Physicists may have finally spotted elusive clusters of four neutrons

Physicists have discovered the strongest sign so far from the legendary four.

For six decades, researchers have been looking for clusters of four neutrons called tetraneutrons. But the evidence for their existence is unstable. Scientists now say they have observed neutron clusters that look like tetraneutrons. The result reinforces the claim that the great four is more than a figment of the imagination of physicists. But some scientists doubt that the alleged tetraneutrons are really what they seem.

Unlike an atomic nucleus, in which protons and neutrons are tightly bound together, putative tetraneutrons appear to be quasi-connected or resonant states. This means that the lumps last only fleeting moments – in this case less than one billionth of a trillionth of a second, researchers said in a June 23 publication. nature.

Tetraneutrons fascinate physicists because, if confirmed, clusters will help scientists isolate and study the mysterious neutron-neutron forces and the inner workings of atomic nuclei. All atomic nuclei contain one or more protons, so scientists do not have a complete understanding of the forces acting in groups composed only of neutrons.

The final observation of the four-neutron assembly would be the first. “So far, there has been no real observation of a neutron-only system,” said nuclear physicist Metal Dwer of the Technical University of Darmstadt in Germany.

To create the neutron quartets, Dwer and his colleagues began with a beam of radioactive, neutron-rich helium, called helium-8, created at RIKEN in Waco, Japan. The team then struck this beam at a target containing protons. When a helium-8 nucleus collides with a proton, the proton kills a group of two protons and two neutrons, also known as alpha particles. Since each original helium-8 nucleus had two protons and six neutrons, it left four neutrons alone.

By measuring the momentum of the alpha particle and the ricocheting proton, the researchers determined the energy of the four neutrons. The measurement revealed irregularities in the neutron energy graph in multiple collisions – the resonance signature.

In the past, “there were indications, but it was never very clear,” whether tetraneutrons existed, said nuclear physicist Marlene Assier of the Laboratoire de Physique des 2 Infinis Irène Joliot-Curie in Orsay, France. In 2016, Assié and his colleagues reported hints of only a few tetraneutrons (SN: 2/8/16). In the new study, researchers reported observing about 30 clusters. The inequalities in the new plot are much clearer, she says. “I have no doubt about that measurement.”

But theoretical calculations of what happens when four neutrons collide have raised skepticism about whether tetraneutron resonance could exist. If the forces between the neutrons were strong enough to create a tetraneutron resonance, there would have to be certain types of atomic nuclei that are known not to exist, says nuclear nuclear physicist Natalia Timofeyuk of Surrey University in Guildford, England.

Due to this contradiction, she believes that the researchers did not observe a real resonance, but another effect that has not yet been understood. For example, she says, the impact may be the result of “memory” that neutrons retain for how they were arranged inside the helium-8 nucleus.

Other types of theoretical calculations are closer to the new results. “Indeed, the theoretical results are very contradictory, as they either predict tetraneutron resonance in good agreement with the results presented in this article or do not predict any resonance at all,” said nuclear theoretical physicist Stefano Gandolfi of the Los Alamos National Laboratory in New Mexico. . Additional calculations will be needed to understand the results of the experiment.

New experiments can also help. Because detecting neutrons without an electric charge is more difficult than detecting charged particles, the researchers did not directly observe the four neutrons. In future experiments, Dwer and colleagues hope to spot neutrons and better determine the properties of tetraneutrons.

Future work may reveal once and for all whether tetraneutrons are the real deal.

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