Physicists may have finally discovered elusive clusters of four neutrons

Physicists have found the strongest sign yet of a legendary four of its kind.

For six decades, researchers have searched for groups of four neutrons called tetratrons. But the evidence for its existence is shaky. Now, scientists say they’ve noticed neutron clusters that look like tetratrons. The result reinforces the case that the Fab Four is more than a figment of physicists’ imaginations. But some scientists are skeptical that the so-called tetrotrons are really what they sound like.

Unlike the atomic nucleus, where protons and neutrons are strongly bound to each other, The so-called tetrotrons appear to be semi-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 reported on June 23. temper nature.

Neutrons fascinate physicists because, if confirmed, the groups will help scientists isolate and investigate the mysterious neutron and 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 at play within groups made only of neutrons.

The discovery of the cluster of four neutrons will be definitively first. “Until now, there has been no real observation … such a system consisting only of neutrons,” says nuclear physicist Mittal Doer of the Technical University of Darmstadt in Germany.

To create the neutron quatrain, Duer and colleagues started with a beam of a neutron-rich type of helium radioactive called helium-8, created at RIKEN in Wako, Japan. The team then smashed that beam into a target containing protons. When a helium-8 nucleus and a proton collided, the proton ejected a group of two protons and two neutrons, also known as an alpha particle. Because each elementary helium-8 nucleus contains protons and six neutrons, that left four neutrons alone.

By measuring the momenta of the alpha particle and the bouncing proton, the researchers determined the energy of the four neutrons. The measurement revealed a bump in a sliver of neutron energy across multiple collisions – an indication of an echo.

In the past, “there were indications, but it was never clear” whether tetratrons existed, says nuclear physicist Marlene Assier of Infinis Irène Joliot-Curie 2 Physics Laboratory in Orsay, France. In 2016, Assi and colleagues reported Hints of a few tetraneutrons (SN: 2/8/16). In the new study, the researchers reported that they observed about 30 groups. She says the bump in the new plot is much clearer. “I have no doubts about this analogy.”

But theoretical calculations of what happens when four neutrons collide have raised doubts about the possibility of a four-neutron resonance. If the forces between neutrons are strong enough to form a tetraneutron resonance, then certain types of unknown atomic nuclei must exist, says theoretical nuclear physicist Natalia Timofeuk of the University of Surrey in Guildford, England.

Because of this discrepancy, she believes, the researchers didn’t notice an actual resonance, but rather another effect that isn’t yet understood. For example, she says, the bump could result from a “memory” held by neutrons of how they are arranged inside a helium-8 nucleus.

Other types of theoretical calculations closely match the new results. says theoretical nuclear physicist Stefano Gandolfi of Los Alamos National Laboratory in New Mexico. Further calculations will be required to understand the results of the experiment.

New experiences can also help. Because neutrons, which have no electric charge, are more difficult to detect than charged particles, the researchers did not directly observe the four neutrinos. In future experiments, Doerr and his colleagues hope to identify neutrons and better characterize tetrotrons.

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