Theoretical calculations now predicted confirmed tetraneutron, an exotic state of matter

Andrey Shirokov, left, from Moscow State University in Russia, who has been a visiting researcher at Iowa State, and James Vary from Iowa State are part of an international team of nuclear physicists who theorized, predicted and announced a four-neutron structure in 2014 and 2016. Credit: Christopher Gannon / Iowa State University College of Liberal Arts and Sciences

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 article in the autumn of 2016.

“When we present a theory, we must always say that we are waiting for experimental confirmation,” said Vary, a professor of physics and astronomy at Iowa State University.

When it comes to four neutrons (very, very) briefly bound together in a temporary quantum state or resonance, that day for Vary and an international team of theorists is now here.

The recently announced experimental discovery of a tetranutron by an international group led by researchers from the German University of Technology in Darmstadt opens the door to new research and may lead to a better understanding of how the universe is composed. This new and exotic state of matter can also have properties that are useful in existing or new technologies.

Neutrons, you probably remember from the science class, are subatomic particles without charge that are combined with positively charged protons to form the nucleus of an atom. Individual neutrons are not stable and are converted into 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 it decays.

Enter the tetra neutron. Using supercomputer power at the Lawrence Berkeley National Laboratory in California, theorists calculated that four neutrons could form a resonant state with a lifetime of only 3×10-22 seconds, less than one billionth of a billionth of a second. It’s hard to believe, but it’s long enough for physicists to study.

Theorists’ calculations say that the tetra neutron should have an energy of around 0.8 million electron volts (a unit of measurement common in high energy and nuclear physics – visible light has energies of around 2 to 3 electron volts.) The calculations also said the width of the plotted energy peak which shows a tetra neutron will be about 1.4 million electron volts. Theorists published subsequent studies that indicated that the energy would probably 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 by using different available candidates for the interaction between the neutrons.

A recently published article in the journal Nature reports that experiments at the Radioactive Isotope Beam Factory at the RIKEN Research Institute in Wako, Japan, found that tetraneutron energy and latitude were around 2.4 and 1.8 million electron volts, respectively. These are both greater than the theoretical results, but Vary said uncertainty in the current theoretical and experimental results may cover these differences.

“A tetranutron has such a short lifespan that it is quite a shock to the nuclear world that its properties can be measured before it breaks up,” said Vary. “It’s a very exotic system.”

It is, in fact, “a whole new state of matter,” he said. “It’s short-lived, but points to opportunities. What happens if you put two or three of these together? Can you get more stability?”

Experiments trying to find a tetranutron began in 2002 when the structure was proposed in certain reactions involving one of the elements, a metal called beryllium. A team at RIKEN found clues about a tetranutron in experimental results published in 2016.

“The tetra neutron will join the neutron as just the second uncharged element on the nuclear map,” Vary wrote in a project summary. It “provides a valuable new platform for theories about the strong interactions between neutrons.”

Meytal Duer at the Department of Nuclear Physics at the Technical University of Darmstadt is the corresponding author of Nature paper, entitled “Observation of a Correlated Free Four-Neutron System” and announces the experimental confirmation of a tetranutron. The results of the experiment are considered a five-sigma statistical signal, which indicates a definitive discovery with a chance of 3.5 million that the finding is a statistical anomaly.

The theoretical prediction was published on October 28, 2016, in Physical review letters, entitled “Prediction for a Four-Neutron Resonance.” Andrey Shirokov of the Skobeltsyn Institute of Nuclear Physics at Moscow State University in Russia, who has been a visiting researcher at Iowa State, is the first author. Vary is one of the corresponding authors.

“Can we make a small neutron star on Earth?” Vary called a summary of the tetraneutron project. A neutron star is what is left when a massive star runs out of fuel and collapses into a super-dense neutron structure. The tetra neutron is also a neutron structure, a Vary says is a “short-lived, very light neutron star.”

Varys personal reaction? “I had pretty much given up the experiments,” he said. “I had not heard anything about this during the pandemic. This came as a big shock. My God, here we are, we can actually have something new.”


Physicists demonstrate the existence of new subatomic structure


More information:
M. Duer et al, Observation of a correlated free four-neutron system, Nature (2022). DOI: 10.1038 / s41586-022-04827-6

Provided by Iowa State University

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