Andrey Shirokov, left, of Moscow State University in Russia, who has been a visiting scientist at Iowa State, and James Vary of Iowa State are part of an international team of nuclear physicists who theorized, predicted and announced a four-neutron structure in 2014 and 2016. Archive photo by Christopher Gannon for the College of Liberal Arts and Sciences.

Tetraneutron – an exotic state of matter discovered

A long-standing question in nuclear physics is whether chargeless nuclear systems can exist. Only neutron stars represent nearly pure neutron systems, where neutrons are compressed to very high densities by gravity. The experimental search for isolated multi-neutron systems has been an ongoing quest for decades, with particular reference to the four-neutron system called the tetraneutron. system for six decades.

A recently announced experimental discovery of a tetraneutron by an international group led by scientists 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.

The first announcement of tetraneutron was made by theoretical physicist James Vary during a presentation in the summer of 2014, followed by a research paper in the fall of 2016. He has been waiting to confirm reality through nuclear physics experiments.

Now his wait is finally over when four neutrons are briefly bound together in a temporary quantum state.

What are neutrons?

Neutrons are subatomic particles with no charge that, together with positively charged protons, form the nucleus of an atom. Individual neutrons are unstable and convert to protons after a few minutes.

Why tetraneutrons?

The system made up of two neutrons, the dineutron, is known to be only about 100 keV unbound. Whether multi-neutron systems can exist as weakly bound states or very short-lived unbound resonance states is a long-standing question. The next simplest system of three neutrons is less likely due to the odd number of nucleons and therefore weaker bonding; yet a recent calculation has suggested its existence. In light of these considerations, the four-neutron system, the tetraneutron, is a suitable candidate to answer this question.

Road to the tetraneutron.

Using the supercomputing power of the Lawrence Berkeley National Laboratory in California, the theorists calculated that four neutrons could form a resonance 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.

Details of the study

The theorists’ calculations say that 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.) The calculations also said that the width of the expanded energy peak that a tetraneutron shows 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.

Recently published article in the journal 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.

Importance of the study

“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 is short-lived, but points to possibilities. What happens if you put two or three together? Can 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.”

“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 “short-lived, very bright neutron star.”

“I had pretty much given up on the experiments,” Vary said. “I hadn’t heard anything about this during the pandemic. This came as a big shock. Oh my God, here we are, maybe we’ll have something new.”

“We presented the experimental observation of a resonance-like structure consistent with a tetraneutron state near the threshold after 60 years of experimental efforts to clarify the existence of this state.” Study ends.

Magazine reference

  1. M. Duer, T. Aumann, R. Gernhäuser, V. Panin, S. Paschalis, DM Rossi, NL Achouri, D. Ahn, H. Baba, CA Bertulani, M. Böhmer, K. Boretzky, C. Caesar, N Chiga, A. Corsi, D. Cortina-Gil, CA Douma, F. Dufter, Z. Elekes, J. Feng, B. Fernández-Domínguez, U. Forsberg, N. Fukuda, I. Gasparic, Z. Ge, J.M. Gheller, J. Gibelin, A. Gillibert, KI Hahn, Z. Halász, MN Harakeh, A. Hirayama, M. Holl, N. Inabe, T. Isobe, J. Kahlbow, N. Kalantar-Nayestanaki, D. Kim, S. Kim, T. Kobayashi, Y. Kondo, D. Körper, P. Koseoglou, Y. Kubota, I. Kuti, PJ Li, C. Lehr, S. Lindberg, Y. Liu, FM Marqués, S. Masuoka, M. Matsumoto, J. Mayer, K. Miki, B. Monteagudo, T. Nakamura, T. Nilsson, A. Obertelli, NA Orr, H. Otsu, SY Park, M. Parlog, PM Potlog, S. Reichert, A Revel, AT Saito, M. Sasano, H. Scheit, F. Schindler, S. Shimoura, H. Simon, L. Stuhl, H. Suzuki, D. Symochko, H. Takeda, J. Tanaka, Y. Togano, T. Tomai, HT Törnqvist, J. Tscheuschner, T. Uesaka, V. Wagner, H. Yamada, B. Yang, L. Yang, ZH Yang, M. Yasuda, K. Yo ne da, L. Zanetti, J. Zenihiro & MV Zhukov Observation of a correlated free four-neutron system. Nature 606, 678-682 (2022). DOI: 10.1038/s41586-022-04827-6

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