The researchers, as part of a major international collaboration, have reported evidence hinting at the existence of a never-before-seen but predicted exotic bound state known as an η′-mesic nucleus. The findings are published in Physical Review Letters.

Physicists have theorized that under certain conditions, short-lived particles called mesons—which only exist for less than a ten-millionth of a second—can become temporarily trapped inside a nucleus, forming an exotic bound system. Measuring mesic nuclei could help scientists understand how the strong nuclear force, which binds atomic nuclei together, behaves and how the vacuum structure changes in extremely high-density environments.

"One particle of particular interest is the η′ meson," says senior author Kenta Itahashi. "It is unusually heavy compared with related particles, and physicists expect that its mass changes when it exists inside nuclear matter. Observing this phenomenon would provide valuable information about how particle masses are generated in the universe."

To search for the η′-mesic nuclei, the international collaboration carried out a high-precision experiment using a powerful particle accelerator in GSI Helmholtzzentrum für Schwerionenforschung in Germany.

The team utilized a beam of high-energy protons bombarded on a carbon target to produce η′-mesic states. The energetic proton beam excites the carbon nucleus, producing η′ mesons, which form a bound state with the carbon nucleus with a certain probability. The excitation energies of the carbon nuclei were measured by analyzing the energy of deuterons—the simplest atomic nucleus made of one proton and one neutron—produced forward in the reaction using a high-resolution spectrometer, fragment separator (FRS).