed that beam at 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 initial 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 numerous collisions.

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 during the collision formed the long-sought tetraneutron cluster.

According to nuclear physicist Marlène Assié of the Laboratoire de Physique des 2 Infinis Irène Joliot-Curie in Orsay, France, "there were signs, but it was never particularly obvious" whether tetraneutrons occurred in the past. Tetraneutron suggestions of only a few were observed by Assié and colleagues in 2016 (SN: 2/8/16). The researchers said they saw about 30 clusters in the new investigation. She claims that the new plot's bump is considerably more obvious. "I am confident in this measurement," you said.

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 forces between neutrons were strong enough to produce a tetraneutron resonance.

She interprets this contradiction as evidence that the researchers did not actually notice a resonance but rather some other, as-yet-unidentified effect. She suggests that the bump could be caused, for instance, by the neutrons' ability to "remember" how they were ordered inside the helium-8 nucleus.

The new findings are more in line with other kinds of theoretical computations. According to theoretical nuclear physicist Stefano Gandolfi of the Los Alamos National Laboratory in New Mexico, "theoretical results are actually 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." To fully comprehend the experiment's findings, more calculations will be required.

Additional research could 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 real or not.

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

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