For almost six decades, various groups of scientists have been on the hunt for mysterious and elusive particles consisting only of four neutrons, appropriately named tetraneutrons. Despite some successes, the results previously obtained did not reach the level of a full-fledged discovery, and some of them looked very controversial at all. And recently, researchers at the Technical University in Darmstadt, Germany, announced that they had managed to observe tetraneutrons directly for the first time, and the intrigue is further heightened by the fact that some experimental results indicate that tetraneutrons are not at all what they seem, nor what was previously thought.
Unlike stable atomic nuclei, in which neutrons and protons are bound by nuclear interactions, tetraneutrons are formed by resonant quasi-bonds. This means that the specific resonant state holding the neutrons remains stable for a very short time. According to available data, a tetraneutron can exist for less than one billionth of one trillionth of a second.
Tetraneutrons attract physicists because the mysterious forces of interaction between neutrons that manifest themselves in tetraneutrons can also take part in the “work” of the nuclei of conventional atoms. However, at present, scientists do not yet have a complete understanding of the nature of such interactions, nor of the effects caused by them.
In order to obtain a quartet of neutrons, scientists created a beam from the nuclei of the radioactive isotope helium-8, which was previously synthesized by scientists from the RIKEN Institute in Japan. The beam of helium-8 nuclei was directed into a region of space filled with free protons. When a helium-8 nucleus collides with a proton, it causes an alpha particle consisting of two protons and two neutrons to be knocked out of the nucleus. And, after that, the proton that provoked the collision ricochets away in one direction, the alpha particle in the other, and something consisting of four neutrons, i.e. a tetraneutron, continues to move in the direction of the original beam.
By measuring the trajectories and momentum of the alpha particle and the “ricocheted” proton, the researchers calculated the energy of the remaining tetraneutron. These measurements showed signatures characteristic of the resonant state, confirming the existence of the tetraneutron and its further decay into individual neutrons.
In contrast to all previous experiments, in this case scientists managed to register and measure some parameters of 30 tetraneutrons. This had some very unexpected consequences; due to some differences in practice from theory, the scientists think that they were not observing a true neutron resonance, but something completely different. “It could have been something like a short-term memory effect by which the neutrons were kept in the same order as they were inside the helium-8 nucleus,” the researchers write.
However, there are other types of theoretical calculations and mathematical models whose results are in excellent agreement with recent experimental data. “Some theories predict strong manifestations of neutron resonance. According to other theories, there should be no such resonance at all,” says Stefano Gandolfi, a nuclear scientist at Los Alamos National Laboratory in New Mexico, USA.
As is often the case in science, only further experiments and simulations will allow to dot all the i’s and find an answer to the question whether tetraneutrons are real quasiparticles or some ephemeral formation? However, detecting and measuring the parameters of the neutron, which has no electric charge, is more difficult than detecting charged particles. This is why scientists could not observe neutrons directly right now, but they plan to eliminate this shortcoming in the future with a specialized detector that will also measure some of the tetraneutron’s properties.
