A long-standing question in nuclear physics is whether chargeless nuclear systems can exist. Only neutron stars represent near-pure neutron systems, where neutrons are squeezed together by the gravitational force to very high densities. The experimental search for isolated multi-neutron systems has been an ongoing quest for several decades, with a particular focus on the four-neutron system called the tetraneutron, resulting in only a few indications of its existence so far, leaving the tetraneutron an elusive nuclear system for six decades.
A recently announced experimental discovery of a tetraneutron by an international group led by scientists from Germany’s Technical University of Darmstadt opens doors for new research and could lead to a better understanding of how the universe is put together. This new and exotic state of matter could also have properties that are useful in existing or emerging technologies.
The first announcement of tetraneutron was done 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 combine with positively charged protons to make up the nucleus of an atom. Individual neutrons aren’t stable and after a few minutes convert into protons.
The system made of two neutrons, the dineutron, is known to be unbound by only about 100 keV. Whether multi-neutron systems can exist as weakly bound states or very short-lived unbound resonant states has been a long-standing question. The next simplest system of three neutrons is less likely to exist owing to the odd number of nucleons and therefore weaker binding; yet, a recent calculation has suggested its existence. Following these considerations, the four-neutron system, the tetraneutron, is an appropriate candidate to address this question.
Way to the tetraneutron.
Using the supercomputing power at the Lawrence Berkeley National Laboratory in California, the theorists calculated that four neutrons could form a resonant state with a lifetime of just 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 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 the width of the plotted energy spike showing a tetraneutron would be about 1.4 million electron volts. The theorists published subsequent studies that indicated the energy would likely lie 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 from adopting different available candidates for the interaction between the neutrons.
Recently published paper in the journal Nature reports that experiments at the Radioactive Isotope Beam Factory at the RIKEN research institute in Wako, Japan, found tetraneutron energy and width to be around 2.4 and 1.8 million electron volts respectively. These are both larger than the theory 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 life it’s a pretty big shock to the nuclear physics world that its properties can be measured before it breaks up,” Vary said. “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 possibilities. What happens if you put two or three of these together? Could you get more stability?”
Experiments trying to find a tetraneutron started in 2002 when the structure was proposed in certain reactions involving one of the elements, a metal called beryllium. A team at RIKEN found hints of a tetraneutron in experimental results published in 2016.
“The tetraneutron will join the neutron as only the second chargeless element of the nuclear chart,” Vary wrote in a project summary. That “provides a valuable new platform for theories of the strong interactions between neutrons.”
“Can we create a small neutron star on Earth?” Vary titled a summary of the tetraneutron project. A neutron star is what’s left when a massive star runs out of fuel and collapses into a super-dense neutron structure. The tetraneutron is also a neutron structure, one Vary quips is a “short-lived, very-light neutron star.”
“I had pretty much given up on the experiments,” Vary said. “I had heard nothing about this during the pandemic. This came as a big shock. Oh my God, here we are, we may actually have something new.”
“We have presented the experimental observation of a resonance-like structure consistent with a tetraneutron state near threshold after 60 years of experimental attempts to clarify the existence of this state.” Study concludes.
- M. Duer, T. Aumann, R. Gernhäuser, V. Panin, S. Paschalis, D. M. Rossi, N. L. Achouri, D. Ahn, H. Baba, C. A. Bertulani, M. Böhmer, K. Boretzky, C. Caesar, N. Chiga, A. Corsi, D. Cortina-Gil, C. A. 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, K. I. Hahn, Z. Halász, M. N. 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, P. J. Li, C. Lehr, S. Lindberg, Y. Liu, F. M. Marqués, S. Masuoka, M. Matsumoto, J. Mayer, K. Miki, B. Monteagudo, T. Nakamura, T. Nilsson, A. Obertelli, N. A. Orr, H. Otsu, S. Y. Park, M. Parlog, P. M. Potlog, S. Reichert, A. Revel, A. T. 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, H. T. Törnqvist, J. Tscheuschner, T. Uesaka, V. Wagner, H. Yamada, B. Yang, L. Yang, Z. H. Yang, M. Yasuda, K. Yoneda, L. Zanetti, J. Zenihiro & M. V. Zhukov. Observation of a correlated free four-neutron system. Nature 606, 678–682 (2022). DOI: 10.1038/s41586-022-04827-6