Theoretical calculations predicted now-confirmed tetraneutron, an exotic state of matter
James Vary has been waiting for nuclear physics experiments to confirm the reality of a “tetraneutron” that he and his colleagues theorized, predicted and first announced during a presentation in the summer of 2014, followed by a research paper in the fall of 2016.
“Whenever we present a theory, we always have to say that we are waiting for experimental confirmation,” said Vary, a professor of physics and astronomy at Iowa State University.
In the case of four neutrons (very, very) briefly bound together in a temporal quantum state or resonance, that day is now here for Vary and an international team of theorists.
The just-announced experimental discovery of a tetraneutron by an international group led by researchers from the German Technical University of Darmstadt opens doors for new research and could lead to a better understanding of how the universe works. This new and exotic state of matter may also have properties useful in existing or emerging technologies.
Neutrons, as you probably remember from science class, are uncharged subatomic particles that, along with positively charged protons, make up the nucleus of an atom. Individual neutrons are unstable and convert to protons after a few minutes. Combinations of double and triple neutrons also do not form what physicists call a resonance, a state of matter that is temporarily stable before decaying.
Enter the tetraneutron. Using the supercomputing power of the Lawrence Berkeley National Laboratory in California, the theorists calculated that four neutrons could form a resonant state with a lifetime of only 3×10.-22 seconds, less than a billionth of a billionth of a second. It’s hard to believe, but that’s long enough for physicists to study.
The theorists’ calculations say the tetraneutron should have an energy of about 0.8 million electron volts (a unit of measurement common in high-energy and nuclear physics — visible light has energies of about 2 to 3 electron volts.) also said the width of the expanded energy peak showing a tetraneutron would be about 1.4 million electron volts. The theorists published later studies indicating that the energy would likely be between 0.7 and 1.0 million electron volts, while the width would be between 1.1 and 1.7 million electron volts. This sensitivity arose through the use of several available candidates for the interaction between the neutrons.
A just published article in the magazine Nature reports that experiments at the Radioactive Isotope Beam Factory of the RIKEN Research Institute in Wako, Japan, found the tetraneutron energy and width to be about 2.4 and 1.8 million electron volts, respectively. These are both larger than the theoretical results, but Vary said uncertainties in the current theoretical and experimental results could cover these differences.
“A tetraneutron has such a short lifespan that it’s quite a big shock to the world of nuclear physics that its properties can be measured before it decomposes,” Vary said. “It’s a very exotic system.”
It is, in fact, “an entirely new state of matter,” he said. “It’s short-lived, but points to possibilities. What happens if you put two or three together? Could you get more stability?”
Experiments trying to find a tetraneutron started in 2002 when its structure was proposed in certain reactions involving one of the elements, a metal called beryllium. A team from RIKEN found hints of a tetraneutron in experimental results published in 2016.
“The tetraneutron will join the neutron as just the second chargeless element of the nuclear map,” Vary wrote in a project summary. That “provides a valuable new platform for theories about the strong interactions between neutrons.”
Meytal Duer of the Institute of Nuclear Physics at the Technical University of Darmstadt is the corresponding author of the Nature paper entitled “Observation of a correlated free four-neutron system” and announcing the experimental confirmation of a tetraneutron. The results of the experiment are considered a five sigma statistical signal, indicating a definitive discovery with a one in 3.5 million chance that the finding is a statistical anomaly.
The theoretical forecast was published on October 28, 2016 in Physical Assessment Letterstitled “Prediction for a Four Neutron Resonance.” Andrey Shirokov of the Skobeltsyn Institute of Nuclear Physics at Moscow State University in Russia, who was a visiting scientist at Iowa State, is the lead author. Vary is one of the corresponding authors.
“Can we create a small neutron star on Earth?” Vary mentioned a summary of the tetraneutron project. A neutron star is what remains when a massive star runs out of fuel and collapses into a super-dense neutron structure. The tetraneutron is also a neutron structure, a joke of Vary is a “ephemeral, very bright neutron star.”
Vary’s personal reaction? “I had pretty much given up on the experiments,” he said. “I hadn’t heard about this during the pandemic. This came as a big shock. Oh my God, here we are, maybe we have something new.”
Physicists demonstrate the existence of a new subatomic structure
M. Duer et al, Observation of a correlated free four-neutron system, Nature (2022). DOI: 10.1038/s41586-022-04827-6
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