Physicists have found the strongest sign yet of a legendary four of a kind.
For six decades, researchers have hunted for clusters of four neutrons, called tetraneutrons. But the evidence for their existence is shaky. Now scientists say they have observed neutron clusters that appear to be tetraneutrons. The result reinforces the argument that the fab four is more than a figment of the imagination of physicists. But some scientists question whether the purported tetraneutrons are really what they seem.
Unlike an atomic nucleus, in which protons and neutrons are tightly bound together, the putative tetraneutrons appear to be quasi-bonded or resonant states† That means the clumps last only fleeting moments — in this case, less than a billionth of a trillionth of a second, the researchers report on June 23. Nature†
Tetraneutrons fascinate physicists because, if confirmed, the clusters would help scientists isolate and investigate mysterious neutron-neutron forces and the 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 made up of only neutrons.
It would be a first to definitively spot the assembly of four neutrons. “Until now, there has been no real observation of … such a system consisting only of neutrons,” says nuclear physicist Meytal Duer of the Technical University of Darmstadt in Germany.
To create the neutron quartets, Duer and colleagues started with a beam of a radioactive, neutron-rich type of helium called helium-8, made at RIKEN in Wako, Japan. The team then smashed that beam into a target with protons. When a helium-8 nucleus and a proton collided, the proton knocked out a group of two protons and two neutrons, known as an alpha particle. Because each initial helium-8 nucleus had two protons and six neutrons, 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 on a graph of the neutron’s energy across multiple collisions — the signature of a resonance.
In the past, “there was evidence, but it was never very clear” whether tetraneutrons existed, says nuclear physicist Marlène Assié of Laboratoire de Physique des 2 Infinis Irène Joliot-Curie in Orsay, France. In 2016, Assié and colleagues reported hints of just a few tetraneutrons †SN: 2/8/16† In the new study, the researchers report observing about 30 clusters. The bump on the new lot is much clearer, she says. “I have no doubts about this measurement.”
But theoretical calculations of what happens when four neutrons collide have led to 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, says theoretical nuclear physicist Natalia Timofeyuk of the University of Surrey in Guildford, England.
Because of that contradiction, she thinks the researchers did not observe a real resonance, but a different effect that is not yet understood. For example, she says the bump may be the result of a “memory” the neutrons keep about how they were arranged in the helium-8 nucleus.
Other types of theoretical calculations are more in line with the new results. “Indeed, theoretical results are highly controversial, because they either predict a tetraneutron resonance that agrees well with the results presented in this paper, or they predict no resonance at all,” said theoretical nuclear physicist Stefano Gandolfi of Los Alamos National Laboratory. in New Mexico. Further calculations are needed to understand the results of the experiment.
New experiments can 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 colleagues hope to spot the neutrons and better determine the properties of the tetraneutrons.
Future work may reveal once and for all whether tetraneutrons are real.