Six decades have been devoted to the quest for tetraneutrons, which are clusters of four neutrons. However, evidence supporting their existence has been inconsistent. Now, scientists report seeing neutron clusters that resemble tetraneutrons. The finding bolsters the argument that the fab four is more than a physicists’ creation. However, some scientists are skeptical that the alleged tetraneutrons are what they seem to be.
Tetraneutrons seem to be quasi-bound or resonant states, in contrast to the firmly coupled nature of protons and neutrons in an atomic nucleus. In this scenario, the clusters exist for less than one quadrillionth of a trillionth of a second, the researchers write in the June 23 issue of Nature.
Tetraneutrons look quasi-bound or resonant, unlike protons and neutrons in an atomic nucleus. Since all atomic nuclei include one or more protons, scientists do not have a thorough grasp of the forces at work inside neutron-only groups.
Finding the quartet of neutrons conclusively would be a first. Meytal Duer, a nuclear physicist at the Technical University of Darmstadt in Germany, explains, “Up until now, there has been no actual observation of… such a neutron-only system.”
Duer and colleagues began the creation of neutron quartets using a beam of helium-8, a radioactive, neutron-rich form of helium produced at RIKEN in Wako, Japan. The scientists then fired this beam towards a proton-containing object. The alpha particle, which is made up of two protons and two neutrons, was released from the proton when it collided with a helium-8 nucleus. Each original helium-8 nucleus included two protons and six neutrons, leaving four neutrons unaccounted for.
The researchers measured the energy of the four neutrons by measuring the momenta of the alpha particle and the proton ricocheting. The test indicated a spike on a map of the neutrons’ energy over several collisions, which is the telltale indicator of a resonance.
Marlène Assié, a nuclear physicist notes that in the past “there were signs, but it was never particularly obvious” if tetraneutrons existed. Assié and colleagues reported just a few tetraneutron clues in 2016. In the latest investigation, researchers saw around thirty clusters. She reports that the rise on the new plot is considerably obvious. “I am confident in this measurement.”
However, theoretical studies of what occurs when four neutrons collide have raised doubts about the existence of tetraneutron resonance. Natalia Timofeyuk, a theoretical nuclear physicist at the University of Surrey in Guildford, England, believes that if the interactions between neutrons were strong enough to produce a tetraneutron resonance, certain sorts of atomic nuclei that are not known to exist should exist.
Due to this inconsistency, she believes that the researchers have not detected a real resonance, but rather an unidentified effect. She explains that the hump might be the consequence of the neutrons’ “remembering” how they were positioned inside the helium-8 nucleus.
Other types of theoretical calculations correlate to the new results more closely. To quote Stefano Gandolfi, a nuclear physicist at the Los Alamos National Laboratory in New Mexico, “conceptual findings are quite contentious,” since they either anticipate a tetraneutron frequency in excellent accord with the data described in this study, or they predict no resonance at all. Additional computations will be required to comprehend the experiment’s outcomes.
New experiments might also be useful. Because it is more difficult to detect neutrons, which have no electric charge, than charged particles, the researchers did not immediately examine the four neutrons. In future tests, Duer and his colleagues plan to detect neutrons and further determine the features of tetraneutrons. Future research may definitively determine if tetraneutrons are the actual thing.