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Successful experiment by an international research team at GSI/FAIR in Darmstadt
An international research team has found evidence of the existence of an exotic atomic nucleus state in an experiment at the GSI/FAIR research center in Darmstadt. Although such a state has long been predicted theoretically, it has never been observed before. It involves a system consisting of an atomic nucleus of the carbon isotope ¹¹C and an η′ meson (etaprime meson) – a short-lived particle composed of a quark and an antiquark. This system is bound exclusively by the strong interaction, i.e., the force that also holds protons and neutrons together in the atomic nucleus.
The research team, which consists of the η PRiME collaboration and the Super Fragment Separator Experiment collaboration, also includes physicists from Justus Liebig University Giessen (JLU) with their working groups: Prof. Dr. Volker Metag and Prof. Dr. Christoph Scheidenberger (GSI/FAIR and Helmholtz Research Academy for FAIR).
In physics, a distinction is made between four fundamental forces (interactions): gravity, electromagnetism, strong interaction, and weak interaction. Many “bound systems” are held together by these forces. For example, the Earth and Moon are bound by gravity, and in atoms, the electromagnetic interaction holds the positively charged atomic nucleus and the negatively charged electrons together. Atomic nuclei consist of protons and neutrons and are bound together by the strong interaction. In addition to protons and neutrons, each of which consists of three quarks, there are other particles that are subject to the strong interaction, including mesons.
Some mesons are electrically negative. In rare cases, they can therefore replace an electron in atoms and are then bound to the atomic nucleus by the electromagnetic interaction, similar to electrons. However, there are also electrically neutral mesons such as the η′ meson. Because it carries no electric charge, it cannot be bound to the nucleus electromagnetically, but only via the strong interaction. Such a state, in which only the strong interaction binds, is particularly interesting because it allows conclusions to be drawn about the properties of this force.
As early as 2005, Japanese scientists predicted the existence of such a meson-nucleus system bound solely by the strong interaction. However, all experiments searching for this exotic nuclear state remained unsuccessful until recently. Only now, some 20 years later, have researchers succeeded in finding corresponding experimental evidence.
The significance of this experimental result goes far beyond the first detection of an exotic nuclear state. At the same time, it was shown that the mass of the η′ meson decreases in the matter of an atomic nucleus. The result helps to understand how the mass of mesons arises: if you add up the masses of the quarks in the η′ meson, you only get about 1 percent of the mass that a free η′ meson has. By far the largest part comes from the energy of the strong interaction between its components – corresponding to Einstein's relationship between mass (m) and energy (E): m=E/c², where c is the speed of light.
The GSI/FAIR experiment shows that this interaction changes when the η′ meson is in the atomic nucleus. The situation is similar for protons and neutrons: there, too, around 99 percent of the mass arises from the strong interaction. So when we measure our body weight, we are also indirectly measuring this interaction. That is why understanding the mass formation of strongly interacting particles is fundamental, and measurement is an important step on this path.
The experiment was proposed by Prof. Dr. Kenta Itahashi (Osaka, Japan) and his research group and carried out at the GSI fragment separator as part of the FAIR Phase 0 research program. A proton beam strikes a ¹²C atomic nucleus at around 96 percent of the speed of light and snatches a neutron from it, which forms a deuteron together with the proton and moves away in forward direction. The remaining ¹¹C nucleus is placed in a highly energetic state. This excitation energy can give rise to an η′ meson, which in rare cases binds to the ¹¹C nucleus – a short-lived bound quantum state. A special setup was developed to filter out these rare cases from many other reactions: Simultaneously with the forward going deuteron, the typical decay products of this short-lived state are measured in a detector that almost completely encloses the reaction zone. This allows the formation, short existence, and decay of the η′ meson-nucleus system to be measured.
The collaboration plans to conduct an improved follow-up experiment with significantly more measurement data in order to determine the spectroscopic properties of the bound η′ meson-nucleus system more precisely, in particular energy levels, binding energy, and decay width. This should allow further conclusions to be drawn about the origin of the mass of strongly interacting particles. In the future, the international accelerator center FAIR will open up further new research opportunities in this field thanks to higher ion beam intensities. FAIR is currently being built at GSI as part of an international collaboration to gain new insights into the structure of matter and the evolution of the universe.
Prof. Dr. Christoph Scheidenberger (Christoph.Scheidenberger@exp2.physik.uni-giessen.de),
Prof. Dr. Volker Metag (Volker.Metag@exp2.physik.uni-giessen.de)
R.Sekiya, K. Itahashi, Y.K. Tanaka, S. Hirenzaki, N. Ikeno, V. Metag, M. Nanova, J. Yamagata-Sekihara, V. Drozd, H. Ekawa, H. Geissel, E. Haettner, A. Kasagi, E Liu, M. Nakagawa, S. Purushothaman, C. Raplod, T.R Saito, H. Alibrahim Alfaki, F.Amjad, M. Amstrong, K.-H. Behr, J. Benlliure, Z. Brenic, T. Dickel, S. Dubey, S. Escrig, M. Feijoo-Fontan, H. Fujioka, Y. Gao, F. Goldenbaum, A. Grana Gonzalez, M. N. Harakeh, Y. He, H. Heggen, C. Hornung, N. Hubbard, M. Iwasaki, N. Kalantar-Nayestanaki, M. Kavatsyuk, E. Kazantseva, A. Khreptak, B. Kindler, H. Kollmus, D. Kostyleva, S. Kraft-Bermuth, N. Kurz, B.Lommel, S. Minami, D.J. Morrissey, P. Moskal, I. Mukha, C. Nociforo, H. J. Ong, S. Pietri, E. Rocco, J.L. Rodriguez-Sanchez, P. Roy, R. Ruber, S. Schadmand, C. Scheidenberger, P. Schwarz, V. Serdyuk, M. Skurzok, B,. Streicher, K. Suzuki, B. Szczepanczyk, X. Tang, N.Tortoreli, M. Vencelij, T. Weber, H. Weick, M. Will, K. Wimmer, A. Yamamoto, A. Yanai, and J. Zhao: “Excitation Spectra of the 12C(p,d) Reaction near the η’-Meson Emission Threshold Measured in Coincidence with High-Momentum Protons”, Phys. Rev. Lett. 136, 142501 – Published 7 April, 2026
DOI: https://doi.org/10.1103/6vsl-ng7x
Part of the team of physicists involved in the successful experiment.
Source: G. Otto, GSI/FAIR
Experimental set-up in a position opened for maintenance work.
Source: J. Hosan, GSI/FAIR
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Part of the team of physicists involved in the successful experiment.
Source: G. Otto, GSI/FAIR
Experimental set-up in a position opened for maintenance work.
Source: J. Hosan, GSI/FAIR
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