Scientists using the Superconducting Multiparticle Radioisotope Beam Analyzer (SAMURAI) operated by the RIKEN Nishina Center and the Center for Nuclear Studies, University of Tokyo. in Japan, they have experimentally observed a tetraneutron . If confirmed, it could help scientists better understand nuclear forces in physics.
A history of many decades
For six decades , scientists have searched for clusters of four neutrons called tetraneutrons, but evidence for their existence has been shaky. In fact, as late as 1965 there was a study that concluded that “the existence of tetraneutrons is highly unlikely.” Now it could be different. What they have observed in SAMURAI appear to be tetraneutrons. The result strengthens the case that the four neutrons are more than a figment of physicists’ imaginations.
What exactly is a tetraneutron?
It is an exotic type of matter made up of four neutrons. Neutrons and protons form the nucleus of almost every element in the universe, held together by the strong nuclear force.
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 only for fleeting moments, in this case, less than a trillionth of a trillionth of a second.
“This experimental breakthrough provides a benchmark for testing the nuclear force with a pure system made only of neutrons,” explains Meytal Duer, from the Institute for Nuclear Physics at TU Darmstadt and co-author of the paper published in the journal Nature. “Nuclear interaction between more than two neutrons could not be tested until now, and theoretical predictions give a wide scatter in terms of the energy and width of a possible tetraneutron state.
How have they achieved it?
The team was able to accomplish this by firing an isotope of helium, called helium-8, which has an extra four neutrons compared to the more common version, at a target made of liquid hydrogen. When the particles collided, four neutrons disappeared. Its absence lasted about 1 trillionth of a trillionth of a second before reappearing as particle disintegration. The technique used was different from the one previously used to create such a particular interaction.
To confirm which scenario was most likely, Shimoura and his team measured the energy emitted by the particles in the reaction and found that it would not have been enough to eject each of the missing neutrons independently. “This confirmed that the four neutrons left behind were indeed bound to a tetraneutron particle,” the experts comment.
“A tetraneutron is so short-lived that it’s a huge shock to the world of nuclear physics that its properties can be measured before it breaks apart,” said theoretical physicist James Vary. “It’s a very exotic system.” It is, in fact, “a completely new state of matter,” he said. “It’s short-lived, but it points to possibilities. What happens if you put two or three of these together? Could you get more stability?
This discovery opens the door to new research and could lead to a better understanding of how the universe is made up. This exotic new state of matter could also have useful properties in existing or emerging technologies.
“The key to the successful observation of the tetraneutron was the chosen reaction, which isolates the four neutrons in a fast process, compared to the nuclear scale, and the chosen kinematics of large momentum transfer, which separates the neutrons from the charged particles in momentum space,” said Thomas Aumann, a physicist at the Institute for Nuclear Physics at the Technische Universität Darmstadt. “ The extreme kinematics resulted in a nearly bottomless measurement. We now plan to use the same reaction to make a precise measurement of the low-energy neutron-neutron interaction. A dedicated neutron detector is currently being built for this experiment,” the researchers conclude.
Referencia: M. Duer et al. 2022. Observation of a correlated free four-neutron system. Nature 606, 678-682; doi: 10.1038/s41586-022-04827-6
