Physicists have found the strongest sign yet of a legendary four of a kind.
For six decades, researchers have searched for groups of four neutrons called tetraneutrons. But the evidence for their existence has been shaky. Now scientists say they have observed clusters of neutrons that appear to be tetraneutrons. The result strengthens the case that the Fab Four are more than just a figment of physicists’ imaginations. But some scientists doubt that the claimed tetraneutrons are really what they seem.
Unlike an atomic nucleus, in which protons and neutrons are solidly bound, the so-called tetraneutrons appear to be quasi-bonded or resonant states. That means the clusters last for only fleeting moments—in this case, less than a trillionth of a trillionth of a second, the researchers report in the June 23 issue. Nature.
Tetraneutrons fascinate physicists because, if confirmed, the clusters would help scientists isolate and probe the mysterious neutron and neutron forces and inner workings of atomic nuclei. All atomic nuclei contain one or more protons, so scientists don’t have a full understanding of the forces at play within groups composed solely of neutrons.
Conclusively discovering the set of four neutrons would be a novelty. “Until now, there was no real observation of… such a system made up of only neutrons,” says nuclear physicist Meytal Duer of the Technical University of Darmstadt in Germany.
To create the neutron quartets, Duer and his colleagues started with a beam of a type of radioactive, neutron-rich helium called helium-8, created at RIKEN in Wako, Japan. The team then crashed that beam into a target containing protons. When a helium-8 nucleus and a proton collided, the proton knocked out a group of two protons and two neutrons, also known as an alpha particle. Because each initial nucleus of helium-8 had two protons and six neutrons, that left only four neutrons.
By measuring the moments of the alpha particle and the bouncing proton, the researchers determined the energy of the four neutrons. The measurement revealed a bump in a graph of neutron energy over multiple collisions: the signature of a resonance.
In the past, “there were hints, but it was never very clear” whether tetraneutrons existed, says nuclear physicist Marlène Assié of the Laboratoire de Physique des 2 Infinis Irène Joliot-Curie in Orsay, France. In 2016, Assié and her colleagues reported hints of just a few tetraneutrons (Serial number: 8/2/16). In the new study, the researchers report looking at about 30 clusters. The bump in the new plot is much clearer, she says. “I have no doubts about this measure.”
But theoretical calculations of what happens when four neutrons collide have raised skepticism about whether a tetraneutron resonance can exist. If the forces between neutrons were strong enough to create a tetraneutron resonance, certain types of atomic nuclei should exist that are known not to exist, says theoretical nuclear physicist Natalia Timofeyuk of the University of Surrey in Guildford, England.
Because of that contradiction, he thinks the researchers have not observed a true resonance, but another effect that is not yet understood. For example, he says, the bulge could be the result of a “memory” that neutrons retain of how they were arranged inside the helium-8 nucleus.
Other types of theoretical calculations are more in agreement with the new results. “In fact, the theoretical results are highly controversial, either predicting a tetraneutron resonance in good agreement with the results presented in this paper, or predicting no resonance at all,” says theoretical nuclear physicist Stefano Gandolfi of Los Angeles National Laboratory. poplars. in New Mexico. More calculations will be needed to understand the results of the experiment.
New experiments could also help. Because detecting neutrons, which have no electrical charge, is more difficult than detecting charged particles, the researchers did not directly observe all four neutrons. In future experiments, Duer and his colleagues hope to detect the neutrons and better pin down the properties of tetraneutrons.
Future work may reveal once and for all whether tetraneutrons are the real deal.