Technische Universität München
The tetra-neutron – experiment finds evidence for a long-sought particle comprising four neutrons
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The tetra-neutron – a miniature neutron star
Experiment finds evidence for a long-sought particle comprising four neutrons
While all atomic nuclei except hydrogen are composed of protons and neutrons, physicists have been searching for a particle consisting of two, three or four neutrons for over half a century. Experiments by a team of physicists of the Technical University of Munich (TUM) at the accelerator laboratory on the Garching research campus now indicate that a particle comprising four bound neutrons may well exist.
While nuclear physicists agree that there are no systems in the universe made of only protons, they have been searching for particles comprising two, three or four neutrons for more than 50 years.
Should such a particle exist, parts of the theory of the strong interaction would need to be rethought. In addition, studying these particles in more detail could help us better understand the properties of neutron stars.
"The strong interaction is literally the force that holds the world together at its core. Atoms heavier than hydrogen would be unthinkable without it," says Dr. Thomas Faestermann, who directed the experiments.
Everything now points to the fact that precisely these kinds of particles were created in one of the last experiments carried out at the now decommissioned tandem Van de Graaff particle accelerator on the Garching research campus.
The long search for the tetra-neutron
As early as 20 years ago, a French research group published measurements that they interpreted as the signature of the sought-after tetra-neutron. However, later work by other groups showed that the methodology used could not prove the existence of a tetra-neutron.
In 2016, a group in Japan attempted to produce tetra-neutrons from helium-4 by bombarding it with a beam of radioactive helium-8 particles. This reaction should produce beryllium-8. In fact, they were able to detect four such atoms. From their measurement results, the researchers concluded that the tetra-neutron was unbound and quickly decayed back into four neutrons.
In their experiments, Faestermann and his team bombarded a lithium-7 target with lithium-7 particles accelerated to about 12 percent of the speed of light. In addition to the tetra-neutron, this should produce carbon-10. And indeed, the physicists succeeded in detecting this species. A repetition confirmed the result.
The team's measurement results matched the signature that would be expected from carbon-10 in its first excited state and a tetra-neutron bound by 0.42 megaelectronvolts (MeV). According to the measurements the tetra-neutron would be roughly as stable as the neutron itself. It would then decay by beta-decay with a half-life of 450 seconds. "For us, this is the only physically plausible explanation of the measured values in all respects," explains Dr. Thomas Faestermann.
With their measurements, the team achieves a certainty of well over 99.7 percent, or 3 sigma. But in physics, the existence a particle is only considered conclusively proven once a certainty of 5 sigma is achieved. Thus, the researchers are now eagerly awaiting independent confirmation.
Indications for a bound tetraneutron
Thomas Faestermann, Andreas Bergmaier, Roman Gernhäuser, Dominik Kolla, Mahmoud Mahgoubc
Physics Letters B 824 (2022) 136799 – DOI: 10.1016/j.physletb.2021.136799
The Maier-Leibnitz Laboratory, with its tandem Van de Graaf accelerator, was operated jointly by the Technical University of Munich and the Ludwig Maximillian University of Munich. The facility was shut down for structural reasons in early 2020. All five authors of the publication are graduates or employees of the Technical University of Munich.
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Dr. Thomas Faestermann
Technical University of Munich
Department of Physics, E12
The Technical University of Munich (TUM) is one of Europe’s leading research universities, with more than 600 professors, 48,000 students, and 11,000 academic and non-academic staff. Its focus areas are the engineering sciences, natural sciences, life sciences and medicine, combined with economic and social sciences. TUM acts as an entrepreneurial university that promotes talents and creates value for society. In that it profits from having strong partners in science and industry. It is represented worldwide with the TUM Asia campus in Singapore as well as offices in Beijing, Brussels, Mumbai, San Francisco, and São Paulo. Nobel Prize winners and inventors such as Rudolf Diesel, Carl von Linde, and Rudolf Mößbauer have done research at TUM. In 2006, 2012, and 2019 it won recognition as a German "Excellence University." In international rankings, TUM regularly places among the best universities in Germany.