Elusive tetraneutron may have been captured for the first time

For over fifty years, researchers have searched for the elusive tetraneutron – four neutrons that form a system. Now, for the first time, a signal has been measured that is believed to be precisely this phenomenon. Thomas Nilsson, Professor at the Department of Physics at Chalmers University of Technology, has participated in the experiment and tells us more about the discovery.

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To create a system where the neutrons could interact only with each other, the researchers used nuclear reactions with atomic nuclei that already have a large excess of neutrons. The researchers created a beam of the isotope helium-8 (with two protons and six neutrons) and fired it at half the speed of light at a target of hydrogen. Thus, a collision was created where sometimes only four neutrons remained. In turn, they formed into a fleeting system of four. ​​​Illustration: Yen Strandqvist, Chalmers

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Our world is made up of atoms, whose nuclei consist of protons and neutrons. Whether a system made up entirely of neutrons can exist has long eluded the world of physics, and more than half a century has passed since searching for it began. Twenty years ago, scientists found signs of the tetraneutron after an experiment in which neutron-rich beryllium isotopes collided with carbon atoms, but the result had large margins of error and was difficult to interpret.

During a large-scale experiment carried out by a big international research team at the Radioactive Ion Beam Factory at RIKEN in Japan, it has now been possible for the first time to demonstrate what is believed to be an observation of the tetraneutron; four neutrons that are fleetingly connected. The experiment was carried out in 2016, but it has taken until now to analyse the complex measurements, the results of which have been presented in Nature.

Better understanding of the atomic nucleus

Thomas Nilsson, Professor of experimental subatomic physics and Head of the Department of Physics at Chalmers, is part of the research team. In short, the driving force behind this type of research is the curiosity to better understand the world, he says.
“The tetraneutron can help us understand the microcosm and how the atomic nucleus is built right down to the quark level – the smallest demonstrated building blocks of matter. It would be an extreme system to study and could give us insights into the strong interaction between neutrons and protons in an atomic nucleus, one of the four interaction types found in nature and the most complicated one to study, says Thomas Nilsson.
“The tetraneutron can also give us insights into the processes that take place in the universe's neutron stars. Large parts of them are made up of neutrons alone, and it is believed that heavy elements are created when neutron stars collide.”

A neutron star in the lab

Experimental studies of neutron systems are challenging because free neutrons decay within minutes. Therefore, the researchers cannot start from them. To create a system where the neutrons could interact only with each other during the experiment, the researchers used nuclear reactions with atomic nuclei that already have a large excess of neutrons. The researchers overcame the challenge by creating a beam of the isotope helium-8 (with two protons and six neutrons) and firing it at half the speed of light at a target of hydrogen. Thus, a collision was created where sometimes only four neutrons remained. In turn, they formed into a system of four – albeit as fleetingly as for 10-22 seconds (0.0000000000000000000001 seconds).
By measuring the mass and energy of the particles before and after the collision, the tetraneutron could be detected by the energy that was missing in the measurement after the collision.
“In the past there have been indications of the tetraneutron, but they have not been statistically significant. Now we have received a very clear signal and you can say that we have possibly created a minimal neutron star in the lab”, says Thomas Nilsson.
Further studies will be required to confirm the result of the experiment. In a few years, the German accelerator facility FAIR, Facility for Antiproton and Ion Research, is expected to be completed. There, the researchers will, among other things, be able to produce matter that is usually only found in space.
“At FAIR, you will be able to measure all four neutrons separately. Then we will really be able to say whether it is a four-neutron system that we have found”, says Thomas Nilsson.

More about the scientific article and the research:

  • The article “Observation of a correlated free four-neutron system“, M. Duer, T. Aumann et al.: was published in Nature, June 22, 2022. The research result has involved researchers from, among others, Technische Universität Darmstadt, Technische Universität Munich, Riken Nishina Center, GSI Helmholtz Center for Heavy-ion Research and Chalmers University of Technology.
  • From the Department of Physics at Chalmers, Mikhail Zhukov, Professor Emeritus, Thomas Nilsson, Professor, and Simon Lindberg, former doctoral student, have been involved in the planning and execution of the experiment and the writing of the article, as well as two further colleagues, Dr. Hans Törnqvist and Dr. Matthias Holl, who took part while being at TU Darmstadt. Additional researchers at the department who contributed to the instrumentation that forms the basis of the experiment are Associate Professor Andreas Heinz and research engineer Håkan Johansson. Generations of students have also worked on the subject in bachelor's and master's theses.
  • Chalmers' contribution to the scientific article has been financed by the Swedish Research Council, that recently granted continued funding for the project until 2026.
  • Read more in the press release from Technische Universität Darmstadt​.

Contact

Thomas Nilsson

  • Full Professor, Subatomic, High Energy and Plasma Physics, Physics and Astronomy
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