Physicists have found the strongest sign yet of a legendary square.
For six decades, researchers have searched for clusters of four neutrons called tetraneutrons. But the evidence for their existence has been flimsy. Now, scientists say they have observed clusters of neutrons that appear to be tetraneutrons. The result strengthens the case that fab fours are more than a figment of physicists’ imaginations. But some scientists doubt the claimed tetraneutrons are really what they appear to be.
Unlike an atomic nucleus, in which protons and neutrons are tightly bound, so-called tetraneutrons appear to be quasi-bound or resonant states. This means that the clumps only last for fleeting moments – in this case, less than a billionth of a trillionth of a second, the researchers report in the June 23. Nature.
Tetraneutrons fascinate physicists because, if confirmed, the clusters would help scientists isolate and probe the mysterious neutron-neutron forces and inner workings of atomic nuclei. All atomic nuclei contain one or more protons, so scientists do not have a full understanding of the forces at play within groups composed of only neutrons.
Conclusively locating the four-neutron assembly would be a first. “Until now, there were no real observations of such a system consisting only of neutrons,” says nuclear physicist Meytal Duer from the Technical University of Darmstadt in Germany.
To create the quartets of neutrons, Duer and his colleagues started with a beam of a radioactive, neutron-rich type of helium called helium-8, created at RIKEN in Wako, Japan. The team then projected this beam onto 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 helium-8 nucleus had two protons and six neutrons, only four neutrons remained.
By measuring the momenta of the alpha particle and the ricocheting proton, the researchers determined the energy of the four neutrons. The measurement revealed a bump in a trace of neutron energy through multiple collisions – the signature of a resonance.
In the past, “there were indications, 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 his colleagues reported clues of only a few tetraneutrons (NS: 02/08/16). In the new study, the researchers report observing about 30 clusters. The bump on 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 that a tetraneutron resonance exists. If the forces between neutrons were strong enough to create a tetraneutron resonance, there should be certain types of atomic nuclei known not to exist, says theoretical nuclear physicist Natalia Timofeyuk of the University of Surrey in Guildford, England.
Because of this contradiction, she thinks the researchers did not observe true resonance, but another effect that is not yet understood. For example, she says, the bump could result from a “memory” neutrons retain of 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 controversial, because either they predict a tetraneutron resonance in good agreement with the results presented in this paper, or they predict no resonance at all,” says theoretical nuclear physicist Stefano Gandolfi of the National Laboratory of Los Alamos. in New Mexico. Further calculations will be needed to understand the results of the experiment.
New experiences might also help. Because detecting neutrons, which have no electrical charge, is more difficult than detecting charged particles, the researchers did not observe the four neutrons directly. In future experiments, Duer and his colleagues hope to spot neutrons and better understand the properties of tetraneutrons.
Future work could reveal once and for all whether tetraneutrons are the real deal.